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<table style="width:100%" class="tblblue"><tr><td class="hdrl"> Elliott Sound Products</td>
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<td align="right" class="hdrr">About The Audio Pages </td></tr></table>
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<br /><p>This site was initially created some time in late 1998, and has progressed from a single page (a somewhat shorter version of the bi-amping article) to what you see today. I have gradually built up the content, and the overall site 'map' has changed several times as I have tried to incorporate all the new stuff in a reasonably sensible manner.</p>
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<p>As the site continues to grow, you will see more changes, but I will always keep the user interface as simple as possible to maximise loading speed. This is one reason that you won't see fancy mapped graphics, frames, flash animations or other frills that might make the site look really cool, but at the expense of download times. Likewise, I <i>never</i> have pop-ups that ask you to register before you can view the site contents, and likewise you won't see pop-up requests/ demands to disable your ad blocker. While I'd <i>prefer</i> that you do so, it's not (and never will be) a requirement.</p>
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<p>The overall philosophy of the site has never changed - keep to the facts, stay away from the constant efforts of the subjectivist camp to ever 'improve' on what they have - almost always with expensive 'tweaks' whose performance cannot be measured, or can only be heard by people with 'finely tuned ears' <img src="grin.gif" alt=":-)">. Music is to listen to. Recordings are rarely perfect, the concept of reproduction ever matching a live performance is a myth. Listen to the music, not the equipment.</p>
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<p>Unfortunately, many people respond more readily to rhetoric and 'herd opinion' than to facts and logic, and there are forces (hi-fi reviewers, the market in general, and the political apparatus) that see it as their business to take advantage of this tendency rather than to rectify it. My philosophy is exactly the opposite - I suggest that 'herd opinion' be eschewed, and I always try to provide information based on verifiable engineering principles.</p>
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<p>Good equipment is always something to strive for, since your enjoyment is greater when it sounds good. I love to experiment, and many of the designs are experimental - in some cases just to prove a point (the DoZ is a perfect example). Sometimes these experiments backfire (the DoZ is a perfect example!), and I get a whole bunch of e-mail telling me how great it sounds.</p>
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<p>How much of the great sound is purely the result of the reader having built it himself/herself? I honestly have no idea, but it doesn't matter. If people can get double the enjoyment from building and then listening to equipment then so much the better. In the long run it is all about enjoyment; of music, of making something and of life.</p>
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<p>May you all enjoy building my projects as much as I enjoy bringing them to you.</p>
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<hr /><b>Images</b>
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<p>I have been asked many times about the way I create the circuit diagrams (or schematics, if you insist), and over the time the pages have been running this has changed. I currently use either Protel or (mainly) SIMetrix to draw the diagrams, although I have used other methods before and since. These are simply captured and pasted into the XP version of Paintbrush (which runs fine on later versions of Windows, somewhat surprisingly) for touch-ups, and the final image is then exported as a GIF file. This method is a little time consuming, but I have found that the images are very clear, and I get consistent results. All schematics on the ESP site have unique features that allow me to recognise them even after they have been stolen and re-published elsewhere.</p>
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|
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<hr /><b>Articles & Projects</b>
|
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<p>The content of all the articles and projects is entirely my own unless otherwise stated. This extends to the philosophy of the site itself, which is mine and mine alone. This (of course) does not mean that others will not have similar ideas (many do), nor that I automatically disagree with the opinions of others who might have a slightly different opinion on the same subject. I have been corrected many, many times - for anything from spelling mistakes to errors in diagrams (I have even managed to get a few electrolytics backwards - oops!), and various people have assisted with additional information on a number of occasions.</p>
|
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||||
<p>I do not (knowingly) steal the ideas, drawings or other content of others, and any information from others is reproduced with permission and full credit is given to the original author. Contributions are encouraged, as I am determined to make the best audio web site around, and I cannot do it alone.</p>
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<p class="t_10">There is a very small number of images on these pages that seem to be in the public domain, and I have used some of these where appropriate. If any reader out there sees their image on my pages and is offended that I purloined it, let me know and I will remove it.</p>
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<hr />
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<table width="100%"><tr><td><img SRC="nospam.jpg" alt="No Spam" align="left">
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<p>Sometimes you see an image that is just too wonderful to ignore - the picture * here falls into that category. It was sent to me by a friend, and I am sorry to say that I know not where it came from. I just loved it on sight!</p>
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<p>I do not use (or condone the use of) spam (the web kind or the canned variety), so you will never get bulk e-mail or cans of 'meat-like substance' from me for any reason, so that image is appropriate in its own silly way <img src="bgrin.gif" />.</p>
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<p>I just wish I knew where it came from so I could thank its creator. Whoever you are - my thanks and apologies for 'borrowing' this image.</p>
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<br /><br />
|
||||
<p>Cheers,<br />
|
||||
Rod Elliott</p>
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</table>
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<hr />
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* <small>SPAM is a registered trademark for luncheon meat owned by Hormel Foods LLC. The author of this website has no legal, commercial or financial involvement with Hormel Foods, LLC. Neither the information herein, nor the manner in which it is presented has been authorised, condoned, sanctioned or approved by Hormel Foods LLC. This information has been included to prevent threatening letters from anyone, and is not currently a legal requirement. However, it's more fun to include it than leave it out. Someone else <i>did</i> get monstered by fax, so I thought I'd get in first. <img src="grin.gif" alt=":-)"></small></p>
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<hr /><span class="imgswap"><a href="index.html" style="display:block;"><img src="a1.gif" alt="Home"/><b class="bb">Main Index</b></a></span>
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<br /><span class="imgswap"><a href="https://sound-au.com/contact.htm" style="display:block;"><img src="a1.gif" alt="contact"/><b class="bb">Contact the Author</b></a></span><br />
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<hr /><p class="t_10">Created 09 Aug 2000</p><br />
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<meta name="description" content="A-B Box for amplifier comparisons. Visit my other pages for more audio projects and information">
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<title>A-B Box For Amplifier Comparisons</title>
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<table style="width:100%" class="tblblue"><tr><td class="hdrl"> Elliott Sound Products</td>
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<td align="right" class="hdrr">Project X </td></tr></table>
|
||||
|
||||
<h1>A-B Switch Box For Amplifier Comparisons</h1>
|
||||
<div align="center" class="t_11">© August 2000, Phil Allison, By Rod Elliott</div>
|
||||
|
||||
<p>I have finally been able to add 'Project X', thanks to Phil Allison. Somehow, 'X' just seemed appropriate <img src="grin.gif" alt="" /> Have fun.</p>
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<hr /><span class="imgswap"><a href="index.html" style="display:block;"><img src="a1.gif" alt="Home"/><b class="bb">Main Index</b></a></span>
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<span class="imgswap"><a href="projects.htm" style="display:block;"><img src="a1.gif" alt="Projects"/><b class="bb">Projects Index</b></a></span>
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<span class="imgswap"><a href="https://sound-au.com/srticles.htm" style="display:block;"><img src="a1.gif" alt="articles"/><b class="bb">Articles Index</b></a></span><br />
|
||||
|
||||
|
||||
<hr /><b>Introduction</b>
|
||||
<p>This is probably going to be a very controversial device. Its purpose is to prove people wrong and that is very confronting. If you don't wish to have your cherished beliefs about amplifiers and audio generally challenged then do not build or use this unit.</p>
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||||
<p>Many of you will know about the ABX system for doing audio comparisons. No doubt it is a very fine piece of design but out of reach for the average person. Some years ago I felt that a much simpler device would at least allow me to do comparisons on power amplifiers while the music played in a similar way to ABX. This device was the outcome. After using it for a few <i>seconds</i> my attitude to audio listening tests changed forever. If you are game for a challenge then try it yourself. It will cost you under $50 to build. It may be the best or worst fifty bucks you ever spent depending on your attitude.</p>
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||||
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||||
<p>The idea is that the listener, you, sits in the 'hot seat' and concentrates on familiar music on your favourite speakers in your own lounge room with the ability swap power amps over without moving more than one finger. If there is even a small change in timbre or definition it should be instantly audible. The stereo image and any other factor can be checked precisely since you can sit totally still during the switchover. Well at least that was what was expected to happen.</p>
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<hr /><b>Switch Box Design</b>
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<p>The design is very simple and I lashed mine up in a couple of hours and put it to use immediately. A couple of good quality relays, some hefty terminals and banana plugs and a long wire finishing in a hand held push button are the ingredients. The circuit is as shown and is self-explanatory. What is does however, is staggering.</p>
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<p>Providing the two amps to be compared are of high quality (why would you be interested in anything else?) and of course fault free, the gains are carefully matched and have similar bandwidth the device permits instant and <i>seamless</i> switching of the amplifier outputs to the speakers at the push of the button in the listeners hand.</p>
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<p>Because the relays switch in a couple of milliseconds there is <i>no</i> audible interruption. This surprised me at first and I tried my sine wave generator set to 100 Hz and pushed the switch. About half the time I could hear a faint 'tick' sound but this was not audible on musical programme.</p>
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<p class="t-pic"><img src="absw-f1.gif" alt="Figure 1" border="1" /><br />Figure 1 - The A-B Switch Box Schematic</p>
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<p>A description of the circuit is hardly needed (but I'll give a brief one anyway). The DC voltage must be matched to the relays, and 12V is suggested as the most practical. The relays should be high quality, high current types, and DPDT relays can be used with the contacts paralleled for lowest resistance. Gold flashed contacts are desirable, but standard silver contacts should be quite adequate unless there is severe atmospheric pollution in your area.</p>
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||||
<p>The speaker grounds for the two amps were connected together at the box and the speaker grounds were coupled as well. Use short thick leads for connection to the amps. It is possible this might cause an earth loop hum with some combinations.</p>
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<blockquote>
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<table>
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||||
<tr><td valign="top"><img src="note.gif" alt="note" /><td><b>Warning!</b> Never attempt to use this switching device with amplifiers that have a BTL (bridge-tied-load)
|
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output arrangement. If there is a warning that neither speaker terminal may be earthed then this describes your amplifier. If you connect it to the switching unit
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||||
you will almost certainly cause severe damage to the amplifier. Both amplifiers under test must be conventional amps that have the -ve speaker terminal connected to
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earth (ground).
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</table>
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</blockquote>
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<p>The switch (marked 'Push-On / Push-Off') is the remote switch, and needs a lead that is long enough to reach the listening position. There is no real limit to the length, even with light duty figure-8 'speaker' lead, but in excess of 10 metres or so may cause some voltage loss. The LED is optional, and may be omitted. If fitted, be very careful that the LED cannot be seen from the hot seat, as it may dim slightly when the relays are energised, thus giving a visual clue - this you don't need!</p>
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<hr><b>Setting up the test</b>
|
||||
<ol>
|
||||
<li>Build the box as shown in Figure 1 and place the two amplifiers close together. If one is on top of the other, then place some cardboard in between
|
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to prevent metallic contact and possible hum loops. (Make sure that you don't obstruct any air vents.)<br /><br /></li>
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||||
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||||
<li>Make 'Y' leads to connect the left and right input signals to both amplifiers.<br /><br /></li>
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||||
<li>Use an audio generator or test CD to set gains to read equally ±1% on a digital multimeter at about 400 Hz. Leave the speakers unconnected while
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you do this.<br /><br /></li>
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||||
<li>Place the pushbutton on the 'hot seat' or at the place where you would normally sit to listen to stereo.<br /><br /></li>
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<li>Check operation of the box by switching with no music and listen for the click of the relays as they changeover. There may be a faint click from the
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speakers too, due to small DC offsets. Trim these out if possible - the idea is to get a seamless change with no audible cues.<br /><br /></li>
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<li>Put your favourite track on the CD player or turntable if you prefer vinyl.<br /><br /></li>
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||||
<li>Listen, and gently push the switch whenever you want to change amps.<br /><br /></li>
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<li>Go and check why the box is not working.<br /><br /></li>
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||||
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||||
<li>If step 8 is not needed check your wiring for correct phase and levels again.<br /><br /></li>
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||||
<li>When you have everything right try it on your friends and family. They will likely think you have gone mad and/ or are playing tricks on them. The most
|
||||
likely comment is simply "The switch is not working."<br /><br /></li>
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||||
<li>Sit quietly and contemplate what you have just found, and all its implications.<br /><br /></li>
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||||
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||||
<li>Don't blame me, I did warn you<b><i>!</i></b></li>
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</ol>
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||||
<p><b>Notes:</b>
|
||||
<br>When comparing amps of different power ratings, stay within the capacity of the smaller unit. Valve (vacuum tube) and transistor amps may be compared as long as the valve model has a damping factor greater than 25. Valve amps with a low damping factor (output impedance of 2Ω or more) <i>will</i> sound different (note: different does <i>not</i> mean 'better'!). Any test involving a valve amp is at your own risk! Be careful, as many valve amps don't like an open-circuit output. Amps with subsonic filters may be distinguishable from those without on some material.</p>
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||||
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||||
<p>Matching the gain may be difficult if the amps do not have level controls. Solder a 10kΩ multiturn trim pot to the back of each RCA plug on the amp with more level to set the gain.</p>
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||||
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||||
<p>You <b><i>must</i></b> use a push-on, push-off hand held switch. Never use one that needs to be held down to make the relays change over. The switch must be one that does not change 'feel' from one position to the other (some feel slightly different depending on the latching mechanism).</p>
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||||
|
||||
<p>After the initial surprise (shock) wears off, try the old 'stop and restart A-B test' method again. See what happens!</p>
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||||
<hr><b>Editor's Notes</b>
|
||||
<p>This is a contributed article, and the author (Phil Allison) has made this information available for the sake of audio.</p>
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||||
|
||||
<p>I do suggest that if you want to test this technique that Phil's instructions must be followed to the letter - even the smallest variation in level can invalidate any A-B test. Comparisons between valve and transistorised amplifiers are likely to show differences, only partly due to the higher output impedance of a valve amp, and extra care is needed to balance the levels with this combination.</p>
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||||
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||||
<p>It is also important that there are no visual cues that might alert you that one amp or the other is in operation. To be safe, place a screen of some sort between you and the amplifiers and switch box. As Phil has stated, if amplifiers are of different power ratings, make sure that neither amp clips (distorts) at any point during the tests. This will be immediately audible, and is not a valid test for an amplifier's sound quality (since it has none when clipping!)</p>
|
||||
|
||||
<p>You might find it hard to get a push-on/ push-off switch that has no difference in feel between states. If this is the case, you can use the circuit shown in <a href="project166.htm" target="_blank">Project 166</a>.This allows a momentary switch to be used, and there will be absolutely no difference in feel either way. If you decide to include an indicator LED (which is a good idea so you can verify the switching action), it must have a switch in series so it can be turned off for testing. Before you start, you might want to get someone else to press the button a (random) number of times to make sure that there's no in-built bias. This is recommended regardless of the switch used.</p>
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||||
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||||
<p>If you don't want to be confronted by this switch box, build one anyway. It will allow you to make comparisons between amplifiers that are otherwise impossible to do with accuracy or repeatability. You might find that there are audible differences, or you might not. Either way, it gives you the ability to <i>know</i> (rather than assume or imagine) that one amplifier is different from another.</p>
|
||||
|
||||
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||||
<hr /><b>Testing Speakers</b>
|
||||
<p>This tester can also be used to change speakers for comparisons. These are the most inaccurate of all electronic components, and there are some interesting traps that can affect the result, leading to a very wrong conclusion. I have discussed this elsewhere, but most people will be unaware that some things are decidedly counter-intuitive. If one speaker (A) has a notch (aka 'suck-out') at some frequency, if you listen to it for 30 seconds or so, then switch over to another speaker (B) that has (comparatively) flat response, speaker B will sound wrong!</p>
|
||||
|
||||
<p>It will seem to the listener that speaker B has a <i>peak</i> at the frequency where the notch was situated in speaker A. This can be verified either with hardware (a notch filter that you can switch in and out of circuit), or you can use Audacity or similar to insert a notch. This is human hearing (ear-brain interface) at work, and it's surprisingly easy to be fooled by the way our auditory systems work. This is always active, and (perhaps surprisingly) it doesn't matter much if the test is blind or sighted. The test <i>should</i> be blind to prevent visual cues that can lead to other issues, but it will still work even when you know which is which. Avoiding the experimenter-expectancy effect is still important though.</p>
|
||||
|
||||
<p>There's a discussion of this issue at <a href="https://harbeth.co.uk/harbeth-blog-home-section/" target="_blank"><u>Harbeth</u></a>, with four videos. The first two discuss this issue in some depth. Never underestimate the apparent 'problems' you might hear that are caused by our hearing mechanism. Despite the (fallacious) claims that only listening can reveal the 'truly best' audio, it becomes very obvious that test equipment is your friend. Measurements have no inbuilt prejudices - provided the measurement is set up properly of course.</p>
|
||||
|
||||
<p>If comparing speakers, the amplifier goes to the 'speaker' terminals, and the speakers are wired to the 'amp 1' and 'amp 2' terminals. Unless the speaker systems have <i>identical</i> sensitivity (dB/W/m) they will sound different, with the louder speaker almost invariably sounding better (even if it's not).</p>
|
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<hr /><span class="imgswap"><a href="index.html" style="display:block;"><img src="a1.gif" alt="Home"/><b class="bb">Main Index</b></a></span>
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<span class="imgswap"><a href="projects.htm" style="display:block;"><img src="a1.gif" alt="Projects"/><b class="bb">Projects Index</b></a></span>
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<table class="tblblk" cellpadding="5">
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<tr><td class="t-wht"><a id="copyright"></a><b>Copyright Notice.</b> This article, including but not limited to all text and diagrams, is the intellectual property of Phil Allison and Rod Elliott, and is © 2000. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro- mechanical, is strictly prohibited under International Copyright laws. The author (Phil Allison) and editor (Rod Elliott) grant the reader the right to use this information for personal use only, and further allow that one (1) copy may be made for reference. Commercial use of this published material is prohibited without express written authorisation from Phil Allison and Rod Elliott.</td></tr>
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<table style="width:100%" class="tblblue"><tr><td class="hdrl"> Elliott Sound Products</td>
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<td align="right" class="hdrr">Project ABX </td></tr></table>
|
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|
||||
<h1>ABX Double Blind Audio Tester</h1>
|
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<div align="center" class="t_11">© August 2002, Steven Hill, Rod Elliott</div>
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<hr><b>Introduction</b>
|
||||
<p>This project describes the construction of test equipment for double-blind or ABX testing of source components - preamplifiers, tuners, DACs etc. or even, if that is your particular vice, interconnects. It builds on the work done by Phil Allison described in Project X. I recommend that you read that project description before you commence this one. If you do not like what you read there, then you might as well stop reading at this point.</p>
|
||||
|
||||
<p>Double-blind and ABX tests do not allow the listener to know which component they are listening to, and furthermore don't allow the test controller to know either. This guards against visual cues to the audience (including body language).</p>
|
||||
|
||||
<p>There is information on the principles behind ABX testing elsewhere on the Net, therefore, I intend to give only the briefest description here.</p>
|
||||
|
||||
<p>An ABX test allows the listener to select either A or B as many times as they like, and ultimately decide which of these is X, where X is randomly selected by the equipment to be either A or B, and the responses are logged for correlation when the test is complete. For example, a 1000Hz tone is assigned as A, and a 1500Hz tone as B. Random selection determines which of these is X. Listen to A then X, listen to B then X, and decide if A or B sounds like X (this would be a rather easy test for anyone to get right, of course- but if B were to be 1001Hz it may be more of a challenge).</p>
|
||||
|
||||
<p>True ABX testing is normally not easy and uses a microcontroller or a PC to interface to the switching module. This was not an option as the project is meant to be simple and inexpensive. The simple remote control (part 2) requires an operator to control switching between A and B and to whom the active channel is known. However, if you proceed to build the double-blind remote (part 3), then the unit is a true ABX comparator - you select the next test in the sequence with the 12-position rotary switch and then have the opportunity to decode if X is input A or input B. This is 'double-blind' because the sequence is not known until the test has been completed.</p>
|
||||
|
||||
<p>With the basic unit, two pieces of equipment (for example, two preamplifiers) are under comparison (A and B). They are fed from the same source. The audience hears first A and then B. Thereafter, the test controller selects either A or B as X and repeats the test for up to (about) twenty iterations, changing from A to B at his or her whim, and the audience is required to write down which they think it is for each iteration. Thereafter, the results are analysed to find out whether the audience was able accurately to identify which piece of equipment was in use at each iteration of the test.</p>
|
||||
|
||||
<p>There is no requirement that A and B are selected the same number of times during any single test. In fact, a test controller once ran a whole test with A. None of the members of the audience selected 'A' for each iteration. I leave you to consider what that result says about people's confidence in their ability to identify different components.</p>
|
||||
|
||||
<p>The project is in three parts of which you must build at least two. I recommend that you build only parts 1 and 2 to start with. You might find that your experience of this piece of test equipment is so infuriating that you will regret the time spent on building part 3 if you proceed with it immediately.</p>
|
||||
|
||||
<p>It is vitally important that the output level of the devices under test is equal. A difference of 1dB is normally sufficient to bias the test, normally in favour of the louder channel. Therefore, you will need a test-tone CD for calibration purposes if using CD as your source or an LP with test tones if using vinyl. If comparing tuners, you will probably find that tuning in the white noise between stations is the only way you can calibrate the two pieces of equipment. You will also need a multimeter. Unless you are using an expensive true RMS digital multimeter, you should not use a test tone above 500Hz. With an analogue meter you can use a higher pitch but I do not recommend that you go higher than 1kHz. 1dB represents a voltage between 891.25mV and 1.122V, assuming a reference level of 1V (RMS). You should aim for better than ±50mV variation for a 1V signal. This equates to ±25mV for a 500mV signal.</p>
|
||||
|
||||
|
||||
<hr><b>Part 1: The Controller Box</b>
|
||||
<p>The circuit diagram should be fairly easy to understand. There is nothing difficult about it, and no electronics are involved in the audio path.</p>
|
||||
|
||||
<div class="t-pic"><img src="abx-f1.gif" alt="Figure 1" border="1" /><br />Figure 1 - The Switching Unit</div>
|
||||
|
||||
<p>Connections 1 through 4 go to either of the remotes and should be wired to a four-pin connector. The relays K1-3 can be low current types. The battery must match the voltage of the relays. I used 6V DPDT relays. K3 and K2 are used to select input A or B respectively. K1 mutes the output for calibration purposes. You could replace K1 with a switch if you wish but you then have to run more wires inside the box. The voltmeter connects externally to the two pins marked "Calibrate". Pots are used on both inputs solely for consistency - if a pot is used on only one of the inputs, then some people may use this as "proof" that the device modifies one channel and not the other, and this can be used as an excuse as to why the results failed to correlate "correctly".</p>
|
||||
|
||||
<p>The circuit should be built on a piece of Veroboard or similar and mounted in a shielded metal case. SW1 and D4 are mounted on the outside of the case, as are the connectors for attaching the voltmeter, the 4-pin connector for connection to the remote and the RCA plugs for the A and B inputs and the X output.</p>
|
||||
|
||||
<p>If you are proposing to use only the remote described in part 2 you do not need to take any precautions to ensure that K2 and K3 are inaudible at the listening position. However, testing using the part 3 controller requires that the relays be inaudible at the listening position. To do this you will have to mechanically decouple the circuit board from the case and also use some acoustic dampening material in the case. See note 2 below.</p>
|
||||
|
||||
<p><b>WARNING:</b> I do not recommend that you omit the muting part of the circuit. When you come to use this device, you will first choose the music you wish to hear for the test and establish a realistic listening level Then you mute the output and use the 0dB level test tone to adjust the levels of both channels to the same voltage. You will probably find that the steady-state voltage is greater than 2V RMS. Unless you have monster loudspeakers, a tone at this level, fed through a power amplifier with a 30dB gain (which is common), will probably destroy your speakers and will do nothing for your hearing.</p>
|
||||
|
||||
<p>I have found that different pieces of music require different volume settings to establish a realistic listening level. An AB test where you are wincing at the volume or straining to hear the music is of no use. The calibration mute circuit has been included to speed up the recalibration between tests using different pieces of music.</p>
|
||||
|
||||
<p>That said, however, if you wish to go through the process of either (a) turning off your power amplifier(s) or (b) disconnecting your loudspeakers each time you need to recalibrate, you can omit this part of the circuit. However, you have been warned. (And you have forgotten to reconnect the speakers and/or turn on the amp(s) before proceeding with the test.)</p>
|
||||
|
||||
<p>VR1 (and optionally VR2) is included for trimming of the voltages during calibration. Obviously, the volume should be set using the preamps' volume controls (if those are what you are comparing). However, if you are testing preamps, both of which have stepped volume controls, you might find it difficult to match the voltages at your realistic listening level. That is what VR1 is for. If you don't expect to be doing this, VR1 can be omitted. For A-B testing of other equipment, such as tuners, VR1 will be required.</p>
|
||||
|
||||
<hr><b>Part 2: The Manual Remote And Its Operating Procedure</b>
|
||||
<p>This is a simple selector which can be built in a small plastic box and is connected to the controller using four-core cable. Heavy duty cable is not required. The cable should be long enough that the operator can sit further away from the source equipment than a person in the normal listening position. SW1 is a 3-position rotary selector switch used to select A or B or null (centre position). The channel selected is indicated by the relevant LED. SW2 is a DPDT toggle switch which flips the channel from A or B or vice versa. This can be used for simple A-B testing. I have found that there is an audible break when a toggle switch is used for SW2. You might find no break is audible if you use a rotary selector.</p>
|
||||
|
||||
<div class="t-pic"><img src="abx-f2.gif" alt="Figure 2" border="1" /><br />Figure 2 - The Basic Remote Schematic</div>
|
||||
|
||||
<p>To do an AB test with this remote you will need a (patient) assistant. The procedure (using as an example two preamps as the devices under evaluation) is as follows:</p>
|
||||
|
||||
<table cellpadding="2" cellspacing="2" style="text-align: left; width:100%">
|
||||
<tr><td width=25 valign="top">a)</td><td valign="top">Use Y-splitters to connect the source equipment to each of the preamps. Connect the output of one
|
||||
preamp to input A and the other to input B on the controller box. Connect output X to your crossover or power amplifier.</td></tr>
|
||||
<tr><td>
|
||||
<tr><td valign="top">b)</td><td valign="top">Select one of the channels and play some music adjusting the volume on that channel to the desired realistic
|
||||
listening level.</td></tr>
|
||||
<tr><td>
|
||||
<tr><td valign="top">c)</td><td valign="top">On the controller box, mute the output using the switch provided. Connect a voltmeter to the calibration
|
||||
terminals. Insert the test tone CD, play a 0dB calibration tone and note the voltage. Use SW1 on the remote to change to the other channel and adjust
|
||||
the volume so that the same voltage is displayed. If stepped volume controls are in use, you might need to trim using VR1 on the controller. (Set VR1
|
||||
to the maximum before calibrating and use it to attenuate the voltage.) Once the voltage is adjusted, remove the test-tone CD and replace the music CD
|
||||
and turn off the muting switch. Do not touch the volume controls again for the duration of the test.</td></tr>
|
||||
<tr><td>
|
||||
<tr><td valign="top">d)</td><td valign="top">Assume your normal listening position. You will need a pencil and paper to record your choices.</td></tr>
|
||||
<tr><td>
|
||||
<tr><td valign="top">e)</td><td valign="top">The person conducting the test (the operator) should sit out of view of the listener with the remote to hand.
|
||||
The listener must not be able to see the LEDs on the remote. Decide which one of you controls the source and how many iterations of the test will take place.</td></tr>
|
||||
<tr><td>
|
||||
<tr><td valign="top">f)</td><td valign="top">The operator selects first channel A and the listener familiarizes him/herself with the particular
|
||||
characteristics of the preamp connected to that channel. Channel B is then selected and familiarization of that channel takes place.</td></tr>
|
||||
<tr><td>
|
||||
<tr><td valign="top">g)</td><td valign="top">The operator now selects either channel, the music is played and the listener to writes down whether A or
|
||||
B is his choice for X. The listener does not divulge to the operator his choice. The test is then continued for the number of iterations agreed upon,
|
||||
with the operator selecting either A or B. I recommend that, to avoid any unintentional bias, the sequence of changes be determined in advance of the
|
||||
test by some random operation (tossing a coin, throwing a die, for example). The operator should carry out this random operation prior to conducting
|
||||
the test.</td></tr>
|
||||
<tr><td>
|
||||
<tr><td valign="top">h)</td><td valign="top">The last iteration of the test having been concluded you may either check the results or return to step (b)
|
||||
with another piece of music.</td></tr>
|
||||
</table>
|
||||
|
||||
<p>The listener's choices are then compared with the operator's notes of the actual channel in use during each iteration. It should be immediately apparent whether any there is any correlation between the listener's choices and the actual selections.</p>
|
||||
|
||||
<p>If you do not have a patient assistant, or patience has run out, you might wish to proceed to ...</p>
|
||||
|
||||
<hr /><b>Part 3: The Double-Blind Remote And Its Operating Procedure</b>
|
||||
<p>This remote uses cascaded SPDT slide switches to set up an A-B sequence which can be used blind for testing purposes and then read back to check the results. It should be apparent from the schematic how this works. The separate poles of SW1 and SW2 must each connect to lines 3 (A) and 4 (B). SW3 selects between SW1 and SW2. For each position on the rotary selector SW4, three SPDT slide switches are required. I have built this with a twelve-position selector using 36 slide switches.</p>
|
||||
|
||||
<div class="t-pic"><img src="abx-f3.gif" alt="Figure 3" border="1" /><br />Figure 3 - ABX Remote Random Switching Unit</div>
|
||||
|
||||
<p>In the above, 'n' is the number of poles on the rotary switch. If a 12 position switch is used for SW4, then 'n' is 12, and 'n-1' is 11.</p>
|
||||
|
||||
<p>The schematic, for clarity, shows the switches SW1, SW2 and SW3 connected together in a regular sequence. However, when building this remote, the switches are hooked up in an irregular fashion so that it is not possible from the outside of the box to identify what function any particular switch has, nor how they are wired in relation to one another. The separate poles of each switch (SW1 and SW2), however, must each connect to the A and B buses. If you do not do this you will introduce a bias in favour of one of the channels. The photo shows my own unit. Properly constructed, it should look like a mess of wires, as does mine. The orange and blue wires are the bus lines, the green wires the connections between the centre poles of the SW1s and SW2s and the brown wires go to the rotary selector.</p>
|
||||
|
||||
<div class="t-pic"><img src="abx-f4.jpg" alt="Figure 4" /><br />Figure 4 - The Wiring of the Remote (SW6 Not Visible)</div>
|
||||
|
||||
<p>If you decide to proceed with this part of the project, I must warn that this remote is tedious to construct and you must proceed with great care because trouble-shooting an incorrect hook-up or a dry joint can be difficult. You should first wire between the centre poles of the SW1s and SW2s to the SW3s, then run the bus wires to the outside poles of the SW1s and SW2s and finally connect from the centre pole of the SW3s to the rotary selector. Solid-core or magnet wire is recommended. The remote can be constructed in a plastic case.</p>
|
||||
|
||||
<p>So how do we use this abomination?</p>
|
||||
|
||||
<table cellpadding="2" cellspacing="2" style="text-align:left; width:100%">
|
||||
<tr><td width=25 valign="top">i)</td><td valign="top">It is connected to the control box with the same four-core connector that you made up for part 2.</td></tr>
|
||||
<tr><td>
|
||||
<tr><td valign="top">j)</td><td valign="top">Before proceeding with the test, set up the test sequence by moving half of the total number of slide switches on the box. SW5 must be open at
|
||||
this time; no LEDs illuminated.</td></tr>
|
||||
<tr><td>
|
||||
<tr><td valign="top">k)</td><td valign="top">SW6 switches selects A or B for calibration as per (b) and (c). SW5 is closed (LEDs in circuit) during this step.</td></tr>
|
||||
<tr><td>
|
||||
<tr><td valign="top">l)</td><td valign="top">Open SW5 so that no LEDs are illuminated.</td></tr>
|
||||
<tr><td>
|
||||
<tr><td valign="top">m)</td><td valign="top">Assume your normal listening position and use SW6 to select A and B as in step (f)</td></tr>
|
||||
<tr><td>
|
||||
<tr><td valign="top">n)</td><td valign="top">Move SW6 to the X position. Is it A or B? Write down your choice. Proceed through all the other positions on SW4 marking down your choice each time.</td></tr>
|
||||
<tr><td>
|
||||
<tr><td valign="top">o)</td><td valign="top">When all positions have been sampled, close SW5, rotate the selector SW4 and observe which channel was in use for each iteration of the test. Correlate
|
||||
your results.</td></tr>
|
||||
<tr><td>
|
||||
<tr><td valign="top">p)</td><td valign="top">If you wish to repeat the test, proceed again from step (j).</td></tr>
|
||||
</table>
|
||||
|
||||
<p><b>Notes</b></p>
|
||||
<ol>
|
||||
<li>It is probably legitimate to build part 3 with only an 8-position switch (24 slide switches) and run the test twice for 16 iterations (or a 10-position
|
||||
switch with 30 slide switches for 20 iterations). In this case, you would read back the results as in (o) note them down without correlating them, stop
|
||||
the music, perform step (j) and then proceed again from (m).<br /><br />
|
||||
|
||||
<li>I mentioned in part 1 that, if you were considering building part 3 of the project you should ensure that the relays are not audible from the listening
|
||||
position. This is because, with an 'off-the shelf' rotary switch, even thought they are normally break-before-make, you will probably find that there is
|
||||
no interruption of the sound when changing positions even if there is a change of channel. If this is the case and if there is no change of channel, the
|
||||
relevant relay will not drop out. However, a change of channel will cause the relays to switch and you will hear it at the listening position, which is
|
||||
something of a give-away! If this troubles you, specify true 'break-before-make' switches for SW1 and SW2 and then the relays will always click. (Actually,
|
||||
this is probably less troublesome than trying to insulate the controller box but has the downside that there might be an audible break in the music.)<br /><br />
|
||||
|
||||
<li>You should be able to acquire all of the parts, including the cases, for less than $100. Parts 1 and 2 are simple to build. Part 3 will take a fair
|
||||
amount of time. However, the results of using this equipment may well astound or confound you and those to whom you demonstrate it. Did you ever wonder
|
||||
why your local high-end audio store does not have something so inexpensive like this so that you can compare your modest preamp, or whatever, with the mega
|
||||
-bucks piece of kit you have been drooling over ever since some reviewer's panegyric: "The best preamp I have ever heard for (only) the cost of a small
|
||||
family car."?<br /><br />
|
||||
|
||||
<li>I mentioned at the beginning that you could use this to compare interconnects. How? Simply connect it in reverse: source into X, the cables under
|
||||
comparison to A and B then via a Y-splitter to the preamp. The presence of the Y-splitter and the controller box does not invalidate the test because the
|
||||
same 'degradation' of the signal occurs to both cables. The author accepts no responsibility for domestic discord or other unpleasantness arising out of
|
||||
the use of this test equipment for such purposes.<br /><br />
|
||||
|
||||
<li>ABX testing of CD players, turntables and cartridges etc. is problematic because of the difficulty of synchronizing them. However, the flip switch
|
||||
on the manual controller can be used to make a direct comparison between the two pieces of equipment.
|
||||
</ol>
|
||||
|
||||
<p>I hope that this little project will amuse you.</p>
|
||||
|
||||
<p>Steven Hill, August 2002</p>
|
||||
|
||||
|
||||
<hr /><b>Editor's Notes</b>
|
||||
<p>Steven has done something I had thought would only really be possible using a microcontroller or a PC to achieve. The random switching is of such complexity that it would be virtually impossible for anyone to know and remember the combinations created by each switch. Especially if assembled according to the instructions - the switches are not only capable of giving an excellent randomisation of their own accord, but if they are wired in a random fashion as you build the unit, the likelihood of being able to remember the combinations is extremely low.</p>
|
||||
|
||||
<p>Will it be worth the effort? Only you can answer that, and it depends on how serious you are about being able to tell the difference between pieces of equipment. Just like Phil's original Project "X", your first reaction may be that the unit is not working - the LED indicator that Steven included in the design to allow you to correlate the results will certainly prove that A and B are being selected, and if you are <i>really</i> unsure, you can always switch off one of the units under test.</p>
|
||||
|
||||
<p>All in all, this is an ambitious project, but one that every hi-fi reviewer should make (or have made) - I expect that if this were done, a great many of the glowing reviews we currently see would diminish. They may even vanish altogether.</p>
|
||||
|
||||
<p>Needless to say, the tester can be also used to verify that the expensive capacitors you bought really don't make any difference, or that all well constructed interconnects sound the same. This is all very confronting, but it is necessary if we are to get hi-fi back on track, and eliminate the snake oil.</p>
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<tr><td class="t-wht"><a id="copyright"></a><b>Copyright Notice.</b> This article, including but not limited to all text and diagrams, is the intellectual property of Steven Hill and Rod Elliott, and is © 2002. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro-mechanical, is strictly prohibited under International Copyright laws. The author (Steven Hill) and editor (Rod Elliott) grant the reader the right to use this information for personal use only, and further allow that one (1) copy may be made for reference while constructing the project. Commercial use is prohibited without express written authorisation from Steven Hill and Rod Elliott.</td></tr>
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<p>ESP has subscribed to Google advertising for some time now. This might come as a surprise to many people, but unfortunately the economics of running such a large website mean that alternative sources of revenue are the only way that I can continue to keep the site operating. In case you were wondering, the sale of PCBs is not a 'goldmine'. I keep prices as reasonable as possible, and the small additional income from the ads is essential for survival of the site.</p>
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<p>I know that many people use ad blocking add-ons with their browsers, and I ask that you place an exception for the ESP site. I have very deliberately avoided placing ads in the middle of pages, and they are restricted to small sections at the top and bottom of each page, identical to what you see here. They may not be the greatest revenue raisers (as Google tells me regularly), but they are as un-intrusive as possible.</p>
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<p>Unlike many other sites, you will <i>never</i> be blocked from accessing the material here, even if you do use an ad blocker. As noted above, I prefer that you disable it for the ESP site, but it's not (and will never be) a requirement.</p>
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<p>You will <i>never</i> see pop-up ads that open a new window, nor will you ever be asked if you really want to leave a page (or the site). Nor do I request that you 'register' to see the information - everything is freely available to everyone, with no restrictions other than to the secure section. That is restricted to purchasers only, and that's been the case for nearly 20 years.</p>
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<p>The primary goal of the site is to provide unbiased information to readers, and everything published is (or is believed to be) accurate and free of snake-oil or other contaminants.</p>
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<tr><td class="t-wht"><a id="copyright"></a><b>Copyright Notice.</b> This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is © 2016. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro- mechanical, is strictly prohibited under International Copyright laws. The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference. Commercial use is prohibited without express written authorisation from Rod Elliott.</td>
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<table style="width:100%" class="tblblue"><tr><td class="hdrl"> Elliott Sound Products</td>
|
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<td align="right" class="hdrr">Amplifier Basics - How Amps Work (Intro) </td></tr></table>
|
||||
|
||||
<h1>Amplifier Basics - How Amps Work (Intro)</h1>
|
||||
<div align="center" class="t_11">© 1999 - Rod Elliott (ESP)
|
||||
<br />Page Last Updated 06 Apr 2005</div>
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<hr /><a id="introduction"></a><b>Introduction</b>
|
||||
<p>The term 'amplifier' is somewhat 'all-encompassing', and is often thought (by many users in particular) to mean a power amplifier for driving loudspeakers. This is not the case (well, it <i>is</i>, but it is not the <i>only</i> case), and this article will attempt to explain some of the basics of amplification - what it means and how it is achieved. This article is not intended for the designer (although designers are more than welcome to read it if they wish), and is not meant to cover all possibilities. It is a primer, and gives fairly basic explanations (although some will no doubt dispute this) of each of the major points.</p>
|
||||
|
||||
<p>I will explain the basic amplifying elements, namely valves (vacuum tubes), bipolar transistors and FETs, all of which work towards the same end, but do it differently. This article is based on the principles of audio amplification - radio frequency (RF) amplifiers are designed differently because of the special requirements when working with high frequencies.</p>
|
||||
|
||||
<p>Not to be left out, the opamp is also featured, because although it is not a single 'component' in the strict sense, it is now accepted as a building block in its own right.</p>
|
||||
|
||||
<p>This article is not intended for the complete novice (although they, too, are more than welcome), but for the intermediate electronics or audio enthusiast, who will have the most to gain from the explanations given.</p>
|
||||
|
||||
<hr /><a id="contents"></a>
|
||||
<b>Contents</b>
|
||||
<ul>
|
||||
<li>Introduction
|
||||
<li><a href="amp-basics.htm#terminology">Basic Terminology</a>
|
||||
<ul>
|
||||
<li><a href="amp-basics.htm#Z">Impedance</a></li>
|
||||
<li><a href="amp-basics.htm#Units">Units</a><br /><br />
|
||||
</ul>
|
||||
<li><a href="amp-basics.htm#amp-basics">Amplification Basics</a>
|
||||
<ul>
|
||||
<li><a href="amp-basics.htm#Zin">Input Impedance</a>
|
||||
<li><a href="amp-basics.htm#Zout">Output Impedance</a>
|
||||
<li><a href="amp-basics.htm#Feedback">Feedback</a>
|
||||
<li><a href="amp-basics.htm#Signal">Signal Inversion</a>
|
||||
<li><a href="amp-basics.htm#designphase">Design Phase</a><br /><br />
|
||||
</ul>
|
||||
|
||||
<li><a href="amp-basics.htm#amp-types">Types Of Amplifier Devices</a>
|
||||
<li><a href="amp-basics.htm#limiting">Common Limiting Ratings</a>
|
||||
<li><a href="amp-basics.htm#essential">Essential Electronics Formulae</a><br /><br />
|
||||
|
||||
<li><a href="amp-basics1.htm"><b>Part 1 - Valves (Vacuum Tubes)</b></a>
|
||||
<ul>
|
||||
<li>1.1 Valve Characteristics
|
||||
<li>1.2 Valve Current Amplifier
|
||||
<li>1.3 Valve Power Amplifiers
|
||||
<li>Summary<br /><br />
|
||||
</ul>
|
||||
|
||||
<li><a href="amp-basics2.htm"><b>Part 2 - Bipolar Transistors</b></a>
|
||||
<ul>
|
||||
<li>2.1 Transistor Characteristics
|
||||
<li>2.2 Transistor Current Amplifier
|
||||
<li>2.3 Transistor Common Base Amplifier
|
||||
<li>2.4 Transistor Combined Voltage + Current Amplifier
|
||||
<li>2.5 Transistor Power Amplifiers
|
||||
<li>Summary<br /><br />
|
||||
</ul>
|
||||
|
||||
<li><a href="amp-basics3.htm"><b>Part 3 - Field Effect Transistors and MOSFETs</b></a>
|
||||
<ul>
|
||||
<li>3.1 FET Characteristics
|
||||
<ul>
|
||||
<li>3.1.1 Junction FETs
|
||||
<li>3.1.2 MOSFETs
|
||||
</ul>
|
||||
<li>3.2 FET Current Amplifier
|
||||
<li>3.3 FET Power Amplifiers
|
||||
<li>Summary<br /><br />
|
||||
</ul>
|
||||
|
||||
<li><a href="amp-basics4.htm"><b>Part 4 - Operational Amplifiers (Opamps)</b></a>
|
||||
<ul>
|
||||
<li>4.1 Power Opamps<br /><br />
|
||||
</ul>
|
||||
|
||||
<li><a href="amp-basics5.htm"><b>Part 5 - Some Basic Linear Circuit Building Blocks</b></a>
|
||||
<ul>
|
||||
<li>5.1 Current Sources and Sinks
|
||||
<li>5.2 Current Mirror
|
||||
<li>5.3 Long Tailed Pair
|
||||
<li>5.4 Grounded Grid (Gate or Base) Circuits
|
||||
<li>5.5 Cascode Circuits<br /><br />
|
||||
</ul>
|
||||
|
||||
<li><a href="amp-basics6.htm"><b>Part 6 - Conclusions</b></a>
|
||||
<li><a href="amp-basics6.htm#references">References</a>
|
||||
<li><a href="amp-basics6.htm#copyright">Copyright & Update Info</a><br /><br />
|
||||
</ul>
|
||||
|
||||
<hr /><a id="terminology"></a><b>Basic Terminology</b>
|
||||
<p>Before we continue, I must explain some of the terms that are used. Without knowledge of these, you will be unable to follow the discussion that follows.</p>
|
||||
|
||||
<table style="width:700px" align="center" border="1">
|
||||
<colgroup span="4" width="25%">
|
||||
<tr class="tbldark"><td colspan=4 align="center"><b>Electrical Units</b>
|
||||
<tr class="tbldark"><td><b>Name</b></td><td><b>Measurement of</b></td><td><b><acronym title="Also Known As">aka</acronym></b></td><td><b>Symbol</b></td></tr>
|
||||
<tr><td>Volt</td><td>electrical 'pressure'</td><td>voltage</td><td>V, U, E (EMF)</td></tr>
|
||||
<tr><td>Ampere</td><td>the flow of electrons</td><td>current</td><td>A, I</td></tr>
|
||||
<tr><td>Watt</td><td>power</td><td></td><td>W, P</td></tr>
|
||||
<tr><td>Ohm</td><td>resistance to current flow</td><td></td><td>Ω, R</td></tr>
|
||||
<tr><td>Ohm</td><td>impedance, reactance</td><td></td><td>Ω, Z, X</td></tr>
|
||||
<tr><td>Farad</td><td>capacitance</td><td></td><td>F, C</td></tr>
|
||||
<tr><td>Henry</td><td>inductance</td><td></td><td>H, L</td></tr>
|
||||
<tr><td>Hertz</td><td>frequency</td><td></td><td>Hz</td></tr>
|
||||
</table>
|
||||
|
||||
<p>Note: 'aka' means 'Also Known As'. Although the Greek letter omega (Ω) is the symbol for Ohms, I often use the word Ohm or the letter 'R' to denote Ohms. Any resistance of greater than 1,000 Ohms will be shown as (for example) 1k5, meaning 1,500 Ohms, or 1M for 1,000,000 Ohms. The second symbol shown in the table is that commonly used in a formula.</p>
|
||||
|
||||
<p>When it comes to Volts and Amperes (Amps), we have alternating current and direct current (AC and DC respectively). The power from a wall outlet is AC, as is the output from a CD or tape machine. The mains from the wall outlet is at a high voltage and is capable of high current, and is used to power the amplifying circuits. The signal from your audio source is at a low voltage and can supply only a small current, and must be amplified so that it can drive a loudspeaker.</p>
|
||||
|
||||
|
||||
<p><a id="Z"></a><b>Impedance</b>
|
||||
<br />A derived unit of resistance, capacitance and inductance in combination is called impedance, although it is not a requirement that all three be included. Impedance is also measured in Ohms, but is a complex figure, and often fails completely to give you any useful information. The impedance of a speaker is a case in point. Although the brochure may state that a speaker has an impedance of 8Ω, in reality it will vary depending on frequency, the type of enclosure, and even nearby walls or furnishings.</p>
|
||||
|
||||
|
||||
<p><a id="Units"></a><b>Units</b>
|
||||
<br />In all areas of electronics, there are smaller and larger amounts of many things that would be very inconvenient to have to write in full. For example, a capacitor might have a value of 0.000001F or a resistor a value of 150,000Ω. Because of this, there are conventional units that are applied to make our lives easier (well, once we are used to using them, anyway). It is much easier to say 1uF or 150k (the same as above, but using standard units). These units are described below.</p>
|
||||
|
||||
<table style="width:700px" align="center" border="1">
|
||||
<colgroup span="3" width="33%">
|
||||
<tr class="tbldark"><td align="center" colspan=3><b>Conventional Metric Units</b>
|
||||
<tr class="tbldark"><td><b>Symbol</b></td><td><b>Name</b></td><td><b>Multiplication</b></td></tr>
|
||||
<tr><td>p</td><td>pico</td><td>1 x 10<sup><font size=-2>-12</font></sup></td></tr>
|
||||
<tr><td>n</td><td>nano</td><td>1 x 10<sup><font size=-2>-9</font></sup></td></tr>
|
||||
<tr><td>μ</td><td>micro</td><td>1 x 10<sup><font size=-2>-6</font></sup></td></tr>
|
||||
<tr><td>m</td><td>milli</td><td>1 x 10<sup><font size=-2>-3</font></sup></td></tr>
|
||||
<tr><td>k</td><td>kilo</td><td>1 x 10<sup><font size=-2>3</font></sup></td></tr>
|
||||
<tr><td>M</td><td>Mega</td><td>1 x 10<sup><font size=-2>6</font></sup></td></tr>
|
||||
<tr><td>G</td><td>Giga</td><td>1 x 10<sup><font size=-2>9</font></sup></td></tr>
|
||||
<tr><td>T</td><td>Tera</td><td>1 x 10<sup><font size=-2>12</font></sup></td></tr>
|
||||
</table>
|
||||
|
||||
<p>Although commonly written as the letter 'u', the symbol for micro is actually the Greek letter mu (μ) as shown. In audio, Giga and Tera are not commonly found (not at all so far - except for specifying the input impedance of some opamps!). There are also others (such as femto - 1x10<sup>-15</sup>) that are extremely rare and were not included. Of the standard electrical units, only the Farad is so large that the defacto standard is the microfarad (µF). Most of the others are reasonably sensible in their basic form.
|
||||
|
||||
<p class="t_11b">It is important to understand that the symbol for microfarad is µF (or uF), <i>not</i> mF - that's a millifarad, and is 1,000 µF.</p>
|
||||
|
||||
|
||||
<hr /><a id="amp-basics"></a><b>Amplification Basics</b>
|
||||
<p>The term 'amplify' basically means to make stronger. The strength of a signal (in terms of voltage) is referred to as amplitude, but there is no equivalent for current (curritude?, nah, sounds silly). This in itself is confusing, because although 'amplitude' refers to voltage, it contains the word 'amp', as in ampere. Maybe we should introduce 'voltitude' - No? Just live with it.</p>
|
||||
|
||||
<p>To understand how any amplifier works, you need to understand the two major types of amplification, and a third 'derived' type:</p>
|
||||
|
||||
<ul>
|
||||
<li>Voltage Amplifier - an amp that boosts the voltage of an input signal</li>
|
||||
<li>Current Amplifier - an amp that boosts the current of a signal</li>
|
||||
<li>Power Amplifier - the combination of the above two amplifiers</li>
|
||||
</ul>
|
||||
|
||||
<p>In the case of a voltage amplifier, a small input voltage will be increased, so that for example a 10mV (0.01V) input signal might be amplified so that the output is 1 Volt. This represents a 'gain' of 100 - the output voltage is 100 times as great as the input voltage. This is called the voltage gain of the amplifier.</p>
|
||||
|
||||
<p>In the case of a current amplifier, an input current of 10mA (0.01A) might be amplified to give an output of 1A. Again, this is a gain of 100, and is the current gain of the amplifier.</p>
|
||||
|
||||
<p>If we now combine the two amplifiers, then calculate the input power and the output power, we will measure the power gain:</p>
|
||||
|
||||
<table style="width:700px">
|
||||
<tr><td width="5%"><br /></td><td width="20%" class="t_12">P = V × I</td><td>(where I = current, note that the symbol changes in a formula)</td></tr></table>
|
||||
|
||||
<p>The input and output power can now be calculated:</p>
|
||||
|
||||
<table style="width:700px">
|
||||
<tr><td width="5%"></td><td width="20%" class="t_12">P<sub>in</sub> = 0.01 × 0.01</td><td>(0.01V and 0.01A, or 10mV and 10mA)</td></tr>
|
||||
<tr><td></td><td class="t_12">P<sub>in</sub> = 100µW</td><td></td></tr>
|
||||
<tr><td></td><td class="t_12">P<sub>out</sub> = 1 × 1</td><td>(1V and 1A)</td></tr>
|
||||
<tr><td></td><td class="t_12">P<sub>out</sub> = 1W</td><td><br /></td></tr>
|
||||
</table>
|
||||
|
||||
<p>The power gain is therefore 10,000, which is the voltage gain multiplied by the current gain. Somewhat surprisingly perhaps, we are not interested in power gain with audio amplifiers. There are good reasons for this, as shall be explained in the remainder of this page. Having said this, in reality all amplifiers are power amplifiers, since a voltage cannot exist without power unless the impedance is infinite or zero. This is never achieved, so some power is always present. It is convenient to classify amplifiers as above, and no harm is done by the small error of terminology.</p>
|
||||
|
||||
<p>Note that a voltage or current gain of 100 is 40dB, and a power gain of 10,000 is also 40dB.</p>
|
||||
|
||||
|
||||
<p><a id="Zin"></a><b class="under">Input Impedance</b>
|
||||
<br />Amplifiers will be quoted as having a specific input impedance. This only tells us the load it will place on preceding equipment, such as a preamplifier. It is neither practical nor useful to match the impedance of a preamp to a power amp, or a power amp to a speaker. This will be discussed in more detail later in this article.</p>
|
||||
|
||||
<p>The load is that resistance or impedance placed on the output of an amplifier. In the case of a power amplifier, the load is most commonly a loudspeaker. Any load will require that the source (the preceding amplifier) is capable of providing it with sufficient voltage and current to be able to perform its task. In the case of a speaker, the power amplifier must be capable of providing a voltage and current sufficient to cause the speaker cone(s) to move the distance required. This movement is converted to sound by the speaker.</p>
|
||||
|
||||
<p>Even though an amplifier might be able to make the voltage great enough to drive a speaker cone, it will be unable to do so if it cannot provide enough current. This has nothing to do with its output impedance. An amplifier can have a very low output impedance, but only be capable of a small current (an operational amplifier, or opamp is a case in point). This is very important, and needs to be fully understood before you will be able to fully appreciate the complexity of the amplification process.</p>
|
||||
|
||||
|
||||
<p><a id="Zout"></a><b class="under">Output Impedance</b>
|
||||
<br />The output impedance (Z<sub>out</sub>) of an amplifier is a measure of the impedance or resistance 'looking' back into the amplifier. It has nothing to do with the actual loading that may be placed at the output.</p>
|
||||
|
||||
<p>For example, an amplifier has an output impedance of 10Ω. This is verified by placing a load of 10Ω across the output, and the voltage can be seen to decrease to ½ that with no load. However, unless this amplifier is capable of substantial output current, we might have to make this measurement at a very low output voltage, or the amplifier will be unable to drive the load. If the output clips (distorts) the measurement is invalid.</p>
|
||||
|
||||
<p>Another amplifier might have an output impedance of 100Ω, but be capable of driving 10A into the load. Output impedance and current are completely separate, and must not be seen to be in any way equivalent. Both of these possibilities will be demonstrated later in this series.</p>
|
||||
|
||||
<p>It is very rare that you will ever be able to perform a direct measurement of output impedance. An opamp configured for a gain of 10 (20dB) will usually have such a low Z<sub>out</sub> that it's almost impossible to measure it directly, other than by using an input level of a few microvolts. Most power amps will be stressed badly by attempting to drive close to a short circuit, and will show their displeasure by blowing up or triggering their protection circuits (if fitted).</p>
|
||||
|
||||
<p>The output impedance is also independent of the power supply impedance. This causes the maximum undistorted power to fall with lower impedance loads, so an amp may be able to deliver 50W into 8Ω but only 80W into 4Ω (continuous power - peak power can be higher for short transients). Failure to understand that all of these factors are independent from each other will lead to false conclusions. It's easy to fall into the traps, and some manufacturers make this worse by claiming that their 'XyZ-5000' 50W amplifier can deliver 100 amps to the load, but fail to tell buyers that no sensible (or even non-sensible) load can <i>ever</i> draw that much current.</p>
|
||||
|
||||
<p>The output impedance is (roughly) equal to the open-loop (zero feedback) output impedance, divided by the feedback ratio. An amplifier may have an open-loop Z<sub>out</sub> of 5Ω, with 46dB of feedback (a factor of 200). Closed-loop Z<sub>out</sub> is then 5 / 200, or 25mΩ. However, the feedback ratio is almost always frequency dependent, so unless the frequency is specified, the Z<sub>out</sub> figure may not be meaningful.</p>
|
||||
|
||||
|
||||
<p><a id="Feedback"></a><b class="under">Feedback</b>
|
||||
<br />Feedback is a term that creates more and bloodier battles between audio enthusiasts than almost any other. Without it, we would not have the levels of performance we enjoy today, and many amplifier types would be unlistenable without it.</p>
|
||||
|
||||
<p>Feedback in its broadest sense means that a certain amount of the output signal is 'fed back' into the input. An amplifier - or an element of an amplifying device - is presented with the input signal, and compares it to a 'small scale replica' of the output signal. If there is any difference, the amp corrects this, and ideally ensures that the output is an exact replica of the input, but with a greater amplitude. Feedback may be as a voltage or current, and has a similar effect in either case.</p>
|
||||
|
||||
<p>In many designs, one part of the complete amplifier circuit (usually the input stage) acts as an error amplifier, and supplies exactly the right amount of signal (with correction as needed) to the rest of the amp to ensure that there is no difference between the input and output signals, other than amplitude. This is (of course) an ideal state, and is never achieved in practice. There will always be some difference, however slight. Note that any amplifier that suffers from crossover (aka notch) distortion <i>cannot</i> be made linear with feedback, because at zero output (where this distortion occurs) there is also (almost) zero gain. You can't have feedback unless there is some 'excess' gain<b>!</b></p>
|
||||
|
||||
|
||||
<p><a id="Signal"></a><b class="under">Signal Inversion</b>
|
||||
<br />When used as voltage amplifiers, all the standard active devices invert the signal. This means that if a positive-going signal goes in, it emerges as a larger - but now negative-going - signal. This does not actually matter for the most part, but it is convenient (and conventional) to try to make amplifiers non-inverting. To achieve this, two stages must be used (or a transformer) to make the phase of the amplified signal the same as the input signal.</p>
|
||||
|
||||
<p>The exact mechanism as to how and why this happens will be explained as we go along.</p>
|
||||
|
||||
|
||||
<p><a id="designphase"></a><b class="under">Design Phase</b>
|
||||
<br />The design phase of an amplifier is not remarkably different, regardless of the type of components used in the design itself. There is a sequence that will be used most of the time to finalise the design, and this will (or should) follow a pattern.</p>
|
||||
|
||||
|
||||
<ul>
|
||||
<li><b>Power Output vs. Impedance</b>
|
||||
<br />The power output is determined by the load impedance and the available voltage and current of the amplifier. An amplifier that is capable of a maximum of
|
||||
1.414A output current will be unable to provide more just because you want it to. Such an amp will be limited to 16W 'RMS' into 8Ω, regardless of the supply
|
||||
voltage. Likewise, an amp with a supply voltage of ±16V (11.32V RMS) will be unable to provide more than 16W RMS into 8Ω, regardless of the available
|
||||
current. Having more current available will allow the amp to provide (for example) 32W into 4Ω (4A peak current) or 64W into 2Ω (8A peak current), but
|
||||
will give no more power into 8Ω than the supply voltage and load impedance will allow.<br /><br /></li>
|
||||
|
||||
<li><b>Driver Current</b>
|
||||
<br />Especially in the case of bipolar transistors, the driver stage must be able to supply enough current to the output transistors - with MOSFETs, the driver
|
||||
must be able to charge and discharge the gate-source capacitance quickly enough to allow you to get the needed power at the highest frequencies of interest. With
|
||||
valves, the driver needs to be able to supply enough current to supply the bias resistors only, since the valve grid draws little or no current (except for the
|
||||
special case of Class-AB2).
|
||||
<br /><br />For the sake of simplicity, if bipolar output transistors have a gain of 20 at the maximum current into the load, the drivers must be able to supply
|
||||
enough base current to allow this. If the maximum collector current is 4A, then the drivers must be able to supply 200mA of base current to the output devices.
|
||||
<br /><br /></li>
|
||||
|
||||
<li><b>Prior Stages</b>
|
||||
<br />The stages that come before the drivers must also be able to supply sufficient current for the load imposed. The Class-A driver of a bipolar or MOSFET amp
|
||||
must be able to supply enough current to satisfy the base current needs of bipolar drivers, or the gate capacitance of MOSFETs.<br /><br />
|
||||
|
||||
Again, using the bipolar example from above, the maximum base current for the output transistors was 200mA. If the drivers have a minimum specified gain of 50, then
|
||||
their base current will be ...
|
||||
|
||||
<blockquote>
|
||||
200 / 50 = 4mA.
|
||||
</blockquote>
|
||||
|
||||
Since the Class-A driver must operate in Class-A (what a surprise), it will need to operate with a current of 1.5 to 5 times the expected maximum driver
|
||||
current, to ensure that it never turns off. The same applies with a MOSFET amp that will expect (for example) a maximum gate capacitance charge (or discharge)
|
||||
current of 4mA at the highest amplitudes and frequencies.<br /><br />
|
||||
|
||||
This is not normally an issue with valve amps, as the early stages of the amp are not loaded with any significant impedance. No further determinations are needed
|
||||
(other than the normal loading effects of valve stages in general), although the undistorted <i>voltage</i> swing may become a limiting factor.<br /><br /></li>
|
||||
|
||||
<li><b>Input Stages</b>
|
||||
<br />The input stages of all transistor amps must be able to supply the base current of the Class-A driver. This time, a margin of between 2 and 5 times the
|
||||
expected maximum base current is needed. If the Class-A driver needs to supply a quiescent current of (say) 8mA, the maximum current will be 12mA (quiescent +
|
||||
driver base current). Assuming a gain of 50 (again), this means that the input stage has to be able to supply 12 / 50 = 240µA, so it must operate at a minimum
|
||||
current of 240µA × 2 = 480µA to preserve linearity. <br /><br /></li>
|
||||
|
||||
<li><b>Input Current</b>
|
||||
<br />The input current of the first stage determines the input impedance of the amplifier. Using the above figures, with a collector current of 480uA, the base
|
||||
current will be 4.8µA for input devices with a gain of 100. If maximum power is developed with an input voltage of 1V, then the impedance is 208k (R = V/I).
|
||||
|
||||
<br /><br />Since the stage must be biased, we apply the same rules as before - a margin of between 2 and 5, so the maximum value of the bias resistors should
|
||||
be 208 / 2 = 104k. A lower value is preferred, and I suggest that a factor of 5 is more appropriate, giving 208 / 5 = 42k (47k can be used without a problem).</li>
|
||||
</ul>
|
||||
|
||||
<p>These are only guidelines (of course), and there are many cases where currents are greater (or smaller) than suggested. The end result is in the performance of the amp, and the textbook approach is not always going to give the expected result. Note that there are some essential simplifications in the above - it is an overview, and is only intended to give you the basic idea.</p>
|
||||
|
||||
|
||||
<hr /><a id="amp-types"></a><b>Types Of Amplifier Devices</b>
|
||||
<p>For the purposes of this article, there are three different types of amplifying devices, and each will be discussed in turn. Each has its strengths and weaknesses, but all have one common failing - they are not perfect.</p>
|
||||
|
||||
<p>A perfect amplifier or other device (known generally as 'ideal') will perform its task within certain set limits, without adding or subtracting anything from the original signal. No ideal amplifying device exists, and as a result, no ideal amplifier exists, since all must be built with real-life (non-ideal) devices.</p>
|
||||
|
||||
<p>The amplifying devices currently available are:</p>
|
||||
|
||||
<ul>
|
||||
<li>Vacuum Tube (Valve)</li>
|
||||
<li>Bipolar Junction Transistor (BJT)</li>
|
||||
<li>Field Effect Transistor (FET)</li>
|
||||
</ul>
|
||||
|
||||
<p>There are also some derivatives of the above, such as Insulated Gate Bipolar Transistors (IGBT), and Metal Oxide Semiconductor Field Effect Transistors (MOSFET). Of these, the MOSFET is a popular choice among many designers due to some desirable characteristics, and these will be covered in their own section.</p>
|
||||
|
||||
<p>All of these devices are reliant on other non-amplifying ('support') components, commonly known as passive components. The passive devices are resistors, capacitors and inductors, and without these, we would be unable to build amplifiers at all.</p>
|
||||
|
||||
<p>All the devices we use for amplification have a variable current output, and it is only the way that they are used that allows us to create a voltage amplifier. Valves and FETs are voltage controlled devices, meaning that the output current is determined by a voltage, and no current is drawn from the signal source (in theory). Bipolar transistors are current controlled, so the output current is determined by the input current. This means that no voltage is required from the signal source, only current. Again, this is in theory, and it is not realisable in practice.</p>
|
||||
|
||||
<p>Only by using the support components can we convert the current output of any of these amplifying devices into a voltage. The most commonly used for this purpose is a resistor.</p>
|
||||
|
||||
|
||||
<hr /><a id="limiting"></a><b>Common Limiting Ratings</b>
|
||||
<p>All active devices have certain parameters in common (although they will have different naming conventions depending on the device). Essentially these are ...</p>
|
||||
|
||||
<ul>
|
||||
<li><b>Maximum Voltage</b> - The maximum voltage that may be applied between the main terminals of the device. This varies from perhaps as low as 25V (sometimes
|
||||
even less) for small signal transistors and FETs, and up to 1,200V or more for some valves and high voltage transistors. MOSFET voltages are typically up to about
|
||||
600 to 800V for switching devices for use in power supplies.<br />
|
||||
<li><b>Maximum Current</b> - The maximum current that the device may pass safely. Ranges from a few mA up to many amps. This will <u>never</u> be while the device
|
||||
also has the maximum voltage across it, as this would result in power dissipation far in excess of ...<br />
|
||||
<li><b>Maximum Power Dissipation</b> - The maximum power that the device may dissipate (in mW or W), under any condition of voltage and current. (Called plate
|
||||
dissipation for valves).<br />
|
||||
<li><b>Heater Voltage/Current</b> - (Valves). The operating voltage and / or current for the filament (directly heated cathodes) or heater (for indirectly
|
||||
heated cathodes). This should always be within 10% of the quoted value, or cathode life will be severely shortened.<br />
|
||||
<li><b>Maximum Junction Temperature</b> - (Semiconductors) The maximum temperature that the semiconductor die will tolerate without failing. At this
|
||||
temperature, most semiconductors will be unable to perform any work, as this would raise the temperature above the maximum permissible.<br />
|
||||
<li><b>Temperature Derating</b> - (Semiconductors). Above a specified temperature, the allowable power rating of semiconductor devices must be reduced to
|
||||
remain below the maximum allowable junction temperature. The power is normally derated above 25° C.<br />
|
||||
<li><b>Thermal Resistance</b> - (Semiconductors). The thermal resistance between junction and case (high power) or junction and air (low power). Measured
|
||||
in Degrees C/W, This allows a suitable heatsink to be determined.
|
||||
</ul>
|
||||
|
||||
<p>This is by no means all of the ratings, there are many more, and vary from device to device. Some MOSFETs for example will have Peak Current ratings, which will be many times the continuous rating, but only for very limited time. Bipolar transistors have a Safe Operating Area (SOA) graph, which indicates that in some circumstances you must not operate the device anywhere near its maximum power dissipation, or it will fail due to a phenomenon called second breakdown (described later).</p>
|
||||
|
||||
<p>With most semiconductors, in many cases it will not be possible to operate them at anywhere near the maximum power dissipation, because thermal resistance is such that the heat simply cannot be removed from the junction and into the heatsink fast enough. In these cases, it might be necessary to use multiple devices to achieve the performance that can (theoretically) be obtained from a single component. This is very common in audio amplifiers.</p>
|
||||
|
||||
|
||||
<hr /><a id="essential"></a><b>Essential Electronics Formulae</b>
|
||||
<p>There are some things that you just can't get away from, and maths is one of them. (Sorry.) I will only include the essentials here, but will describe any others that are needed as we go. I am not about to give a lesson in algebra, but the best reason for ever doing the subject is to learn how to transpose electronics formulae ! Transposition is up to you (unless I am forced to do it for a calculation here or there).</p>
|
||||
|
||||
<p><b>Ohm's Law</b>
|
||||
<br />The first of these is Ohm's Law, which states that a voltage of 1V across a resistance of 1 Ohm will cause a current of 1 Amp to flow. The formula is ...</p>
|
||||
|
||||
<blockquote>
|
||||
R = V / I (where R = resistance in Ohms, V = Voltage in Volts, and I = current in Amps)
|
||||
</blockquote>
|
||||
|
||||
<p>Like all such formulae, this can be transposed (oops, I said I wasn't going to do this, didn't I).</p>
|
||||
|
||||
<blockquote>
|
||||
V = R × I (× means multiplied by), and<br />
|
||||
I = V / R
|
||||
</blockquote>
|
||||
|
||||
<p><b>Reactance</b>
|
||||
<br />Then there is the impedance (reactance) of a capacitor, which varies inversely with frequency (as frequency is increased, the reactance falls and vice versa).</p>
|
||||
|
||||
<blockquote>
|
||||
Xc = 1 / ( 2<span class="times">π</span> × f × C )
|
||||
</blockquote>
|
||||
|
||||
<p>where Xc is capacitive reactance in Ohms, <span class="times">π</span> (pi) is 3.14159, f is frequency in Hz, and C is capacitance in Farads.</p>
|
||||
|
||||
<p>Inductive reactance, being the reactance of an inductor. This is proportional to frequency.</p>
|
||||
|
||||
<blockquote>
|
||||
X<sub>l</sub> = 2<span class="times">π</span> × f × L
|
||||
</blockquote>
|
||||
|
||||
<p>where X<sub>l </sub>is inductive reactance in Ohms, and L is inductance in Henrys (others as above).</p>
|
||||
|
||||
<p><b>Frequency</b>
|
||||
<br />There are many different calculations for this, depending on the combination of components. The -3dB frequency for resistance and capacitance (the most common in amplifier design) is determined by ...</p>
|
||||
|
||||
<blockquote>
|
||||
f<sub>o</sub> = 1 / ( 2<span class="times">π</span> × R × C ) where f<sub>o</sub> is the -3dB frequency
|
||||
</blockquote>
|
||||
|
||||
<p>When resistance and inductance are combined, the formula is</p>
|
||||
|
||||
<blockquote>
|
||||
f<sub>o</sub> = R / (2<span class="times">π</span> × L)
|
||||
</blockquote>
|
||||
|
||||
<p><b>Power</b>
|
||||
<br />Power is a measure of work, which can be either physical work (moving a speaker cone) or thermal work - heat. Power in any form where voltage, current and resistance are present can be calculated by a number of means:</p>
|
||||
|
||||
<blockquote>
|
||||
P = V × I<br />
|
||||
P = V² / R<br />
|
||||
P = I² × R
|
||||
</blockquote>
|
||||
|
||||
<p>where P is power in watts, V is voltage in Volts, and I is current in Amps.</p>
|
||||
|
||||
|
||||
<p><b>Decibels (dB)</b>
|
||||
<br />It has been known for a very long time that human ears cannot resolve very small differences in sound pressure. Originally, it was determined that the smallest variation that is audible is 1dB - 1 decibel, or 1/10 of 1 Bel. It seems fairly commonly accepted that the actual limit is about 0.5dB, but it is not uncommon to hear that some people can (or genuinely believe they can) resolve much smaller variations. I shall not be distracted by this!</p>
|
||||
|
||||
<blockquote>
|
||||
dB = 20 × log ( V1 / V2 )<br />
|
||||
dB = 20 × log ( I1 / I2 )<br />
|
||||
dB = 10 × log ( P1 / P2 )
|
||||
</blockquote>
|
||||
|
||||
<p>As can be seen, dB calculations for voltage and current use 20 times the log (base 10) of the larger unit divided by the smaller unit. With power, a multiplication of 10 is used. Either way, a drop of 3dB represents half the power and vice versa.</p>
|
||||
|
||||
<p>There are many others, but these will be sufficient for now. I do not intend this to be a complete electronics course, so I will give you that which is needed to understand the remainder of the article - for the rest, there are lots of excellent books on electronics, and these will have every formula you ever wanted.</p>
|
||||
|
||||
<hr /><p align="right" class="t_12b"><a href="amp-basics1.htm">Next (Part 1 - Valves)</a></p>
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<tr><td class="t-wht"><a id="copyright"></a><b>Copyright Notice.</b> This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is Copyright © 1999, 2005. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro-mechanical, is strictly prohibited under International Copyright laws. The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference. Commercial use is prohibited without express written authorisation from Rod Elliott.</td>
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<table style="width:100%" class="tblblue"><tr><td class="hdrl"> Elliott Sound Products</td>
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<td align="right" class="hdrr">Amplifier Basics - How Amps Work (Part 1) </td></tr></table>
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<h1>Amplifier Basics - How Amps Work (Part 1)</h1>
|
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<div align="center" class="t_11">© 1999 - Rod Elliott (ESP)
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<br />Page Last Updated 06 Apr 2005</div>
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<hr /><a id="contents"></a>
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<ul>
|
||||
<li><a href="amp-basics.htm">Introduction</a>
|
||||
<li><b>Part 1 - Valves (Vacuum Tubes)</b>
|
||||
<ul>
|
||||
<li><a href="amp-basics1.htm#s11">1.1 Valve Characteristics</a>
|
||||
<li><a href="amp-basics1.htm#s12">1.2 Valve Current Amplifier</a>
|
||||
<li><a href="amp-basics1.htm#s13">1.3 Valve Power Amplifiers</a>
|
||||
<li><a href="amp-basics1.htm#sum">Summary</a>
|
||||
</ul>
|
||||
<li><a href="amp-basics2.htm"><b>Part 2 - Bipolar Transistors</b></a>
|
||||
<li><a href="amp-basics3.htm"><b>Part 3 - Field Effect Transistors and MOSFETs</b></a>
|
||||
<li><a href="amp-basics4.htm"><b>Part 4 - Operational Amplifiers (Opamps)</b></a>
|
||||
<li><a href="amp-basics5.htm"><b>Part 5 - Some Basic Linear Circuit Building Blocks</b></a>
|
||||
<li><a href="amp-basics6.htm"><b>Part 6 - Conclusions</b></a>
|
||||
</ul>
|
||||
|
||||
<hr /><a id="valve"></a><b>Part 1 - The Valve (Vacuum or Thermionic Tube)</b>
|
||||
<p>In the beginning the vacuum tube was the only way to amplify, and valves (or 'tubes') survive to this day, with a dedicated following of 'believers' who are convinced that the development of the transistor (or indeed, any semiconductor) was fundamentally a bad idea. This is not a discussion I intend to follow - I intend simply to state how these devices amplify a signal, and the factors that determine voltage and current gain. For more information about valve circuits, look at the material shown in the <a href="valves/index.html" target="_blank">ESP Valve Pages</a>.</p>
|
||||
|
||||
<p>The basic amplifying valve (there are many different types with higher complexity) has three elements. These are ...</p>
|
||||
|
||||
<ul>
|
||||
<li>The anode (Plate) - Collects the electrons released by the cathode</li>
|
||||
<li>Cathode (indirectly or directly heated) - The source of the electron flow</li>
|
||||
<li>Control Grid (Grid) - controls the flow of electrons</li>
|
||||
</ul>
|
||||
|
||||
<p>When a positive voltage is applied to the anode with respect to the cathode, an electron stream is emitted from the cathode and flows to the anode, completing the circuit. The grid is a fine coil of wire, suspended between the other two elements. A negative voltage on the grid (with respect to the cathode) will repel some of the electron stream, causing the current to be reduced. If the voltage on the grid were to be varied, then the cathode to anode current must also vary, and an amplifier is born. Figure 1.1 shows the basic circuit of a valve voltage amplifier.</p>
|
||||
|
||||
<div class="t-pic"><img src="ab-f1-1.gif" alt="Figure 1.1" border="1" /><br />Figure 1.1 - A Basic Valve Voltage Amplifier</div>
|
||||
|
||||
<p>This circuit configuration is known as 'common cathode', because the cathode reference point (earth) is common to both input and output. By placing a resistor 'Rk' in the cathode circuit, a voltage is developed because of the current flow. If the grid is referenced to earth (ground), then the grid is negative with respect to the cathode. The voltages shown on the circuit are typical of a single element of a 12AX7 twin triode. Note that the two valve pins that are not connected are for the heater. This is used to heat the cathode so that it emits electrons more readily.</p>
|
||||
|
||||
<p>The cathode resistor will cause the circuit to reach a stable current, where any attempt at increasing the cathode current will cause a greater voltage across the resistor, making the grid effectively more negative and reducing the current. A point of equilibrium is quickly reached, where the circuit operates in a stable manner. This is known as cathode biasing, and is most common with signal level and low power amps.</p>
|
||||
|
||||
<p>By applying a varying voltage (the signal) to the grid, the current between cathode and anode will vary too. Since the anode load is a resistance, a varying voltage will be developed which will (hopefully) be greater than the voltage applied to the grid. The input voltage has been amplified.</p>
|
||||
|
||||
<p>Because the signal voltage on the grid is 'fighting' the attempt of the cathode resistor to maintain the current through the valve at a constant value, this is a form of feedback. It is also known as cathode degeneration. The name is of no consequence, because as local feedback, it will improve the linearity of the stage but reduce the gain. In reality, the improvement in linearity is only minor, and leaving out the capacitor can increase noise in sensitive circuits - especially hum that is induced into the cathode from the heater, which was nearly always operated from AC in the past, but DC heater supplies are now common.</p>
|
||||
|
||||
<p>Where the cathode is directly heated (the filament has the oxide coatings directly applied), DC operation is mandatory or hum would result. Directly heated cathodes will always emit electrons unevenly, because of the voltage gradient across the filament. The only common directly heated valves used in any number these days are rectifiers.</p>
|
||||
|
||||
<p>Note that for indirectly heated cathodes, the heating element is called the heater, but for directly heated cathodes it is more commonly referred to as the filament (as in the heated filament of a light bulb).</p>
|
||||
|
||||
<p>Because valves have a relatively low voltage gain, it is common to bypass the cathode resistor with a capacitor to defeat the local feedback and extract as much gain as possible, as is shown in Figure 1.1. The gain (or more correctly, the transfer characteristic) of a valve is sometimes measured in mA/V - which tells us how many milliamps change in anode current will occur with a grid voltage change of 1 Volt. Another common way to describe this is 'mu' (µ) or amplification factor. Yet another value is common with valves - the 'conductance' (aka mutual conductance or transconductance), which is the opposite of resistance, and is expressed in Mhos (Ohms backwards - seriously!) or Siemens.</p>
|
||||
|
||||
<p>One problem with valves has always been the number of different methods used to describe what is essentially the same thing. Depending entirely which book you happen to be reading, you will see the effective gain quoted as mA/V, mutual conductance ('g<sub>m</sub>', in Mhos or more commonly µMhos), or the equally obscure term 'Amplification Factor', none of which has any direct relevance to the gain you can expect without further calculation.</p>
|
||||
|
||||
<p>The output impedance of the circuit of Figure 1.1 is about 44k - it's the value of the plate resistor in parallel with the internal plate resistance. Rg2 is the grid resistor for the following stage, and at 1M, loads the output and reduces gain.</p>
|
||||
|
||||
|
||||
<hr /><a id="s11"></a><b>1.1 Valve Characteristics</b>
|
||||
<p>There are four main characteristics that are quoted for any given valve. These are:</p>
|
||||
|
||||
<ul>
|
||||
<li>Amplification Factor (mu or µ) - This parameter compares the effectiveness of the control grid voltage to the plate voltage in changing the plate current</li>
|
||||
<li>Plate Resistance - This is the equivalent resistance of the internal cathode to plate circuit, since the plate voltage is applied across the valve, and the current flows through it</li>
|
||||
<li>Mutual Conductance (aka Transconductance) in µMhos - This shows how effective the grid is in controlling the plate current.</li>
|
||||
<li>Plate Resistance - The value of the internal resistance between the plate and cathode, assuming normal biasing. It cannot be ignored!</li>
|
||||
</ul>
|
||||
|
||||
<p>One important thing to realise about valves is that everything changes. The characteristics vary widely with plate voltage, load resistance, bias current and just about everything else you can think of. Despite this, it is still possible to design a circuit using valves that will be repeatable from one unit to the next, provided the designer knows what s/he is doing.</p>
|
||||
|
||||
<p>A typical signal valve (such as the 12AX7 high mu dual triode) has a plate resistance of 80k, an amplification factor (mu) of 100, and a g<small>m</small> (using the circuit of Figure 1.1) of about 1250µMhos, which can (by simple mathematics) be converted into a figure of 1.25mA/V, meaning that a change of 1V on the grid will cause a change of 1.6mA in the anode current. This does not actually mean what it says, since the valve might be quite incapable of sustaining an anode current of 1.25mA under all circumstances. However, a change of 0.1V at the grid <i>can</i> cause a change in plate current of 0.125mA - the measurement is typically 'normalised' to make comparison easier.</p>
|
||||
|
||||
<p>Let us now have a look at how the valve amplifies the signal. The transfer curve in Figure 1.2 shows the input waveform applied to the grid, at any convenient frequency. As the signal becomes more positive, the valve draws more current, until at the peak of the waveform, the grid voltage has been made 0.1V more positive than it was before. Therefore, the anode current is 0.125mA greater that it was before. Using Ohm's Law, 0.125mA with a resistance of 100k means that the anode voltage should be 12.5 Volts lower than when in the idle (or quiescent) state.</p>
|
||||
|
||||
<p>This would seem to imply that the valve has a gain of (12.5 / 0.1) 125 - not a chance! The circuit of Figure 1 will have a typical voltage gain (Av) of much less than this. Why? Because the valve's internal plate resistance wasn't considered. This is effectively in parallel with the plate load resistor and external load (the grid resistor of the next stage). When these are taken into consideration, the gain can be calculated at around 55 - somewhat shy of the figure obtained before considering the complete circuit.</p>
|
||||
|
||||
<p>The only way to be certain what a valve will actually do is consult the manufacturer's data, and refer to the transfer curves for the mode of operation and cathode current you wish to use. Valve characteristics, supply voltage, plate current, plate voltage and the impedance of the next stage all have a profound effect on the performance of any valve.</p>
|
||||
|
||||
<div class="t-pic"><img src="ab-f1-2.gif" alt="Figure 1.2" border="1" /><br />Figure 1.2 - Typical Valve Transfer Curve</div>
|
||||
|
||||
<p>As can be seen from Figure 1.2, the transfer curve is not linear, which means that as the valve approaches cut-off (turned off completely) or saturation (turned on completely) the characteristics change, and distortion is introduced. A (very) rough estimation of maximum RMS output voltage to keep distortion below 1% is about 0.1 of the quiescent plate voltage, but often less. Thus, with a plate voltage of 125V at idle, the maximum output voltage will be 12.5V RMS. This assumes that the valve has been biased correctly in the first place. From the graph we can see that at high values of negative grid voltage the valve will cut off, while at low (or positive) grid voltage, the valve is turned on as hard as it can.</p>
|
||||
|
||||
<p>A valve can be thought of as having an infinite input impedance (although this is never realised in practice). The input impedance is approximately equal to the value of the grid resistor for audio frequencies. The output current is therefore controlled by a voltage at the grid, so the valve might be considered a voltage controlled current source (or VCCS).</p>
|
||||
|
||||
|
||||
<hr /><a id="s12"></a><b>1.2 Valve Current Amplifier</b>
|
||||
<p>Figure 1.3 shows a valve current amplifier, commonly known as a cathode follower, or common plate (because the plate circuit is common to both input and output - for AC signals only). Although this circuit can provide a useful increase of current, and an equally useful decrease in output impedance, it has a voltage gain that is less than unity. Typically, this will be about 0.8 to 0.9, so for every volt of signal applied to the input, we only get about 850mV output.</p>
|
||||
|
||||
<div class="t-pic"><img src="ab-f1-3.gif" alt="Figure 1.3" border="1" /><br />Figure 1.3 - Cathode Follower Current Amplifier</div>
|
||||
|
||||
<p>The cathode follower is typically used where a low impedance output is desired, since the output impedance of most valve circuits is rather high (equal to the value of the plate load resistor in parallel with the internal plate resistance). Simply attaching a low impedance load to a voltage amplifier stage will cause the output level to be dramatically reduced, so the current amplifier (cathode follower) is a useful stage. The output impedance of the circuit of Figure 1.3 can be expected to be about 1/10th the value of the cathode resistance Rk2 - but this is highly dependent on the valve itself and its operating current. The available current is very low, so the circuit will not be able to drive a load much less than Rk2, or 47k. Remember that output impedance and drive capability are not related.</p>
|
||||
|
||||
<p>Note that the grid must still be biased to an appropriate voltage negative with respect to the cathode. The bypassed cathode resistor is used as before, but the grid is connected to the bottom of this resistor, and not ground. If it were connected to ground, the circuit would be capable of only very small signal levels before it distorted.</p>
|
||||
|
||||
|
||||
<hr /><a id="s13"></a><b>1.3 Valve Power Amplifier</b>
|
||||
<p>Finally, we can combine a voltage amplifier stage and a current amplifier stage, and get a power amplifier. Cathode followers are unusual in valve power amplifiers, and it is far more common to use a plate-loaded 'push-pull' output stage, using a transformer in the plate circuit to match the high voltage and relatively high impedance of the output valves to the impedance of the speaker. In a few cases, output stages have been configured to use part of the transformer winding in the anode circuit, and some in the cathode circuit. This can improve linearity, but makes the output valves harder to drive.</p>
|
||||
|
||||
<p>'Transformerless' valve output stages had a short period of popularity, but most required high impedance loudspeakers which were expensive and disappeared only a few years after they were introduced. The high voltage requirement and comparatively low current capabilities make valves unsuited to direct-coupling to 'normal' speaker impedances.</p>
|
||||
|
||||
<div class="t-pic"><img src="ab-f1-4.gif" alt="Figure 1.4" border="1" /><br />Figure 1.4 - Basic Valve Power Amplifier</div>
|
||||
|
||||
<p>Figure 1.4 shows a very basic valve power amplifier, using a triode in 'single-ended' mode. The output transformer converts the high voltage, high impedance plate circuit of the valve to a low voltage, low impedance signal for the loudspeaker. Because the primary of the output transformer must carry the full DC quiescent current of the valve (which will be a large, high current unit), it needs a very large core of laminated steel with an air gap to minimise saturation effects and distortion.</p>
|
||||
|
||||
<p>Interestingly, these inefficient and high distortion amplifiers have made a comeback in recent years. However in the heyday of the valve, the inefficiency and high distortion of these circuits was such that they were replaced in nearly all installations by more efficient and lower distortion circuits, such as that shown in Figure 1.5.</p>
|
||||
|
||||
<div class="t-pic"><img src="ab-f1-5.gif" alt="Figure 1.5" border="1" /><br />Figure 1.5 - Push-Pull Valve Power Amplifier</div>
|
||||
|
||||
<p>The valves shown for the output are called pentodes (from penta - five), having 5 electrodes instead of the three for a triode. The second grid (called the screen grid, or just screen) increases the gain of the valve dramatically, while the third grid, the suppressor, prevents what is called 'secondary emission' from the plate. The screen accelerates the electron flow so much that electrons bounce off the plate, or dislodge others. The addition of the screen gives the valve some nice characteristics, such as much higher gain, but also some nasty ones (lower linearity, more distortion), which the suppressor counteracts to some degree. The suppressor grid is almost always connected internally to the cathode. It is not uncommon for designers to connect pentodes as triodes, by connecting the screen and plate together.</p>
|
||||
|
||||
<p>The first stage of the circuit is interesting, and is called a phase splitter. It is a combination of a voltage amplifier and a current amplifier, having equal values of resistance in each circuit (i.e. Rp = Rk2). Because all valves have the same 'polarity', they cannot be used like transistors or MOSFETs, but must be driven with their own signal of the correct polarity.</p>
|
||||
|
||||
<p>The incoming signal is therefore sent 'as is' to one valve (from the cathode circuit), and is inverted for the other - hence the term push-pull. As one valve 'pulls' the anode current lower, the other simultaneously 'pushes' it higher. In a properly designed circuit, the two output valves will pass the signal between them with little disturbance. Any disturbance in this region is called crossover distortion, because it happens as the signal crosses over from one valve to the other.</p>
|
||||
|
||||
<p>Notice something else quite different. The cathodes of the output valves are connected directly to earth, and the grids are supplied with a negative bias from a separate negative power supply. This is the most common method of biasing output valves in high power circuits, having a much greater efficiency than cathode biasing.</p>
|
||||
|
||||
<p>For many large output valves, it is not even considered a good idea to use cathode biasing, because the amount of negative grid voltage required is too high. Voltages of up to -60V are not uncommon with high power pentodes or another common type, beam power tetrodes (I will not cover these in more detail, but there is much information to be found on the web). Using cathode bias for this sort of voltage and current is inefficient and reduces the output power dramatically.</p>
|
||||
|
||||
|
||||
<hr /><a id="sum"></a><b>Valves - A Summary</b>
|
||||
<p>The above is but a very small offering from the world of the vacuum tube. As I said in the introduction, this is not designed to be a complete electronics training course. The circuits presented are basic only, which is to say that they will all work, but are not optimised in any way.</p>
|
||||
|
||||
<p>For further reading, the most highly recommended work is the rather old (but still considered <i>the</i> reference manual) "Radiotron Designer's Handbook", by F. Langford-Smith and originally published by Amalgamated Wireless Valve Co. Pty. Ltd. in Australia. My copy is dated 1957, but it has recently been republished (although I think it is quite expensive, unfortunately).</p>
|
||||
|
||||
<p>Overall, the valve is still an almost mystical thing, but in all honesty, modern amplifiers using transistors or MOSFETs are so vastly superior in terms of fidelity, efficiency and reliability, that I really don't see what all the fuss is about. Having said this, I was using a valve preamplifier on my own system until recently.</p>
|
||||
|
||||
<p>There is no doubt that valves do have some very nice characteristics, and for guitar amplifiers there are few guitarists who would argue otherwise. A 'soft' overload behaviour means that a valve amp does not sound as harsh as a transistor amp when it is overdriven - which is great for guitar, but a hi-fi should never be overdriven anyway, so the point is moot.</p>
|
||||
|
||||
<p>The problems that befall valves are many, and include ...</p>
|
||||
|
||||
<ul>
|
||||
<li><b>Fragile</b> - The glass envelopes are very thin, and are easily broken.</li>
|
||||
<li><b>Limited Life</b> - Even if a valve is operated well within its ratings, it still has a finite life. The main causes of valve failure are cathode emission
|
||||
degradation (happening all the time), and gas, when small amounts of air 'break' the full vacuum.</li>
|
||||
<li><b>Microphonics</b> - All valves tend to be slightly microphonic, which is to say that they act as a microphone. This can cause additional colouration to the
|
||||
signal if the sound from speakers vibrates the amplifier.</li>
|
||||
<li><b>High Voltage</b> - Having to ensure that the 600V DC (or up to 1,200V peak) typical of a high power amp does not 'flash over' valve bases is a constant
|
||||
headache, and ensuring that these voltages are kept well away from small fingers is mandatory. High voltage capacitors are also more expensive than lower voltage
|
||||
ones.</li>
|
||||
<li><b>Heaters</b> - Valve cathodes must be operated at the correct temperature so they emit electrons properly, and 'boil off' contaminants. If the heater voltage
|
||||
is too low, the cathode will become poisoned, and the valve is useless. The heater power used is all wasted, in that none of it is turned into sound.</li>
|
||||
<li><b>Output Transformers</b> - The output transformer for a valve amp is expensive, bulky and heavy. It introduces its own distortion components, which are
|
||||
difficult (or impossible) to eliminate completely.</li>
|
||||
<li><b>Heat</b> - All valve amps run hot. A valve will not work unless it is hot, and the heat causes problems for other components, shortening their life.
|
||||
The heat is all wasted energy.</li>
|
||||
<li><b>Damping</b> - Valve amplifiers nearly all have a low damping factor, caused by a relatively high output impedance. Speakers must be very well damped indeed
|
||||
to work well with any valve amp, or the bass will become poorly defined, and crossover networks (which rely on a very low amp impedance) may not work as well as
|
||||
intended.</li></ul>
|
||||
|
||||
<p>On the positive side, valve amplifiers have a 'warm' sound, partly because of the low order harmonic distortion introduced. A good valve amp will also have a very wide bandwidth, and will have an easy job driving loads that might cause some solid-state equipment to have severe heartburn (or just blow up on the spot).</p>
|
||||
|
||||
<p>At low levels, valve equipment has vanishingly small distortion levels, and when all is said and done, there is something nice about little glass tubes, with little lights inside, making your music. For more on the topic of valves, see the <a href="valves/index.html" target="_blank">Valve Index</a>.</p>
|
||||
|
||||
<p>Overall though, valves are an expensive, fragile and unreliable way to amplify anything these days. Well designed, modern 'solid state' equipment will easily surpass the best valve gear from any era, and even rather pedestrian circuits can easily beat the best valve designs for noise and distortion.</p>
|
||||
|
||||
<hr /><p align="right" class="t_12b"><a href="amp-basics.htm">Previous (Intro)</a> <a href="amp-basics2.htm">Next (Part 2 - Bipolar Transistors)</a></p>
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<tr><td class="t-wht"><a id="copyright"></a><b>Copyright Notice.</b> This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is Copyright © 1999. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro-mechanical, is strictly prohibited under International Copyright laws. The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference. Commercial use is prohibited without express written authorisation from Rod Elliott.</td>
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<table style="width:100%" class="tblblue"><tr><td class="hdrl"> Elliott Sound Products</td>
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<td align="right" class="hdrr">Amplifier Basics - How Amps Work (Part 2) </td></tr></table>
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<h1>Amplifier Basics - How Amps Work (Part 2)</h1>
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<div align="center" class="t_11">© 1999 - Rod Elliott (ESP)
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<br />Page Last Updated Dec 2018</div>
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<hr /><a id="contents"></a>
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<ul>
|
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<li><a href="amp-basics.htm"><b>Introduction</b></a>
|
||||
<li><a href="amp-basics1.htm"><b>Part 1 - Valves (Vacuum Tubes)</b></a>
|
||||
<li><b>Part 2 - Bipolar Transistors</b>
|
||||
<ul>
|
||||
<li><a href="amp-basics2.htm#s21">2.1 Transistor Characteristics</a></li>
|
||||
<li><a href="amp-basics2.htm#s22">2.2 Transistor Current Amplifier</a></li>
|
||||
<li><a href="amp-basics2.htm#s23">2.3 Transistor Common Base Amplifier</a></li>
|
||||
<li><a href="amp-basics2.htm#s24">2.4 Transistor Combined Voltage + Current Amplifier</a></li>
|
||||
<li><a href="amp-basics2.htm#s25">2.5 Transistor Power Amplifiers</a></li>
|
||||
<li><a href="amp-basics2.htm#sum">Summary</a></li>
|
||||
</ul>
|
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<li><a href="amp-basics3.htm"><b>Part 3 - Field Effect Transistors and MOSFETs</b></a>
|
||||
<li><a href="amp-basics4.htm"><b>Part 4 - Operational Amplifiers (Opamps)</b></a>
|
||||
<li><a href="amp-basics5.htm"><b>Part 5 - Some Basic Linear Circuit Building Blocks</b></a>
|
||||
<li><a href="amp-basics6.htm"><b>Part 6 - Conclusions</b></a>
|
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|
||||
|
||||
<br /><hr /><a id="bipolar"></a><b>Part 2 - Bipolar Transistors</b>
|
||||
<p>Since it was invented, the transistor (from 'transfer resistor') has come a long way. Early transistors were made from germanium, which was 'doped' with other materials to give the desired properties required for a semiconductor. In the beginnings of the transistor era, nearly all devices were PNP (Positive Negative Positive), and it was very difficult to make the opposite (NPN) polarity. The NPN transistors that were available at that time were low power and did not work as well as their PNP counterparts.</p>
|
||||
|
||||
<p>When silicon was first used, the opposite was the case, and for quite some time the only really high power devices available were all of silicon NPN construction. More recently, it has become possible to make NPN and PNP transistors that are almost identical in performance. Germanium is rarely used any more, although some examples are still available.</p>
|
||||
|
||||
<p>All transistors have three 'elements':</p>
|
||||
|
||||
<ul>
|
||||
<li>Emitter - analogous to the cathode of a valve. The emitter only emits electrons in an NPN device, but this is of no consequence</li>
|
||||
<li>Base - the controlling terminal. A current at the base controls the current through the transistor</li>
|
||||
<li>Collector - basically, collects the emitted electrons. Somewhat analogous to the plate of a valve</li>
|
||||
</ul>
|
||||
|
||||
<p>A transistor can be represented as two diodes, with a junction in the middle. This is shown for both polarities in Figure 2.1. This is only an analogy, and connecting two discrete diodes in this manner will not produce a transistor, because the point where they meet must be a common junction on the same piece of silicon (or germanium) - hence (in part) the term Bipolar Junction Transistor. The 'bipolar' term means that transistors use 'charge carriers' of both polarities - positive and negative, or minority and majority.</p>
|
||||
|
||||
<p>Since the base to collector junction is reverse biased in normal operation, there will be no current flow. It is the action of injecting current into the base that causes current flow in the collector circuit. I do not intend to explain the exact conduction mechanism, as it is somewhat outside the scope of this article.</p>
|
||||
|
||||
<div class="t-pic"><img src="ab-f2-1.gif" alt="Figure 2.1" border="1" /><br />Figure 2.1 - Analogous Depiction of Transistors</div>
|
||||
|
||||
<p>This is very convenient, because it gives us an easy way to check if a transistor is likely to be good or bad, simply by measuring the 'diodes'. Early PNP germanium devices would actually work equally well if the emitter and collector were reversed, but devices are now optimised to maximum performance, so this trick is not as successful (it does still work, but the device gain is <i>much</i> lower when the terminals are reversed).</p>
|
||||
|
||||
<p>To make the transistor actually do something useful, it is necessary to bias it correctly. This is done (having selected a suitable collector resistance) simply by applying enough base current to ensure that the collector is at 1/2 the supply voltage. In the same way that the plate load resistor determines the output impedance of a valve amplifier, the collector resistor determines the output impedance of a transistor amplifier. Unlike a valve, the transistor is not said to have a 'collector resistance' as in the equivalent resistance between emitter and collector, because this is not relevant to the operation of a transistor.</p>
|
||||
|
||||
<p>Figure 2.2 shows three methods of biasing a transistor, wired in 'common emitter' configuration ¹. Of these Figure 2.2a is the least usable, because there is no mechanism to ensure that the circuit will be repeatable with different devices or with temperature. Variations caused by temperature are (and always have been) a real problem, and it is necessary always to ensure that the circuit has some feedback mechanism for DC operating conditions to ensure stability. Different transistors (of the same type and even from the same manufacturing run) will have different gains, and this, too, must be compensated for.</p>
|
||||
|
||||
<ol>
|
||||
<li>There are three configurations for transistors, referred to as 'common emitter', 'common collector' and 'common base'. The 'common' part simply means that it is common to both input and output from
|
||||
a signal (AC) perspective. The supply rail and ground are equivalent for AC due to the use of bypass capacitors. The same techniques were used with valves, giving 'common cathode', 'common anode' and
|
||||
'common grid' equivalents (respectively).
|
||||
</ol>
|
||||
|
||||
<p>For the three circuits below, assume that the gain of the transistor is 100 (exactly). This means that for 1mA of base current, 100mA of collector current will flow. The emitter current is the sum of the base and collector currents. To bias the transistor we need only meet this criterion (in theory), and everything will be well. With a Supply voltage (Vcc) of 20V, we want to have 10V at the collector, to allow maximum voltage swing. This will allow the voltage to go to +20V or to 0V, however the signal will be badly distorted by then.</p>
|
||||
|
||||
<div class="t-pic"><img src="ab-f2-2.gif" alt="Figure 2.2" border="1" /><br />Figure 2.2 - Biasing a Transistor Voltage Amplifier, Three Methods</div>
|
||||
|
||||
<p>Figure 2.2A is unusable in practice, even though it appears to satisfy the criteria for correct operation. Figure 2.2B is a simple way to achieve (acceptably) stable bias, but has some drawbacks. Because the bias resistor (Rb) is supplied from the collector circuit, it will have some of the collector current flowing in it. This will introduce negative current feedback, which at DC stabilises the circuit, but with the AC signal makes the input impedance very low, as well as reducing gain for any finite value of source impedance. This is not necessarily a drawback, however, as the feedback also reduces the distortion components.</p>
|
||||
|
||||
<p>This problem is overcome with the circuit in Figure 2.2C, with a bias divider providing a fixed voltage reference, and the emitter resistor (Re) providing stabilising feedback as we saw with the valve voltage amplifier. In the same way as with a valve, this also provides feedback, increasing linearity and reducing gain. With a transistor we get one additional effect - the input impedance is increased (more on this subject later). Again, to achieve maximum gain, it is common to place a capacitor in parallel with Re to defeat the feedback for AC signals, allowing maximum gain.</p>
|
||||
|
||||
<p>To bias a PNP device, we use exactly the same circuitry, but the supply polarity is reversed, so the collector (and base) will have a negative voltage with respect to the emitter.</p>
|
||||
|
||||
<p>One of the major differences between valves and transistors is that once we have decided on a suitable biasing circuit (or specified a gain from the amplifier), we can make device substitutions with little or no change in performance, provided the transistors have similar basic parameters. Often the same circuit will work just as well with perhaps 10 or 20 different devices, all from different manufacturers.</p>
|
||||
|
||||
|
||||
<hr /><a id="s21"></a><b>2.1 Transistor Characteristics</b>
|
||||
<p>I shall only discuss the basic characteristics of transistors (as with valves), and there is really only one variable parameter and two fixed parameters (which are the same for every silicon transistor) to deal with. With transistors, the parameters are not as interactive as with valves, and the circuit gain is not as reliant on the device gain as with valves. In the same way as with valves, there are small signal devices (low power), working all the way up to power devices, which can have collector current ratings of 50 to over 100A for some of the very large power transistors.</p>
|
||||
|
||||
<ul>
|
||||
<li>Gain - (aka DC Current Gain, h<sub>FE</sub>, β, beta) - the measure of the base current versus the collector current. This figure is only slightly dependent on the
|
||||
collector voltage, but can be affected considerably by collector current.<br /><br /></li>
|
||||
|
||||
<li>re - Internal emitter resistance. With silicon transistors this is <b>26 / Ie</b> (in mA). Ie is the emitter current.<br /><br /></li>
|
||||
|
||||
<li>Thermal coefficient - the diode junction voltage of a silicon transistor is nominally 0.65V, and falls by 2.0mV / °C (this applies equally to diodes and transistors)</li>
|
||||
</ul>
|
||||
|
||||
<p>As stated before, the gain of a transistor is dependent on collector current, but will normally be applicable over a fairly wide range. The gain normally falls at very low currents (compared to the device maximum), and again at high current (approaching the maximum rated collector current for a given device).</p>
|
||||
|
||||
<div class="t-pic"><img src="ab-f2-3.gif" alt="Figure 2.3" border="1" /><br />Figure 2.3 - Transistor Transfer Characteristics</div>
|
||||
|
||||
<p>The signal transfer curve is similar to that of a valve, and is shown in Figure 2.3. There is generally less distortion in the linear part of the curve, but because of the lower operating voltage, a transistor amp must work closer to the supply rail and earth, so distortion may be higher with simple circuits such as those in Figure 2.2 than with an equivalent valve amplifier.</p>
|
||||
|
||||
<p>The major cause of distortion in small signal transistor amplifiers is the variation in the internal emitter resistance (re). Because transistors can tolerate a wider range of supply voltage and operating current than valves, it was common (when transistors were new and frightfully expensive) to try to extract as much voltage gain as possible from each device. This is no longer an issue, but the underlying problem is still there and it is necessary to take steps to prevent distortion. It's common to operate transistors at a constant current to minimise distortion. Very high gain circuits with global feedback are now the most common with transistor circuits, which renders the circuit immune from almost any variation of the device parameters, whether intrinsic (internally fixed) or manufacture dependent.</p>
|
||||
|
||||
<p>The gain of a transistor stage is approximately equal to the collector resistance divided by the emitter resistance (including the internal resistance re). So for the circuit of Figure 2.2c, the gain will be 9.75 without the emitter bypass capacitor, or about 384 with it installed. The distortion will be much higher with the emitter bypass in place, and it is uncommon to see these circuits any more.</p>
|
||||
|
||||
<p>The input impedance of a transistor voltage amplifier is low, and the output impedance is determined by the collector resistance (ignoring any feedback that may be applied from collector to base).</p>
|
||||
|
||||
<p>The input impedance is essentially determined by the gain of the device, and the value of emitter resistance (including the internal resistance), and in theory (that word again) is approximately equal to the emitter resistance multiplied by gain. The circuit of Fig 2.2a will therefore have an input impedance in the order of 2600 Ohms, Fig 2.2b will be very low because of the feedback, and 2.2c (without the bypass capacitor) will have an input impedance of 100k - but as this is shunted by the bias resistors, the impedance will actually only be about 12k.</p>
|
||||
|
||||
<p>A transistor can be thought of as a current controlled current source (CCCS).</p>
|
||||
|
||||
|
||||
<hr /><a id="s22"></a><b>2.2 Transistor Current Amplifier</b>
|
||||
<p>The current amplifier is much more common in transistor circuits than with valves, and is called an emitter follower (or occasionally common-collector). The emitter follower (like the cathode follower) has a voltage gain of less than 1 (or unity), but the difference is much less. Typically, the gain of an emitter follower circuit will be about 0.95 to 0.99 - depending on the operating current. The use of feedback to lower this further is very common, and output impedances of less than 1 Ohm are quite possible.</p>
|
||||
|
||||
<p>Figure 2.4 shows a standard configuration for an emitter follower current amplifier stage. It is common to bias the base to exactly 1/2 the supply voltage, using equal value resistors. I say 'a' standard because there are many different configurations that can be (and are) used, including direct coupling, which is very common with transistor circuits.</p>
|
||||
|
||||
<div class="t-pic"><img src="ab-f2-4.gif" alt="Figure 2.4" border="1" /><br />Figure 2.4 - Transistor Current Amplifier</div>
|
||||
|
||||
<p>One of the great attractions of transistors is their flexibility, which is considerably enhanced by having two polarities of device to work with. Because of this, circuits such as that shown in Figure 2.5 are common (or they were before the advent of opamps). Indeed, opamps themselves use the flexibility of transistors to the full, as can be seen if you have a look at the 'simplified equivalent circuit' often published as part of the specification sheet for many opamps.</p>
|
||||
|
||||
|
||||
<hr /><a id="s23"></a><b>2.3 Transistor Common Base Amplifier</b>
|
||||
<p>The common base amplifier is something that you rarely see these days. It was also used in valve circuits and was sometimes called a 'grounded grid' amplifier. Input impedance is <i>very</i> low, and the circuit shown has an input impedance of around 50 ohms. It has high gain, and can be used at radio frequencies because there is almost no collector-base feedback (or plate-grid feedback) due to stray (or internal) capacitance. In early designs common base stages were sometimes used for low impedance microphone preamps, or for other low-Z applications. The input capacitor (Cin) needs to be large to pass audio frequencies, due to the very low input impedance. The base capacitor (Cb) connects the base to ground for all AC signals.</p>
|
||||
|
||||
<div class="t-pic"><img src="ab-f2-5.gif" alt="Figure 2.5" border="1" /><br />Figure 2.5 - Common Base Transistor Voltage Amplifier</div>
|
||||
|
||||
<p>As shown, the circuit will have a gain of around 70 times (35dB), but that depends on the source impedance (50 ohms is assumed). It's an interesting circuit overall, but cannot compete with an opamp 'virtual earth' stage, which has an input impedance of close to zero. The common base arrangement was also used in 'cascode' amplifiers, as were common grid valve circuits - indeed, that's where the circuit came from. Cascode designs were mainly used where high gain at radio frequencies was necessary, but have re-emerged in valve audio gear because they (allegedly) sound 'better' than other circuits.</p>
|
||||
|
||||
|
||||
<hr /><a id="s24"></a><b>2.4 Transistor Combined Voltage + Current Amplifier</b>
|
||||
<p>The vast majority of circuit 'blocks' used today are combinations of stages. A combined voltage and current amplifier are very common, and these can be found in IC equivalent circuits, as well as many of the older designs that were in general use before opamps took over for the majority of circuitry.</p>
|
||||
|
||||
<div class="t-pic"><img src="ab-f2-6.gif" alt="Figure 2.6" border="1" /><br />Figure 2.6 - A Typical Direct Coupled Transistor Amplifier</div>
|
||||
|
||||
<p>As can be seen, this amplifier uses an emitter follower for the output, is direct coupled within the circuit itself, uses both NPN and PNP devices, and has feedback to set a gain which is dependent only on the ratio of the two resistors Rfb1 and Rfb2. It is this sort of circuit that the opamp came from in the beginning, and there are still ICs (and small power amplifiers) that use similar circuitry internally. Regular readers may even recognise the basic circuit from the Projects Pages - essentially this is a discrete opamp, and will have a very high gain, which is brought back to something sensible by the feedback.</p>
|
||||
|
||||
<p>The actual gain is almost entirely dependent on the resistor values (for gains less than about 50 or so), and may be calculated by</p>
|
||||
|
||||
<blockquote>
|
||||
Av = (Rfb1 + Rfb2) / Rfb2 where Av is voltage gain (Amplification, voltage)
|
||||
</blockquote>
|
||||
|
||||
<p>So to obtain a gain of 20, Rfb1 would be 22k, and Rfb2 1k2 - this is actually a gain of 19.33, representing an error of 0.3dB. This gain is so stable that a completely different set of transistors from a different manufacturer would make no difference to measured gain performance. Other factors, such as noise or distortion must vary with the quality of the active devices, but the changes will generally be very subtle, and may not be noticeable at all, depending on the similarity of the transistors.</p>
|
||||
|
||||
|
||||
<hr /><a id="s25"></a><b>2.5 Transistor Power Amplifiers</b>
|
||||
<p>A transistor power amplifier uses (typically) another configuration for the input stage. This is called a 'long tailed pair', (LTP) and acts as both the input stage and error amplifier (Q1 and Q2). This circuit operates in current mode, so there is little output voltage to be seen from its output.</p>
|
||||
|
||||
<p>The second stage (Q3) is a Class-A amplifier, and is responsible for a large proportion of the overall gain of the circuit. Notice the current sources that are typically used for the LTP and Class-A amp sections. These are commonly made using transistors and maintain a constant current regardless of the voltage at the collector. If the current were truly constant, this implies that the impedance is infinite (which means that the gain of the transistor stage is also infinite!), and although this is not the case in reality, it will still be remarkably high.</p>
|
||||
|
||||
<p>For more information on how current sources are constructed, see <a href="amp-basics5.htm#5.1">Section 5.1</a>
|
||||
|
||||
<div class="t-pic"><img src="ab-f2-7.gif" alt="Figure 2.7" border="1" /><br />Figure 2.7 - Transistor Power Amplifier</div>
|
||||
|
||||
<p>The output stage (Q4 and Q5) typically is a pair of complementary emitter followers, which must be correctly biased to ensure that as the signal passes from one transistor to another, there is no discontinuity. This form of operation is known as Class-AB, since the amp operates in Class-A for very low level signals, then changes to Class-B at higher levels. Any discontinuity while passing the signal from one transistor to the other is the cause of crossover distortion, and for many years gave transistor amplifiers a bad name in the audio world. With proper biasing, and properly applied feedback, the crossover distortion can be made to go away - although never completely, but amplifiers with distortion levels of well below 0.01% are common.</p>
|
||||
|
||||
<p>The resistors at the emitters of the output transistors help to maintain a stable bias, and also introduce some local feedback to linearise the output stage. This is a simplified circuit, and in reality the output stage will usually consist of multiple transistors, commonly a driver transistor followed by the output transistor itself. This does not change the operation of the circuit, but simply gives the output stage more gain, so it does not load the Class-A driver too heavily (this will result in greatly increased distortion).</p>
|
||||
|
||||
<p>Like the previous example, the gain is entirely dependent on the ratio of Rfb1 and Rfb2. As shown, the amp in Figure 2.6 is DC coupled, meaning that it will amplify any voltage from DC up to its maximum bandwidth. Not shown on this circuit are the various components needed to stabilise the circuit to prevent oscillation at high frequencies - often in the MHz range. Such oscillation is a disaster for the sound, and will quickly overheat and destroy the output transistors.</p>
|
||||
|
||||
<p>There are also transistor amplifiers that operate in Class-A, which means that the output transistors conduct all the time, and are never turned off. This can produce distortion levels that are almost impossible to measure, but this is at the expense of efficiency, and Class-A amplifiers will get very hot while doing nothing. Unlike the more common Class-AB amplifier, they will actually get slightly cooler as they reproduce a signal, since some of the input power is then diverted to the loudspeaker.</p>
|
||||
|
||||
|
||||
<hr /><a id="sum"></a><b>Transistors - A Summary</b>
|
||||
<p>Just as with valve amplifiers, I have only scratched the surface. Entire books are written on the subject, and range from basic texts used in technical schools, to very advanced tomes intended for university students. Since transistors are easy to work with (and safe), there is much to be gained by experimentation, and you will have the satisfaction of having designed and built a functioning amplifier.</p>
|
||||
|
||||
<p>Transistors also have their fair share of problems, and there are some things that they are just not very good at. Some of the major failings include:
|
||||
|
||||
<ul>
|
||||
<li><b>Low Impedance</b> - Bipolar transistors are inherently low impedance, and additional circuitry is needed to make them work satisfactorily in high impedance
|
||||
circuitry. Noise is also a problem when high impedance sources are used.<br /><br /></li>
|
||||
|
||||
<li><b>Heat</b> - Transistors dislike heat, and if it is not removed, they will destroy themselves. Most transistors can operate with junction temperatures up to
|
||||
about 125 degrees C, but at that temperature, can do no work at all. The life of a transistor is severely shortened by operating at high temperatures.<br /><br /></li>
|
||||
|
||||
<li><b>Thermal Stability</b> - Transistors are subject to some major changes in operation, depending on their temperature. This can make the design of high
|
||||
quality amplifiers difficult, because the transistor has a tendency towards 'thermal runaway'. This means that as the device gets hotter, it will draw more current,
|
||||
which makes it get hotter still. This continues until the maximum operating temperature is exceeded, and the transistor(s) fail.<br /><br /></li>
|
||||
|
||||
<li><b>Second Breakdown</b> - This is a version of thermal runaway, but at a molecular level. Parts of the internal structure become hotter than others, causing
|
||||
the hottest part to do the most work. This makes it hotter still until the transistor fails. Second breakdown is the most common cause of output transistor failure
|
||||
in power amps. It also happens very fast, and without warning - transistors can fail from second breakdown even when at ambient temperature.<br /><br /></li>
|
||||
|
||||
<li><b>Short-term Overload</b> - Largely due to second breakdown effects, transistors do not tolerate short term overloads, and in many cases even a momentary short
|
||||
circuit will cause instantaneous failure. Compared to valves, transistor circuits are much less capable with difficult loads, and usually must be over-engineered
|
||||
to sometimes extreme levels to prevent failures.<br /><br /></li>
|
||||
|
||||
<li><b>Hard Overload</b> - when a transistor amp goes into overload, it does so with startling clarity. The sound is altogether unpleasant</li>
|
||||
</ul>
|
||||
|
||||
<p>Again, there are many advantages as well. Transistor amplifiers are very reliable, and can be counted on to give many years of life without requiring even a basic service ( most of the time anyway).</p>
|
||||
|
||||
<p>They are also very quiet (generally much quieter than valve amps) and do not suffer from microphony, so room vibrations are not re-introduced into the music. Efficiency is much higher, with lower voltages and no heaters (its a pity they don't look really nice, though).</p>
|
||||
|
||||
<p>Output impedances of 0.01 Ohm are achievable, so loudspeaker damping can be very high. Because transistor amps are very mechanically rugged, they can be installed in speaker boxes, so speaker lead lengths can be very short.</p>
|
||||
|
||||
<p>Typical transistor amplifiers have much wider bandwidth than valve amps, because there is no transformer, this is especially noticeable at the lowest frequencies - a transistor amp can reproduce 5Hz as easily as 500Hz.</p>
|
||||
|
||||
<hr /><p align="right" class="t_12b"><a href="amp-basics1.htm">Previous (Part 1 - Valves)</a> <a href="amp-basics3.htm">Next (Part 3 - FETs)</a></p>
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<h1>Amplifier Basics - How Amps Work (Part 3)</h1>
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<div align="center" class="t_11">© 1999 - Rod Elliott (ESP)
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<br />Page Last Updated Jan 2017</div>
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<hr /><a id="contents"></a><b>Contents</b>
|
||||
<ul>
|
||||
<li><a href="amp-basics.htm"><b>Introduction</b></a>
|
||||
<li><a href="amp-basics1.htm"><b>Part 1 - Valves (Vacuum Tubes)</b></a>
|
||||
<li><a href="amp-basics2.htm"><b>Part 2 - Bipolar Transistors</b></a>
|
||||
<li><b>Part 3 - Field Effect Transistors and MOSFETs</b>
|
||||
<ul>
|
||||
<li><a href="amp-basics3.htm#s311">3.1.1 FET Characteristics</a>
|
||||
<li><a href="amp-basics3.htm#s312">3.1.2 Junction FETs</a>
|
||||
<li><a href="amp-basics3.htm#s313">3.1.3 MOSFETs</a>
|
||||
</ul>
|
||||
<li><a href="amp-basics3.htm#s32">3.2 FET Current Amplifier</a>
|
||||
<li><a href="amp-basics3.htm#s33">3.3 FET Power Amplifiers</a>
|
||||
<li>Summary
|
||||
<ul>
|
||||
<li><a href="amp-basics3.htm#sum1">Junction FETs</a>
|
||||
<li><a href="amp-basics3.htm#sum2">MOSFETs</a>
|
||||
</ul>
|
||||
<li><a href="amp-basics4.htm"><b>Part 4 - Operational Amplifiers (Opamps)</b></a>
|
||||
<li><a href="amp-basics5.htm"><b>Part 5 - Some Basic Linear Circuit Building Blocks</b></a>
|
||||
<li><a href="amp-basics6.htm"><b>Part 6 - Conclusions</b></a>
|
||||
</ul>
|
||||
|
||||
<hr /><a id="fieldeffect"></a><b>Part 3 - Field Effect Transistors and MOSFETs</b>
|
||||
<p>Now on to FETs and MOSFETs. FET stands for "<i>Field Effect Transistor</i>", and MOSFET means "<i>Metal Oxide Semiconductor Field Effect Transistor</i>". This topic is something of a can of worms, not because of some deficiency in the devices, but because of the huge array of different types. The basic FET types are ...</p>
|
||||
|
||||
<ul>
|
||||
<li>N-Channel Junction FETs
|
||||
<li>P-Channel Junction FETs
|
||||
<li>N-Channel Enhancement Mode MOSFETs
|
||||
<li>P-Channel Enhancement Mode MOSFETs
|
||||
<li>N-Channel Depletion Mode MOSFETs
|
||||
<li>P-Channel Depletion Mode MOSFETs
|
||||
</ul>
|
||||
|
||||
<p>There are a couple of major sub-classes of MOSFET - lateral and vertical. Lateral MOSFETs are particularly suited to audio applications, as they are far more linear than their vertical brethren, although their gain is lower. Vertical MOSFETs (e.g. HEXFETs and their ilk) are ideally suited to switching applications, and this includes Pulse Width Modulated (PWM) amplifiers.</p>
|
||||
|
||||
<p><b>Note: </b>Further to the material here, I suggest you also read the article <a href="articles/jfet-design.htm" target="_blank">Designing With JFETs</a>. It's much more recent than this article, and describes the use of JFETs in some additional detail. It also provides some info that will come in handy when you discover that your favourite JFET is no longer made, something that's depressingly common and it's getting worse.</p>
|
||||
|
||||
<p>The terms 'lateral' and 'vertical' refer to internal fabrication methods, so many others you may come across (such as HEXFETs <sup>®</sup>) are essentially variations of the vertical process. This is still not all the possibilities, because there are additional sub-classes as well, particularly with switching MOSFETs. However, for the purpose of a general article on their characteristics and how they work, I will concentrate on the most commonly used versions. This narrows the field, and we are left with both polarities of junction FETs, and both polarities of enhancement mode MOSFETs. With these, we cover the major proportion of current designs, so even 'though I will be leaving out a lot, the stuff I leave out is not all that common (he says hopefully).</p>
|
||||
|
||||
<p>FETs are 'unipolar' devices, in that they use only one polarity of carrier, in contrast to bipolar transistors, which use both majority and minority charge carriers (electrons or 'holes', depending on the polarity). FETs are far more resistant to the effects of temperature, X-Rays and cosmic radiation - any of these can cause the production of minority carriers in bipolar transistors).</p>
|
||||
|
||||
<p>I shall concentrate only on three terminal FETs, and the terminals are ...</p>
|
||||
|
||||
<ul>
|
||||
<li>Source - The electron 'source' (for N-channel devices), and is the equivalent of the cathode of a valve or the emitter of a transistor</li>
|
||||
<li>Gate - Control terminal - (more or less) equivalent to the grid of a valve or base of a transistor</li>
|
||||
<li>Drain - The terminal from which current is 'drained' - equivalent to the plate of a valve or collector of a transistor</li>
|
||||
</ul>
|
||||
|
||||
<p>There is no simple equivalent circuit for FETs (as there is for transistors), but this is of no consequence. The gate is the controlling element, and affects the electron flow not by amplifying a current (as in the transistor), but by the application of a voltage. The input impedance of junction FETs is very high at all usable frequencies, but MOSFETs are different. They have an almost infinite input resistance, but appreciable capacitance between the gate and the rest of the device. This can make MOSFETs hard to drive, because the capacitive loading makes most amplifier devices unhappy.</p>
|
||||
|
||||
<p>The junction FET is common in the inputs of high performance opamps, and offers extremely high input impedance. Indeed this is the case for discrete FETs as well, and a simple voltage amplifier using a junction FET and a power MOSFET are both shown in Figure 3.1. Both devices are N-Channel, and note that the arrow points in a different direction for each. The arrows point in the opposite direction for a P-Channel device, and all polarities are reversed. Vdd is +20V.</p>
|
||||
|
||||
<div class="t-pic"><img src="ab-f3-1.gif" alt="Figure 3.1" border="1" /><br />Figure 3.1 - Junction FET and Power MOSFET Voltage Amplifiers</div>
|
||||
|
||||
<p>Junction FETs are depletion mode devices, and (like all depletion mode FETs and MOSFETs) can be biased in exactly the same way as a valve. Depletion mode means that without a negative bias signal on the controlling element (the gate), there will be current flow between the drain (equivalent to plate or collector) and source (equivalent to cathode or emitter).</p>
|
||||
|
||||
<p>An enhancement mode device remains turned off until a threshold voltage is reached, after which the device conducts, passing more current as the voltage increases. Although there are MOSFETs made for low power operation, the majority (in audio, anyway) are power devices. These are almost exclusively enhancement mode, and can be capable of very high current.</p>
|
||||
|
||||
<p>In Figure 3.1, the power MOSFET is an enhancement mode device, and the junction FET is depletion mode. These are the most commonly used in audio. Enhancement mode power MOSFETs are also used in switching power supplies, and are far better than bipolar transistors in this role. They are faster, so switching losses are not as great (therefore the MOSFETs run cooler), and they are more rugged, and able to withstand abuses that would kill a bipolar transistor almost instantly.</p>
|
||||
|
||||
<p>This ruggedness (coupled with the freedom from second breakdown effects), means that MOSFETs are very popular as output devices for high power professional amplifiers. In this area, the MOSFET is second to none, and they are firmly entrenched as the device of choice for high power.</p>
|
||||
|
||||
<p>This is not to say that this is the only place MOSFETs are used. There are many fine audiophile power amps (and even preamps) that use power MOSFETs, and there are many claims that they are sonically superior to bipolar transistors (again, a debate that I will not discuss here).</p>
|
||||
|
||||
<p>Somewhat like valves, FETs and MOSFETs are very device dependent, and it is not normally possible to just substitute one device for a different type. Also like valves, the gain that can be expected from a voltage amplifier circuit is device dependent, and the manufacturer's data sheet (or testing) is the only way that one can be sure of obtaining the gain required in a given circuit.</p>
|
||||
|
||||
|
||||
<hr /><a id="s31"></a><b>3.1 FET Characteristics</b>
|
||||
<p>The characteristics of FETs must be covered in two parts, since we are dealing with two quite different devices. The first will be the junction FET, and as with transistors, I shall only describe the N-Channel, but virtually identical P-Channel devices are available (although not as commonly used).</p>
|
||||
|
||||
<div class="t-pic"><img src="ab-f3-2.gif" alt="Figure 3.2" border="1" /><br />Figure 3.2 - Transfer Curves For a Junction FET and MOSFET</div>
|
||||
|
||||
<p>Initially, so the transfer characteristics of the two devices can be seen side by side for comparison, Figure 3.2 shows a fairly typical device from each 'family'. The junction FET data is from a 2N5457, and the MOSFET is an IRFP240 (a vertical MOSFET - more suited to switching applications).</p>
|
||||
|
||||
<p>Rather than show the input and output signals superimposed on a graph, this time I show only the graph itself. These are excerpts from manufacturers data, but with a small catch - Figure 3.2b has the drain current displayed on a logarithmic scale, so the linearity of the device cannot be seen properly. If this graph were redrawn as linear, it will show that linearity is best at higher currents (on the graph shown it looks the other way around), and the device becomes almost perfectly linear with drain currents above about 3A.</p>
|
||||
|
||||
<p>Note that because the junction FET is depletion mode, drain current is highest at 0V gate-source voltage. The (most common) MOSFET on the other hand is enhancement mode, so at 0V gate-source, there is no current. Conduction starts at 4V, and by 6V the drain current is 10A (for example). This varies by MOSFET type, and they are available with low threshold (suitable for driving from 5V logic) or 'normal' threshold, requiring up to 10V or so for full conduction.</p>
|
||||
|
||||
<table style="width:100%">
|
||||
<tr><td valign="top"><img src="note.gif" alt="NOTE" height="37" width="108"></td>
|
||||
<td>The term Siemens (S) is now replacing Mhos as the unit of transconductance in most literature: 1S = Mho (1µS=1 µMho). For the above graphs, it may be worked out that the
|
||||
junction FET has a transconductance of 1,500µS, and for the MOSFET it is approx. 9,000µS (9,000 µMhos)</td></tr>
|
||||
</table>
|
||||
<br />
|
||||
|
||||
|
||||
<hr /><a id="s312"></a><b>3.1.2 Junction FETs</b>
|
||||
<p>Like valves, FET data sheets provide gain information as gm (mutual conductance - in µMhos). The junction FET shown has a gm of (typically) 1,500 µMhos (in the graph shown it is actually closer to 1425 µMhos in the linear section), which translates to about 1.5 mA/V.</p>
|
||||
|
||||
<p>The most common of the quoted parameters for junction FETs are</p>
|
||||
|
||||
<ul>
|
||||
<li>Forward Transfer Admittance (Common Source) - Transconductance - essentially the gain of the device</li>
|
||||
<li>Input Capacitance - The effective capacitance of the gate terminal to the remainder of the FET</li>
|
||||
<li>Gate-Source Cutoff Voltage - The gate voltage at which the FET is turned off</li>
|
||||
</ul>
|
||||
|
||||
<p>The process of amplification is almost identical to that of a valve, except that the voltages are lower. The device is biased in the same way (although fixed bias can also be used). This means that the gate must be reverse biased with respect to the source, with the gate having the opposite polarity of the source-drain voltage.</p>
|
||||
|
||||
<p>FETs offer low noise, especially with high impedance inputs, and in this respect are the opposite of bipolar transistors, which are generally at their best with low source impedance.</p>
|
||||
|
||||
<p>Junction FETs are predominantly low power, although there are some high power devices available. These are uncommon in audio applications.</p>
|
||||
|
||||
<p>It's notable (and regrettable) that many manufacturers have 'rationalised' their range of JFETs. Many of the high performance devices we used to be able to use in (for example) very low noise circuits have disappeared, and you can almost see JFETs vanishing from supplier catalogues as you watch. While I have never believed that JFETs have some 'magical' property that makes them sound better than anything else, it would have been nice if the manufacturers hadn't just decided that we don't need these specialised devices any more. I only have a couple of designs that use FETs, and it's now difficult to find devices that are suitable.</p>
|
||||
|
||||
|
||||
<hr /><a id="s313"></a><b>3.1.3 MOSFETs</b>
|
||||
<p>Again, MOSFET data sheets also provide information similar to junction FETs, but there are more items of importance to the designer. The most useful of these are</p>
|
||||
|
||||
<ul>
|
||||
<li>Forward Transconductance - The device gain characteristic
|
||||
<li>Drain to Source On Resistance - The minimum resistance when the MOSFET is fully on
|
||||
<li>Gate Threshold Voltage - The gate voltage at which the MOSFET will start conduction
|
||||
<li>Gate to Source Voltage - The maximum voltage (of either polarity) that may be applied between source and gate. (This is typically in the order of +/-20V)
|
||||
<li>Input Capacitance - The value of capacitive loading that is placed on the driving circuit
|
||||
</ul>
|
||||
|
||||
<p>Enhancement mode MOSFETs pass virtually no current when there is no gate voltage present. To conduct, a voltage must be applied between source and gate (of the same polarity as the drain voltage). Once the threshold has been reached, the device will start to conduct between drain and source.</p>
|
||||
|
||||
<p>At increasing gate voltages, the drain current increases until either a) the maximum permissible drain current or total dissipation limit is reached, or b) the drain voltage falls to its lowest possible value. In this instance, since the source-drain channel is now fully conducting, the value of R<sub>DS(on)</sub> determines the voltage.</p>
|
||||
|
||||
<p>Typical power MOSFETs offer extremely low on resistance, with values of less than 0.2 Ohm being fairly typical. There are many devices with much lower values (<50mΩ), but this is only important in switching circuits. In an audio amp, the MOSFETs should never be turned completely on, since this means the amplifier is clipping.</p>
|
||||
|
||||
<p>Another area that must be addressed with MOSFETs is the voltage between gate and source. Because the gate is insulated from the channel by a (very) thin layer of metal oxide, it is susceptible to damage by static discharge or other excessive voltage. It is common to include a zener diode between source and gate to ensure that the maximum voltage cannot be exceeded. Voltage spikes in excess of the breakdown voltage of the insulating layer will cause instantaneous failure of the device.</p>
|
||||
|
||||
|
||||
<hr /><a id="s32"></a><b>3.2 FET/ MOSFET Current Amplifier</b>
|
||||
<p>Again, I have shown both a junction FET and a MOSFET in Figure 3.3, both common-drain or source-follower circuits. As can be seen, the junction FET is biased almost identically to a valve, but all voltages are much lower. The MOSFET requires a positive voltage, and this must be greater than the source voltage, by an amount that takes the characteristics of the MOSFET into consideration. For the device characteristics shown in Figure 3.2 this means that at a current of 100mA, the gate must be 4V higher than the source.</p>
|
||||
|
||||
<div class="t-pic"><img src="ab-f3-3.gif" alt="Figure 3.3" border="1" /><br />Figure 3.3 - FET Current Amplifiers</div>
|
||||
|
||||
<p>For the JFET source follower, the bypass capacitor (Cb) is not always used, in which case the output would normally be taken from the source. When Cb is included, the output level is the same at both ends of Rs1, and input impedance is much greater because Rg is bootstrapped. The input impedance increase depends on the transconductance of the FET. For the JFET circuit shown(with Rg being 1MΩ), input impedance is about 5MΩ if Rs1 is not bypassed, rising to around 18MΩ with Cb included.</p>
|
||||
|
||||
<p>Cb needs to be large enough to ensure that the AC voltage across it remain small at the lowest frequency of interest. For example, if Rs1 is 1k, Cb must be at least 10µF (a -3dB frequency of 16Hz). A higher value is recommended to minimise low frequency distortion. For normal audio work, I'd use at least 33µF (still assuming 1k for Rs1).</p>
|
||||
|
||||
<p>Included in the MOSFET version is a zener for protection of the gate insulation. A 10V zener is used, as this gives good protection and is still able to let the maximum possible MOSFET current flow. A 6V zener could have been used, and this would still allow current up to 10A, which is far more than can be achieved from this simple circuit.</p>
|
||||
|
||||
|
||||
<hr /><a id="s33"></a><b>3.3 FET/ MOSFET Power Amplifiers</b>
|
||||
<p>In exactly the same way as a power valve can be used in single-ended Class-A, so too can a MOSFET. A simple circuit is shown in Figure 3.4 which will provide about 10W of audio. Using a constant current source as a load (as shown) gives better efficiency than a resistor, and improves linearity. The distortion from a circuit such as that shown will be roughly the same as that from a single ended triode valve circuit. Overall efficiency will be higher, since there is no cathode bias resistor needed, and no heaters as with a valve. Performance is <i>not</i> up to hi-fi expectations !</p>
|
||||
|
||||
<div class="t-pic"><img src="ab-f3-4.gif" alt="Figure 3.4" border="1" /><br />Figure 3.4 - Single Ended MOSFET Class-A Amplifier</div>
|
||||
|
||||
<p>Although there are a few, all MOSFET power amplifiers are uncommon. Most use a combination of bipolar transistors (for the input and gain stages), and MOSFETs for the output devices. This seems to be the most popular circuit arrangement, so I will concentrate on this. Figure 3.5 shows a fairly typical arrangement (in simplified form), and the operation of this is almost identical to that of an amplifier using bipolar transistors in the output. Note that emitter followers are needed to be able to provide the low impedance drive that MOSFETs need, although in some circuits they are not used. Instead, the Class-A driver stage (Q3) is operated at a higher than normal current to allow it to drive the MOSFETs properly.</p>
|
||||
|
||||
<div class="t-pic"><img src="ab-f3-5.gif" alt="Figure 3.5" border="1" /><br />Figure 3.5 - MOSFET Output Power Amplifier</div>
|
||||
|
||||
<p>One problem with this arrangement is that the gate to source voltage represents a circuit loss, so the power supply voltage needs to be typically ±6V higher than the required peak output voltage to the load to turn on the MOSFETs fully. Although this is not a major problem, it does increase dissipation in the output stage, and the loss increases with lower impedance loads.</p>
|
||||
|
||||
<p>Some (especially very high power) amps get around this by using a low current (but higher voltage) secondary power supply for the drive circuit, and the main high current supply for the MOSFETs. In an amp using +/-50V at 20 Amp main supplies, the secondary supply might be ±60V, but capable of perhaps 1A maximum.</p>
|
||||
|
||||
<p>As with the bipolar amp (did you notice how similar they are?), I have not included components for stability. These are typically the same as for a standard bipolar transistor amp, but will usually include 'stopper' resistors in series with the gates of the MOSFETs, and sometimes additional capacitance to prevent parasitic oscillation - the need for these varies from one device type to the next.</p>
|
||||
|
||||
|
||||
<hr /><a id="sum"></a><b>FETs - A Summary</b>
|
||||
<p><a id="sum1"></a><b>Junction FETs</b>
|
||||
<p>The surface is again, only barely scratched. The junction FET (aka JFET) is ideally suited to circuits where high impedances are expected, and will give the lowest noise. They are an invaluable electronic building block when used where they excel - providing extremely high input impedance.</p>
|
||||
|
||||
<p>Like all devices so far, JFETs have their limitations ...</p>
|
||||
|
||||
<ul>
|
||||
<li><b>Gain</b> - JFETs do not have the high gain of bipolar transistors</li>
|
||||
<li><b>High Frequency Response</b> - Generally, JFETs have a high frequency performance that is not as good as bipolar transistors</li>
|
||||
<li><b>Linearity</b> - The linearity of JFETs is not as good as bipolar transistors (so distortion is greater), but can be improved by using current source
|
||||
loading or feedback.</li>
|
||||
</ul>
|
||||
|
||||
<p>There is generally an ideal (or close to ideal) amplifying device for every application, and when used properly, the JFET is extremely versatile and at its best when high impedances are needed. If you have a need to send an amplifier into space, then JFETs are preferred due to their greater 'radiation hardness'. However, parameter spread is high, so no two JFETs can ever be assumed to be the same, even from the same batch. Where operation is critical, JFETs must be matched or provided with an adjustable source resistance to allow the operating point to be established.</p>
|
||||
|
||||
<p>JFETs (in fact all FETs) are more sensible than bipolar transistors when heated, and problems of thermal runaway are not usually encountered with these devices.</p>
|
||||
|
||||
<p>Most of the 'better' JFETs for audio use have now disappeared from the market. The 2SK170 was revered in some quarters, and was the 'go to' device for very low noise in many different applications. The original and any replacements that were offered subsequently are now obsolete. You might be able to buy JFETs with '2SK170' printed on them, but what's inside is anyone's guess. One thing you can be fairly sure of - it almost certainly will <i>not</i> be a genuine 2SK170. The LSK170 made by Linear Systems is available, as is as good as the original.</p>
|
||||
|
||||
<p>Even many 'pedestrian' JFETs have all but vanished from supplier's inventory, leaving you with limited choices. Some are available if you can handle SOT (small outline transistor, SMD), but even there the range is nothing like it used to be. This situation continues to get worse with each passing year.</p>
|
||||
|
||||
|
||||
<hr />
|
||||
<a id="sum2"></a><b>MOSFETs</b>
|
||||
<p>The MOSFET is one of the most powerful of all the current range of amplifying device, with extraordinary current handling capability. Ideally suited to very high power amplifiers, switchmode power supplies and Class-D amplifiers, where extremes of operating conditions are regularly encountered, the MOSFET has no equal. The possible exception is the Insulated Gate Bipolar Transistor (IGBT) which is a hybrid device as the name implies. IGBTs are not covered in these articles.</p>
|
||||
|
||||
<p>... And, as always, there are limitations ...</p>
|
||||
|
||||
<ul>
|
||||
<li><b>Gain</b> - Like JFETs, MOSFETs have a lower gain than bipolar transistors, which usually means that additional gain must be applied to the driving circuit
|
||||
to ensure that the global feedback is sufficient to maintain low distortion at low levels.</li>
|
||||
<li><b>Gate Capacitance</b> - The capacitance of the gate to source can be as high as 2nF (although more typically around 1.2nF). This is not a lot at low frequencies,
|
||||
but makes the drive circuit work very hard at high frequencies.</li>
|
||||
<li><b>Static Damage</b> - Until installed in a circuit with full protection, the MOSFET is susceptible to damage from static discharge. The voltage and current needed
|
||||
to destroy the device are generally below the threshold of feeling for humans. Some devices have (limited) protection built in.</li>
|
||||
<li><b>Linearity</b> - Most MOSFETs are not very linear at low currents, so for low distortion higher quiescent is needed to ensure that crossover distortion is minimised.</li>
|
||||
</ul>
|
||||
|
||||
<p>To some extent, all the above can be forgiven when you really need the capabilities of a MOSFET. The freedom from second breakdown and the massive current capabilities of MOSFETs are unmatched by any other active device. With a properly designed drive circuit, MOSFETs are also very fast, capable of performance that is generally superior to that of bipolar transistors. This is not very helpful in audio, but is essential for switching circuits. Note that the 'freedom from second breakdown' is (or was) often cited by manufacturers, but there <i>is</i> a failure mechanism that's almost identical, and is invoked when a switching MOSFET is used in linear mode. Most manufacturers state that their MOSFETs are not intended for linear operation. If you decide to do so, then be prepared for unexplained failures.</p>
|
||||
|
||||
<p>Coupled with a positive temperature coefficient that can stop thermal runaway in a linear circuit (when proper precautions are taken), the (lateral) MOSFET is almost indestructible, provided that you ensure the gate voltage is kept below the breakdown voltage. It's also essential to keep the drain voltage below the maximum specified.</p>
|
||||
|
||||
<p>The positive temperature coefficient can be a help in audio circuits, although it can be a problem in switching power supplies, since the 'on' resistance also increases with temperature, and in a switch-mode power supply this can cause thermal runaway (exactly the reverse of bipolar transistors in this application).</p>
|
||||
|
||||
<p>Switching MOSFETs are by far the most common now, with many of the earlier 'lateral' MOSFETs now unavailable. <a href="project101.htm" target="_blank">Project 101</a> was designed to use lateral MOSFETs, and it simply won't work with switching MOSFETs (not only because the gate and source pins are reversed for the two types. Switching MOSFETs are not designed for linear operation, and have to be severely derated to prevent failure.</p>
|
||||
|
||||
<hr /><p align="right" class="t_12b"><a href="amp-basics2.htm">Previous (Part 2 - Bipolar Transistors)</a> <a href="amp-basics4.htm">Next (Part 4 - Opamps)</a></p>
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<tr><td class="t-wht"><a id="copyright"></a><b>Copyright Notice.</b> This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is © 1999. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro-mechanical, is strictly prohibited under International Copyright laws. The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference. Commercial use is prohibited without express written authorisation from Rod Elliott.</td>
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<div align="center" class="t_11">© 1999 - Rod Elliott (ESP)
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<hr /><a id="contents"></a><b>Contents</b>
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<ul>
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<li><a href="amp-basics.htm"><b>Introduction</b></a>
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<li><a href="amp-basics1.htm"><b>Part 1 - Valves (Vacuum Tubes)</b></a>
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<li><a href="amp-basics2.htm"><b>Part 2 - Bipolar Transistors</b></a>
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<li><a href="amp-basics3.htm"><b>Part 3 - Field Effect Transistors and MOSFETs</b></a>
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<li><b>Part 4 - Operational Amplifiers (Opamps)</b>
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<ul>
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<li><a href="amp-basics4.htm#opamp">Operational Amplifiers</a>
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<li><a href="amp-basics4.htm#pwropamp">Power Operational Amplifiers</a>
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</ul>
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<li><a href="amp-basics5.htm"><b>Part 5 - Some Basic Linear Circuit Building Blocks</b></a>
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<li><a href="amp-basics6.htm"><b>Part 6 - Conclusions</b></a>
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</ul>
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<hr><a id="opamp"></a><b>Part 4 - Operational Amplifiers (Opamps)</b>
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<p>No discussion of amplifying devices would be complete without a discussion of opamps (aka op. amps). Although not a single device, the opamp is considered to be a building block, just like a valve or any transistor.</p>
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<p>The format I used for the other discussions is not appropriate for this topic, so will be changed to suit this most versatile of components. I shall not be covering esoteric or special purpose types, only the basic variety, as there are too many variations to cover.</p>
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<p>The operational amplifier was originally used for analogue computers, although at that time they were made using discrete components. Modern (good) opamps are so good, that it is difficult or impossible to achieve results even close with discrete transistors or FETs. However, there are still some instances where opamps are just not suitable, such as when high supply voltages are needed for large voltage swings.</p>
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<p>The majority of power amplifiers (whether bipolar or MOSFET) are in fact discrete opamps, with a +ve input and a -ve input. You tend not to see this, but have a look at Figure 3.5 again. The signal is applied to the +ve input at the base of Q1. The base of Q2 is the -ve input, and is used for the feedback signal, exactly the same as you will see in Figure 4.1a below.</p>
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<p>Unlike the other devices, opamps are primarily designed as voltage amplifiers, and their versatility comes from their input circuitry. Opamps have two inputs, designated as the non-inverting and inverting (or simply + and -).</p>
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<p>When wired into a conventional amplifier circuit, the opamp has one major goal in its little life ...</p>
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<b><i> Make both inputs the same voltage</i></b>
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<p>If, because some swine of a designer has made this impossible (very common with a lot of circuits), the opamp then takes another approach ...</p>
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<b><i> Make the output the same polarity as the most positive input</i></b>
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<p>The latter condition needs a small explanation. If the +ve input is most positive, then the output will swing to the positive supply rail (or as close as it can get). Should the -ve input be more positive, then the output will swing to the negative supply rail. The difference between the two inputs may be less than 1mV! Simple as that.</p>
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<p>I call these "The First and Second Laws of Opamps". These two statements describe everything an opamp does, and just by knowing this, makes the task of working out what most common circuits do a simple process. There is actually nothing especially complex about opamps, unless you look at the 'simplified' circuit diagram often included in data sheets. Don't do this, as it is too depressing. (By the way, the first statement is not strictly true of real-life devices, which will always have some error, however without very specialised equipment you will be unable to measure it.)</p>
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<p>Modern opamps (the good ones, anyway) are as close as anyone has ever got to the ideal amplifier. The bandwidth is very wide indeed, with very low distortion (0.00003% for one of the Burr Brown devices), and low noise. Although it is quite possible to obtain an output impedance of far less than 10 Ohms, the current output is usually limited to about +/-20mA or so. Supply voltage of most opamps is limited to a maximum of about +/-18V, although there are some that will take more, and others less.</p>
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<p>Depending on the opamp used, gains of 100 with a frequency response up to 100kHz are easily achieved, with noise levels being only very marginally worse that a dedicated discrete design using all the noise reducing tricks known. The circuits shown below have frequency response down to DC, with the upper frequency limit determined by device type and gain.</p>
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<div class="t-pic"><img SRC="ab-f4-1.gif" ALT="Figure 4.1" border="1" /><br />Figure 4.1 - Standard Opamp Configurations</div>
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<p>Figure 4.1 shows the two most common opamp amplifier circuits. The first (4.1a) is non-inverting, and is the better connection for minimum noise. The voltage fed back through Rfb1 will cause a voltage to be developed across Rfb2. The output will correct itself until these two voltages are equal at any instant in time. It does not matter if the signal is a sinewave, square wave, or music, the opamp will keep up (provided you stay within its capabilities). Once the speed of the opamp is not significantly higher than the rate of change of the input (generally a factor of 10 is sufficient - i.e. the opamp needs to be 10 times faster than the highest frequency signal it is expected to amplify), the output will become distorted. At voltage gains of 10 or less, almost any opamp will be able to keep up with typical audio signals, but (and be warned) this is no guarantee that they will sound any good.</p>
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<p>Input impedance is equal to Rin, and voltage gain (Av) is calculated from ...</p>
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<blockquote>
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Av = (Rfb1 + Rfb2) / Rfb2 or ...
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Av = Rfb1 / Rfb2 +1
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</blockquote>
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<p>The second circuit (4.1b) is an inverting amplifier, and is commonly used as a 'summing' amplifier - the output is the negative sum of the three (or more) inputs. It is also called a 'virtual earth' mixer, because the -ve input is a virtual earth (remember my 'First law of opamps'). If the +ve input is earthed (grounded), then the opamp must try to keep the -ve input at the same voltage - namely 0V. They are used in many diverse applications, and are common when a signal polarity must be inverted.</p>
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<p>It does this by adjusting its output until the current flowing through Rfb is exactly the same (but of the opposite polarity) as the current flowing into the inputs from each Rin. They must all sum to 0V, as they are equal and opposite. This is done with amazing speed, and good opamps will continue to succeed in fulfilling the First Law up to over 100kHz or more (depending on gain). Lesser devices will start to have trouble, and the appearance of a measurable voltage at the -ve input is an indication that the opamp can no longer keep up with the signal.</p>
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<p>Input impedance is equal to RinX (where X is the number of the input), and voltage gain is calculated from ...</p>
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<blockquote>
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Av = Rfb / RinX
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</blockquote>
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<p>Multiple inputs can all have different gains (and input impedances). There are two catches to this circuit. The first is that if the source does not have an output impedance significantly lower than Rin, then the gain will be lower than expected. The other, not always realised, is that if the circuit is configured for a gain of 1 (actually it is technically correct to refer to it as -1), Rin1, Rin2 etc. will all be equal to Rfb. If the circuit has 10 inputs, then from the opamp's perspective it has a gain of 10, and its frequency response and noise will reflect this.</p>
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<p>There are literally hundreds of different opamp circuit configurations. Feedback circuits with frequency dependent components (capacitors or inductors) make the opamp into a filter, or a phono equaliser, or almost anything else.</p>
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<p>For an in-depth look at opamp circuits, see the <a href="dwopa.htm" target="_blank">Designing With Opamps</a> series.</p>
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<p><a id="pwropamp"></a><b>Power Opamps</b>
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<p>Opamps even come in power versions, using a TO-220 (or other specialised) case, and are typically capable of around 25W to 50W or more into an 8 Ohm speaker load. These devices, while not necessarily considered to be to audiophile standards, are still very capable, and have been used by many domestic appliance manufacturers in such things as high-end TV sets and even 'high end' hi-fi equipment. Some of the more advanced devices are capable of output power up to 80W. It is very doubtful that even the most 'golden eared' reviewer would pick that an amplifier used a monolithic power amp (power opamp) in a double-blind test.</p>
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<p>They typically have distortion figures well below 0.1%, and can be used anywhere a small, convenient and cheap power amp is required. The circuit looks almost identical to that of a small signal opamp, except that a Zobel stabilisation network is used on the output to prevent oscillation. There are several circuits amongst the ESP projects, and PCBs are available for the most popular designs.</p>
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<hr><p align="right" class="t_12b"><a href="amp-basics3.htm">Previous (Part 3 - FETs)</a> <a href="amp-basics5.htm">Next (Part 5 - Building Blocks)</a></p>
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<tr><td class="t-wht"><a id="copyright"></a><b>Copyright Notice.</b> This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is Copyright (c) 1999, 2005. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro-mechanical, is strictly prohibited under International Copyright laws. The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference. Commercial use is prohibited without express written authorisation from Rod Elliott.</td>
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<td align="right" class="hdrr">Amplifier Basics - How Amps Work (Part 5) </td></tr></table>
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<h1>Amplifier Basics - How Amps Work (Part 5)</h1>
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<div align="center" class="t_11">© 1999 - Rod Elliott (ESP)
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<br />Page Last Updated 06 Apr 2005</div>
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<hr /><a id="contents"></a><b>Contents</b>
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<ul>
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<li><a href="amp-basics.htm"><b>Introduction</b></a>
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<li><a href="amp-basics1.htm"><b>Part 1 - Valves (Vacuum Tubes)</b></a>
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<li><a href="amp-basics2.htm"><b>Part 2 - Bipolar Transistors</b></a>
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<li><a href="amp-basics3.htm"><b>Part 3 - Field Effect Transistors and MOSFETs</b></a>
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<li><a href="amp-basics4.htm"><b>Part 4 - Operational Amplifiers (Opamps)</b></a>
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<li><b>Part 5 - Some Basic Linear Circuit Building Blocks</b>
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<ul>
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<li><a href="amp-basics5.htm#s51">5.1 Current Sources and Sinks</a>
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<li><a href="amp-basics5.htm#s52">5.2 Current Mirror</a>
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<li><a href="amp-basics5.htm#s53">5.3 Long Tailed Pair</a>
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<li><a href="amp-basics5.htm#s54">5.4 Grounded Grid (Gate or Base) Circuits</a>
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<li><a href="amp-basics5.htm#s55">5.5 Cascode</a>
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</ul>
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<li><a href="amp-basics6.htm"><b>Part 6 - Conclusions</b></a>
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<hr /><a id="part5"></a><b>Part 5 - Some Basic Linear Circuit Building Blocks</b>
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<p>There are some circuits in the world of electronics that are just too useful. While some have been around for many years, others only became practical with the advent of the transistor. The circuits described are a mixture, some are very old, and others much newer.</p>
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<p>I shall not go into the history (this is an electronics tutorial, not a history lesson), but will show the various stages in their basic form for each type of circuit.</p>
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<p>On the topic of current sources, sinks and mirrors, <a href="ism.htm">click here</a> to see the full article describing how they are most commonly used in audio circuits (and why).</p>
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<hr /><a id="s51"></a><b>5.1 Current Sources And Sinks</b>
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<p>The constant current source (or sink) is one of the most versatile and widely used of the circuits shown in this section. The ideal current source provides a current into a load that is independent of the resistance (or impedance) of the load, from zero to infinity. As always, the ideal does not exist, but within the capabilities of the power supply voltage, it is quite simple to do, and surprisingly accurate.</p>
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<p>There is no real difference between the two circuits - one sources current (or sinks electrons) or vice versa. Sometimes it might help to consider the circuit 'upside down' to see that there is no real difference, only one of terminology.</p>
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<p>As an example, if we wanted to supply a current that were fixed at 1A into any load impedance, then we might use a circuit similar to that in Figure 5.1 - a basic transistor current source. As shown, this will supply 1A into any resistance from zero Ohms up to a little under 50 Ohms. The power supply is the limiting factor - to be able to supply the same current into 1M Ohm would need a 1,000,000V power supply, which is an unrealistic expectation.</p>
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<p>The current source or sink can be imagined as a device with infinite impedance - this must be the case if the current remains unchanged even as the load resistance is varied over a wide range. Naturally, the impedance of actual current sources is not infinite, but can easily reach values of many megohms, even in a simple circuit.</p>
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<div class="t-pic"><img SRC="ab-f5-1.gif" alt="Figure 5.1" border="1" /><br />Figure 5.1 - A Basic Transistor Current Source / Sink</div>
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<p>The operation of the circuit is simple. If the voltage across the emitter resistor of Q2 attempts to exceed 0.65V (the base turn-on voltage for a silicon transistor), then Q1 will turn on, and short out all base current to Q2 except for exactly that amount required to maintain the specified current of 10mA (10mA through 65 ohms develops 0.65V). If the collector current of Q2 falls, then the voltage across the emitter resistor also falls. This turns off Q1 until the current is again stable at the preset value. (This is only one way to make a current source - there are many others.)</p>
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<p>Thermal stability is not good. The emitter-base potential falls at 2mV / degree C, so as the temperature increases, the current will fall from the nominal 1A. At low temperatures, the opposite will occur. A precision voltage reference can be used, or an opamp can monitor the voltage across the resistor, resulting in a much more stable current. Fortunately, in most circuits, it is not that critical, so the circuit of Figure 5.1 is very common.</p>
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<div class="t-pic"><img SRC="ab-f5-2.gif" alt="Figure 5.2" border="1" /><br />Figure 5.2 - A JFET Current Source / Sink</div>
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<p>Junction FETs, being a depletion mode device, can be used as a current source very easily, as shown in Figure 5.2. Because JFETs are mainly low current devices, the useful range is from about 0.1mA up to 10mA or so. This is ideal for many of the circuits that need a current source. The actual current is dependent on the FET's characteristics, but is sufficiently stable for many non-critical applications. Based on the FET curve shown in Figure 3.2a, this current source will supply a current of about 0.4mA into a load from zero to 72k Ohms. The voltage is also lower, because of the lower voltage rating of most FETs.</p>
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<hr /><a id="s52"></a><b>5.2 Current Mirror</b>
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<p>The current mirror is one of the 'new' circuits, and works well with bipolar transistors. It is unusual to see this circuit implemented with valves or FETs, and I will not change this (i.e. I will show transistors only). Figure 5.3 shows a simple current mirror (this version is not very accurate, but is still extremely effective and commonly used).</p>
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<div class="t-pic"><img SRC="ab-f5-3.gif" alt="Figure 5.3" border="1" /><br />Figure 5.3 - A Simple Current Mirror</div>
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<p>Any current injected into the collector/base circuit of Q1 (via Ri) will be 'mirrored' by Q2, which will draw the same current through its load resistor (within the capability of the transistor and power supply). Current mirrors are sometimes used as current sources (one less resistor), and are not as dependent on temperature, since both transistors will ideally be at the same temperature. It is not uncommon to use dual transistors (or thermal bonding) to ensure stability.</p>
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<hr /><a id="s53"></a><b>5.3 Long Tailed Pair</b>
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||||
<p>The long tailed (or differential) pair is an old circuit, and is used with valves, FETs and BJTs. It was originally designed in the valve era, and provides a means for the comparison of two voltages. The long tailed pair (LTP) is used as the input stage of most opamps, and many (if not most) modern power amplifiers.</p>
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<div class="t-pic"><img SRC="ab-f5-4.gif" alt="Figure 5.4" border="1" /><br />Figure 5.4 - The Long Tailed Pair - All Common Devices</div>
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||||
<p>As can be seen in Figure 5.4, the LTP can be made using valves (A), JFETs (B) or bipolar transistors (C). Valves and JFETs can be self biased as shown, but BJT circuits must have external bias resistors. A pair of MOSFETs could be used, but at the typical currents used (less than 5mA), the gain and linearity would be very poor. Although each circuit is shown using a resistor as the 'tail', in FET and bipolar circuits this is most commonly a current source (or sink if you prefer).</p>
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||||
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||||
<p>The use of a current source stabilises the overall current, so the device input current is not affected by supply voltage changes, or variations in the input bias voltages.
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||||
|
||||
<p>In each case, the circuit has an inverting and a non-inverting input, and an inverted and non-inverted output. Application of the same voltage and polarity to both inputs at once results in (theoretically) zero output - this is called the common mode signal, and is commonly quoted for opamps as the common mode rejection ratio. </p>
|
||||
|
||||
<p>The valve and FET versions only require capacitive coupling, as they are self biasing as shown. The bipolar circuit cannot be self biased, and requires the biasing resistors Rb1 and Rb2 for each input.</p>
|
||||
|
||||
<p>The output of each version may be taken from either or both outputs, and may be capacitively or direct coupled. Direct coupling is very common with LTP circuits, especially in opamps and audio power amplifiers, where it is the rule, rather than the exception.</p>
|
||||
|
||||
<p>When used as the input stage of an amplifier, the LTP uses one input as the signal input, and the other is used for the application of feedback, in the same way as in an opamp.</p>
|
||||
|
||||
<p>It's worth noting that the performance of a long tailed pair is dependent on the gain of the active device. BJTs have far greater gain than valves or JFETs, and the performance of a BJT version is generally vastly superior to the valve or FET circuits. Of the three circuits shown, only the BJT will be able to provide close to identical outputs (but with one inverted of course). The other two certainly work, but the level difference between the outputs can be 20% or more.</p>
|
||||
|
||||
|
||||
<hr /><a id="s54"></a><b>5.4 Grounded Grid (Gate or Base) Circuits</b>
|
||||
<p>Sometimes, it is desirable to have an extremely low input impedance, for example where the output impedance of the source is very low. One way to achieve this is to use the control element (grid, gate or base) as the reference, and apply the signal to the cathode, source or emitter (as appropriate). Figure 5.5 shows an example of grounded (or common) grid (A), gate (B) and emitter (C).</p>
|
||||
|
||||
<div class="t-pic"><img SRC="ab-f5-5.gif" alt="Figure 5.5" border="1" /><br />Figure 5.5 - Common Grid, Gate and Base Amplifiers</div>
|
||||
|
||||
<p>Apart from having an extremely low input impedance, this class of amplifier has an additional advantage. The normal capacitance from output to input is bypassed to earth, and no longer acts as a feedback path. Such circuits are therefore capable of a much better high frequency response than when used in the 'conventional' way, and are common in radio frequency circuits. The bypass path is direct for a valve or FET, and is via a capacitance for the bipolar circuit.</p>
|
||||
|
||||
<p>All inputs and outputs must be capacitively coupled, unless the preceding circuit is to be direct coupled (unusual) or the output is direct coupled to a follower (quite common).</p>
|
||||
|
||||
<p>Because the input impedance is so low, there are few applications in audio, except where this circuit is used in conjunction with a 'normal' amplifier stage. This forms a new circuit, called cascode.</p>
|
||||
|
||||
|
||||
<hr /><a id="s55"></a><b>5.5 Cascode</b>
|
||||
<p>This circuit was developed in the valve era, primarily to obtain better response at high radio frequencies. Valves have capacitance between the plate and grid, and this acts as a feedback path at high frequencies, causing a drop in gain as the frequency is increased. This is the so-called 'Miller' effect. Operating in cascode allows the circuit to have a high input impedance (via the normal grid input), and the grounded grid amplifier (the signal is applied to the cathode) means that there is no feedback from plate to grid, and the grid acts as a shield to prevent feedback to the cathode. The lower half of the stage contributes a relatively small amount of gain, and is not subject to the feedback effect since it is operating as a current amplifier (there is very little voltage swing on the plate, so there is little or no signal to feed back).</p>
|
||||
|
||||
<div class="t-pic"><img SRC="ab-f5-6.gif" alt="Figure 5.6" border="1" /><br />Figure 5.6 - Valve Cascode Amplifier</div>
|
||||
|
||||
<p>As can be seen, the grid of V2 is earthed via the capacitor for all signal frequencies, allowing V2 to operate as common grid. The capacitor (C bypass) is used to ensure that there is no gain lost due to cathode degeneration (local feedback). The cathode of V1 will also be bypassed in many cases, especially where low noise is a primary goal.</p>
|
||||
|
||||
<p>The same principles can be applied to FETs or BJTs, and has similar advantages. The capacitance between Drain and gate (or collector and base) is isolated in the same way as with a valve circuit, with the signal being coupled to the source or emitter by the first device. Figure 5.7 shows a composite JFET / BJT cascode circuit, which will have better linearity than a conventional amplifier, and a much better high frequency response.</p>
|
||||
|
||||
<div class="t-pic"><img SRC="ab-f5-7.gif" alt="Figure 5.7" border="1" /><br />Figure 5.7 - Composite FET / BJT Cascode Amplifier</div>
|
||||
|
||||
<p>This type of circuit is not uncommon in high performance opamps, where very wide bandwidth and good linearity before feedback are required. Cascode circuits are also sometimes seen in solid state power amplifier circuits, where the designer is trying to obtain the maximum possible bandwidth from the amp.</p>
|
||||
|
||||
<p>The base of Q2 is grounded to all signal frequencies, so the stage operates as a common base circuit. Using a JFET as the input element means that the circuit has a high input impedance, while the BJT ensures maximum gain. To obtain even more gain, Rc might be replaced by a current source, in which case the gain from this single stage can exceed 1000 times, with wide bandwidth and excellent linearity.</p>
|
||||
|
||||
<hr /><p align="right" class="t_12b"><a href="amp-basics4.htm">Previous (Part 4 - Opamps)</a> <a href="amp-basics6.htm">Next (Conclusions)</a></p>
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<tr><td class="t-wht"><a id="copyright"></a><b>Copyright Notice.</b> This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is Copyright (c) 1999-2005. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro-mechanical, is strictly prohibited under International Copyright laws. The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference. Commercial use is prohibited without express written authorisation from Rod Elliott.</td>
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<td align="right" class="hdrr">Amplifier Basics - How Amps Work (Part 6) </td></tr></table>
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<h1>Amplifier Basics - How Amps Work (Part 6)</h1>
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<div align="center" class="t_11">© 1999 - Rod Elliott (ESP)
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<hr><a id="contents"></a><b>Contents</b>
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<ul>
|
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<li><a href="amp-basics.htm"><b>Introduction</b></a>
|
||||
<li><a href="amp-basics1.htm"><b>Part 1 - Valves (Vacuum Tubes)</b></a>
|
||||
<li><a href="amp-basics2.htm"><b>Part 2 - Bipolar Transistors</b></a>
|
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<li><a href="amp-basics3.htm"><b>Part 3 - Field Effect Transistors and MOSFETs</b></a>
|
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<li><a href="amp-basics4.htm"><b>Part 4 - Operational Amplifiers (Opamps)</b></a>
|
||||
<li><a href="amp-basics5.htm"><b>Part 5 - Some Basic Linear Circuit Building Blocks</b></a>
|
||||
<li><b>Conclusions</b></li>
|
||||
<ul>
|
||||
<li><a href="amp-basics6.htm#references">References</a></li>
|
||||
<li><a href="amp-basics6.htm#copyright">Copyright Info</a></li>
|
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<li><a href="amp-basics6.htm#updates">Update Info</a></li>
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||||
<hr /><a id="conclusion"></a><b>Conclusions</b>
|
||||
<p>Section 5 is the last of the technical pages in this series, and this page finalises the topic at this level - at least until such time as I find (or someone points out) a mistake or major omission that I will then have to fix, otherwise there will be no further updates.</p>
|
||||
|
||||
<p>The articles in this series describe the essential building blocks of nearly all circuits in common use today. There are others (of course) but they are most often combinations of the above - for example, a LTP (long-tailed pair) stage can be built using two cascode circuits, a current source and a current mirror. The resulting circuit looks complex, but is simply a combination of common circuits such as those shown.</p>
|
||||
|
||||
<p>Other circuits are modification of the basic stages to exploit what might otherwise be seen as a deficiency - for example circuits that deliberately exploit the temperature dependency of a BJT can be used as high gain thermal sensors, or to stabilise the quiescent current in a power amplifier.</p>
|
||||
|
||||
<p>There are also some bizarre combinations possible. A valve and BJT operating in cascode would be interesting, and would no doubt have some desirable characteristics (and I have seen this particular combination used in a power amplifier). Likewise, a valve with a transistor current source instead of the load resistor has far better linearity and more gain than a simple resistor loaded version.</p>
|
||||
|
||||
<p>In many cases, ICs are available to accomplish the functions described. Opamps are an obvious one, but there are also IC current sources, transistor arrays (ideal for current mirror applications because of the excellent thermal tracking), plus quite a few others.</p>
|
||||
|
||||
<p>There are countless different IC power amplifiers, many of which have very high performance. There are several ESP projects that use 'power opamps' ... my terminology, because most are used just like any other opamp, but with higher voltages and the ability to drive loudspeaker loads. Complete ICs are even available for Class-D amplifiers, which combine just about every technique described in this series, but with even more circuit concepts. As you'd expect, these are also covered in separate articles.</p>
|
||||
|
||||
<p>None of the techniques described here is just for audio. The same (or very similar) circuitry is used in industrial control systems, radio frequency amplifiers and any number of diverse fields. While you could be forgiven for thinking that <i>everything</i> is now 'digital', that's not the case. Analogue circuitry will be around for a very long time yet, and will probably never go away. Even the most sophisticated digital process controller still has to interface with the 'real world', which is 100% analogue!</p>
|
||||
|
||||
<p>I hope that I have shed some light on the subject, and that you get some benefit from the information presented. Please be aware that this series is intended as a <i>very</i> basic introduction only, and (almost) every configuration discussed here is fully explained elsewhere on the ESP site. There are whole articles on designing with opamps, current sources, sinks and mirrors, and there's even a section dedicated to valves (vacuum tubes).</p>
|
||||
|
||||
|
||||
<hr><a id="references"></a><b>References</b>
|
||||
<ol><li>Philips 'Miniwatt' Technical Data, 7th Edition, 1972</li>
|
||||
<li>RCA Receiving Tube Manual, 1968</li>
|
||||
<li>Basic Electronics - Grob, McGraw Hill, 1971</li>
|
||||
<li>Radiotron Designer's Handbook - Langford-Smith, AWV Pty. Ltd, 1957</li>
|
||||
<li>Analysis and Design of Electronic Circuits - P.M. Chirlean, McGraw Hill, 1965</li>
|
||||
<li>Data Sheets, various</li>
|
||||
</ol>
|
||||
|
||||
<hr><p align="right" class="t_12b"><a href="amp-basics5.htm">Previous (Part 5 - Building Blocks)</a></p>
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<tr><td class="t-wht"><a id="copyright"></a><b>Copyright Notice.</b> This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is Copyright (c) 1999, 2005. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro-mechanical, is strictly prohibited under International Copyright laws. The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference. Commercial use is prohibited without express written authorisation from Rod Elliott.</td>
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<div class="t-sml"><a id="updates"></a>Page created and copyright (c) 20 Dec 1999./ Various updates up to 06 Apr 05./ Dec 2018 - minor format changes, additional info in various sections.</div><br />
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<h1>Amplifier Sound - What Are The Influences?</h1>
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|
||||
<ul>
|
||||
<li><a href="amp-sound.htm#intro">Introduction</a>
|
||||
<li><a href="amp-sound.htm#s1">1.0 The Components of Sound</a>
|
||||
<li><a href="amp-sound.htm#s2">2.0 Measurable Performance Characteristics</a>
|
||||
<li><a href="amp-sound.htm#s3">3.0 Distortion</a>
|
||||
<ul>
|
||||
<li><a href="amp-sound.htm#s31">3.1 Clipping Distortion</a>
|
||||
<li><a href="amp-sound.htm#s32">3.2 Crossover Distortion</a>
|
||||
<li><a href="amp-sound.htm#s33">3.3 Frequency And Phase Distortion</a>
|
||||
<li><a href="amp-sound.htm#s34">3.4 Slew Rate Distortion</a>
|
||||
</ul>
|
||||
<li><a href="amp-sound.htm#s4">4.0 Open Loop Response</a>
|
||||
<li><a href="amp-sound.htm#s5">5.0 Speaker - Amplifier Interface</a>
|
||||
<li><a href="amp-sound.htm#s6">6.0 Impedance</a>
|
||||
<li><a href="amp-sound.htm#outro">Conclusions</a>
|
||||
<li><a href="amp-sound.htm#ref">References</a>
|
||||
</ul>
|
||||
|
||||
|
||||
<hr /><a id="intro"></a><b>Introduction</b>
|
||||
<p>The sound of an amplifier is one of those ethereal things that seems to defy description. I will attempt to cover the influences I know about, and describe the effects as best I can. This is largely hypothesis on my part, since there are so many influences that, although present and audible, are almost impossible to quantify. Especially in combination, some of the effects will make one amp sound better, and another worse - I doubt that I will be able to even think of all the possibilities, but this article might help some of you a little - at least to decipher some of the possibilities.</p>
|
||||
|
||||
<p>I don't claim to have all the answers, and it is quite conceivable that I don't have any (although I do hope this is not the case). This entire topic is subject to considerable interpretation, and I will try very hard to be completely objective.</p>
|
||||
|
||||
<p>Reader input is encouraged, as I doubt that I will manage to get everything right first time, and there are some areas where I do not really know what the answers are. The only joy I can get from this is that I doubt that anyone else can do much better. If you can, let me know.</p>
|
||||
|
||||
<p>Unfortunately, it can be extremely difficult for the novice to figure out what on-line information is reliable, what is unmitigated drivel, and which material has a random mixture of both. There are some extraordinarily dubious claims made, and as an example I offer the following gem (reproduced verbatim) ...</p>
|
||||
|
||||
<blockquote class="t_11i">
|
||||
"A modern high-quality audio system has excellent specifications and sounds almost perfect. Almost perfect, but not quite. There is one very important attribute missing in audio
|
||||
systems - the attribute we call 'presence'. This article discusses an alternative power amplifier design with sound that often lacks in conventional amplifiers. Even the best
|
||||
commercially available audio systems lack real presence - while the sound can be crystal clear, you would never mistake the recorded voices for real voices, or the recorded piano for a real
|
||||
piano. The human ear immediately knows the difference.
|
||||
<br /><br />
|
||||
As listeners, even as audiophile listeners, we don't fuss about this lack of presence because we have come to accept that what we hear from a modern audio system is as good as it gets.
|
||||
Yet this just isn't true, and it doesn't have to be accepted.
|
||||
<br /><br />
|
||||
The lack of presence occurs almost entirely as a result of distortions inherent in the fundamental design of all commercial power amplifiers. Have you noticed how much clearer headphones
|
||||
sound? It's due to the fact that they are driven by low-powered amplifiers."
|
||||
</blockquote>
|
||||
|
||||
<p>This nonsense has <i>just</i> enough (semi) truth to appear plausible, but as it continues the claims become less coherent. A recorded sound is different from a live sound because there's a microphone and speakers between the source and your ears. It has nothing to do with the amplifier, and especially nothing to do with the amplifier's <i>power</i>. Headphones sound clearer (except when they don't) because of the headphone drivers and intimate coupling with our hearing mechanism. The amplifier power is utterly irrelevant, and the third paragraph is unmitigated drivel!</p>
|
||||
|
||||
<p>I could dissect the claims (which continue ad nauseam in the full text) in greater detail, but frankly it's not worth the electrons that would be used to transport the text. The article goes on to extol the 'virtues' of a rather odd amplifier topology that saw daylight for perhaps 30 seconds or so back in 1971, and never saw commercial production. It was published in Wireless World, but doesn't appear to have ever been re-published elsewhere. The amp used a single supply, so was capacitor coupled to the speaker, and while the basic design works well enough (or so it's claimed), almost no-one wants capacitively coupled speakers any more.</p>
|
||||
|
||||
|
||||
<hr /><a id="s1"></a><b>1.0 The Components of Sound</b>
|
||||
<p>When people talk about the sound of an amplifier, there are many different terms used. For a typical (high quality) amplifier, the sound may be described as 'smeared', having 'air' or 'authoritative' bass. These terms - although describing a listener's experience - have no direct meaning in electrical terms. The term 'presence' referred to above is created in guitar amps (for example) by boosting the frequencies around 3kHz - it's <i>not</i> something found in power amplifiers.</p>
|
||||
|
||||
<p>Electrically, we can discuss distortion, phase shift, current capability, slew rate and a myriad of other known phenomena. I don't have any real idea as to how we can directly link these to the common terms used by reviewers and listeners.</p>
|
||||
|
||||
<p>Some writers have claimed that all amplifiers actually sound the same, and to some extent (comparing apples with apples) this is 'proven' in double-blind listening tests. I am a great believer in this technique, but there are some differences that cannot be readily explained. An amp that is deemed 'identical' to another in a test situation, may sound completely different in a normal listening environment. It is these differences that are the hardest to deal with, since we do not always measure some of the things that can have a big influence on the sound.</p>
|
||||
|
||||
<p>For example; It is rare that testing is done on an amplifier's clipping performance - how the amp recovers from a brief transient overload. I have stated elsewhere that a hi-fi amplifier should never clip in normal usage - nice try, but it IS going to happen, and often is more common than we might think. Use a good clipping indicator on the amp, and this can be eliminated, but at what cost? It might be necessary to reduce the volume (and SPL) to a level that is much lower than you are used to, to eliminate a problem that you were unaware existed.</p>
|
||||
|
||||
<p>Different amplifiers react in different ways to these momentary overloads, where their overall performance is otherwise almost identical. I have tested IC power amps, and was dismayed by the overload recovery waveform. My faithful old 60W design measures about the same as the IC in some areas, a little better in some, a little worse in others (as one would expect).</p>
|
||||
|
||||
<p>Were these two amps compared in a double blind test (avoiding clipping), it is probable that no-one would be able to tell the difference. Advance the level so that transients started clipping, and a fence post would be able to hear the difference between them. What terms would describe the sound? I have no idea. The sound might be 'smeared' due to the loss of detail during the recovery time of the IC amp. Imaging might suffer as well, since much of the signal that provides directional cues would be lost for periods of time.</p>
|
||||
|
||||
|
||||
<hr /><a id="s2"></a><b>2.0 Measurable Performance Characteristics</b>
|
||||
<p>A detailed description of the more important (from a sound perspective) of the various amplifier parameters is given later in this article, but a brief description is warranted first. Items marked with a * are problem areas, and the effect should be minimised wherever possible. The parameters that should normally be measured (although for those marked # this is rare indeed) are as follows:</p>
|
||||
|
||||
<p>¹ Important parameter<br />
|
||||
² Rarely measured
|
||||
|
||||
<ul>
|
||||
<li><b>Input Sensitivity : </b>The signal level required to obtain full power at the amplifier's output. This is determined by the gain and power rating of the amp. A 10W
|
||||
amplifier requires far less gain than a 200W amplifier to obtain full power for the same input voltage. It would be useful if all amplifiers had the same gain regardless of power,
|
||||
but this is not the case. Sensitivities vary widely, ranging from about 500mV up to 1.5V or more.<br /><br /></li>
|
||||
|
||||
<li><b>Total Harmonic Distortion (THD) ¹ :</b> This is a measure of the amount of distortion (modification) of the input signal, which adds additional signal frequencies to the output
|
||||
that are not present in the input signal. THD is commonly measured as a percentage, and can range from 0.001% to 0.1% for typical hi-fi amplifiers. A theoretically perfect
|
||||
amplifier contributes no distortion.<br /><br /></li>
|
||||
|
||||
<li><b>Intermodulation Distortion (IMD) ¹ :</b> The most objectionable form of distortion, where signals 'inter-modulate' to create new frequencies that are not harmonically related
|
||||
to any of the input frequencies (other than by accident). It's tied to THD, and you cannot have one without the other. THD does not predict IMD. Various methods are used
|
||||
to measure IMD, but not all tests will reveal the true extent. This is one of the more difficult tests to perform.<br /><br /></li>
|
||||
|
||||
<li><b>Transient Intermodulation Distortion (TIM) ¹ :</b> Also sometimes called slew induced distortion, this is a form of distortion said to occur when the input signal changes so
|
||||
fast that the output cannot keep up with it. When this happens, feedback ceases to be effective, since the output signal is delayed too long. This remains somewhat contentious,
|
||||
and most modern amplifiers are quite capable of handling the normal programme amplitude and frequency range without difficulty.<br /><br /></li>
|
||||
|
||||
<li><b>Crossover Distortion ¹ ² :</b> A form of distortion caused by the power output devices in a push-pull amplifier operating in Class-AB. This occurs in valve and
|
||||
solid state designs, and is caused by one device switching off as the other takes over for its half of the waveform. There are some designs that claim to eliminate this distortion
|
||||
by never turning off the power devices, but in reality, only Class-A amplifiers have zero crossover distortion. This is generally measured as a part of the THD of an amplifier, and
|
||||
becomes worse as power is reduced from the maximum.<br /><br /></li>
|
||||
|
||||
<li><b>Frequency Response ¹ :</b> The amount of frequency versus amplitude distortion in an amplifier. A perfect amplifier will amplify all signals equally, regardless of
|
||||
frequency. Realistically, an amplifier needs a response of about 5Hz to 50kHz to ensure that all audible signals are catered for with minimal modification.<br /><br /></li>
|
||||
|
||||
<li><b>Phase Response :</b> This indicates the amount of time that the input signal is delayed before reaching the output, based on the signal frequency. Variations in absolute
|
||||
phase are not audible in an amplifier system, but are generally considered undesirable by the hi-fi press. Since it is not difficult to ensure phase linearity, this is not generally
|
||||
a design issue except with valve amplifiers.<br /><br /></li>
|
||||
|
||||
<li><b>Output Power :</b> This is most commonly measured into a non-inductive resistive load. This is not done to improve the figures or disguise any possible shortcomings, but to
|
||||
ensure that measurements are accurate and repeatable. Power should only ever be quoted as 'RMS', which although is not strictly correct, is accepted in the industry, and may be
|
||||
measured into 8 Ohms, or other impedances that the amplifier is capable of driving.<br /><br /></li>
|
||||
|
||||
<li><b>Output Current ² :</b> Not often measured, but sometimes quoted by manufacturers, this represents the maximum current the amplifier can supply into any load. It is rare
|
||||
that any amplifier will be called upon to deliver any current greater than about 3 to 5 times the maximum that the nominal speaker impedance would allow for the amplifier's supply
|
||||
voltage. Greater variations may be possible with some speaker designs, but (IMO) this represents a flaw in the design of the loudspeaker.<br /><br /></li>
|
||||
|
||||
<li><b>Power Bandwidth :</b> This is usually taken as the maximum frequency at which the amplifier can produce 1/2 of its rated output power (this is the -3dB frequency). A 100W
|
||||
amplifier that can produce 50W at 50kHz will be deemed as having a 50kHz power bandwidth.<br /><br /></li>
|
||||
|
||||
<li><b>Slew Rate ² :</b> Closely related to power bandwidth, the slew rate is the maximum rate of change (measured in Volts per microsecond) of the amplifier output. The
|
||||
higher the amplifier power, the higher the slew rate must be to obtain the same power bandwidth.<br /><br /></li>
|
||||
|
||||
<li><b>Open Loop Bandwidth ² :</b> The bandwidth of the amplifier with no AC feedback applied. Very few amplifiers will have an open loop bandwidth greater than a few
|
||||
kilo-Hertz, but valve amps and some solid state designs have a comparatively high open loop bandwidth.<br /><br /></li>
|
||||
|
||||
<li><b>Open Loop Gain ² :</b> Rarely quoted except for DIY amps (and few of them as well), this is the gain of the amplifier without any AC signal feedback. It is not really
|
||||
a helpful parameter for most people, but can be used to determine the ...<br /><br /></li>
|
||||
|
||||
<li><b>Open Loop Distortion ¹ ² :</b> The THD of the amplifier with no feedback applied. This should be as low as possible, but realistically will usually be quite high
|
||||
by normal standards. The open loop distortion is reduced by an amount approximately equal to the feedback ratio.<br /><br /></li>
|
||||
|
||||
<li><b>Open Loop Output Impedance ² :</b> The output impedance of the amplifier with no AC feedback applied. This may range from a few Ohms to 10 or more Ohms, depending on
|
||||
the design of the amplifier. Valve amplifiers will normally have an open loop output impedance of around 0.7 of the designed speaker impedance.<br /><br /></li>
|
||||
|
||||
<li><b>Feedback Ratio ² :</b> How much of the open loop gain is fed back to the amplifier's input to obtain the sensitivity figure quoted for the amp. For example if an
|
||||
amplifier has an open loop gain of 100dB, and a gain of 20dB, then the feedback ratio is 80dB. The application of feedback will ...<br /><br />
|
||||
<ul>
|
||||
<li>Increase bandwidth</li>
|
||||
<li>Reduce phase shift</li>
|
||||
<li>Reduce distortion</li>
|
||||
<li>Reduce output impedance<br /><br /></li>
|
||||
</ul></li>
|
||||
|
||||
<li><b>Output Impedance ¹ :</b> This is the actual output impedance of the amplifier, and has no bearing on the amount of current that can be supplied by the output stage.
|
||||
Valve amplifiers usually have a relatively high output impedance (typically 1 to 6 Ohms), while solid state amps will normally have an output impedance of a fraction of an Ohm.
|
||||
By use of feedback, it is possible to increase output impedance (> 200 Ohms is quite easy), or it can be made negative. Negative impedance has been tried by many designers
|
||||
(including the author), but has never gained popularity - possibly because most speakers react very poorly to negative impedances and tend to sound awful.</li>
|
||||
</ul>
|
||||
|
||||
<p>Every amplifier design on the planet has the same set of constraints, and will exhibit all of the above problems to some degree. The only exception is a Class-A amplifier, which does not have crossover distortion, but is still limited by all other parameters.</p>
|
||||
|
||||
<p>The difficulty is determining just how much of any of the problem items is tolerable, and under what conditions. For example, there are many single ended triode valve designs which have very high distortion figures (comparatively speaking), high output impedance and low output current capability. There are many audio enthusiasts who claim that these sound superior to all other amplifiers, so does this mean that the parameters where they perform badly (or at least not as well as other amps) can be considered unimportant? Not at all!</p>
|
||||
|
||||
<p>If a conventional (i.e. not Class-A) solid state amplifier gave similar figures, it would be considered terrible, and would undoubtedly sound dreadful.</p>
|
||||
|
||||
|
||||
<hr />Although all the issues described above are separate in their own right, many can be lumped together under a single general category ....
|
||||
|
||||
|
||||
<p><a id="s3"></a><b>3.0 Distortion</b>
|
||||
<br />Technically, distortion is any change that takes place to a signal as it travels from source to destination. If some of the signal 'goes missing', this is distortion just as much as when additional harmonics are generated.</p>
|
||||
|
||||
<p>We tend to classify distortion in different ways - the imperfect frequency response of an amplifier is not generally referred to as distortion, but it is. Instead, we talk about frequency response, phase shift, and various other parameters, but in reality they are all a form of distortion.</p>
|
||||
|
||||
<p>The bottom line is that amplifiers all suffer from some degree of distortion, but if two amplifiers were to be compared that had no distortion at all, they must (by definition) be identical in both measured and perceived sound.</p>
|
||||
|
||||
<p>Naturally, there is no such thing as a perfect amplifier, but there are quite a few that come perilously close, at least within the audible frequency range. What I shall attempt to do is look at the differences that do exist, and try to determine what effect these differences have on the perceived 'sonic quality' of different amplifiers. I will not be the first to try to unravel this mystery, and I doubt that I will be the last. I also doubt that I will succeed, in the sense that success in this particular area would only be achieved if everyone agreed that I was right - and of that there is not a chance! (However, one lives in hope.)</p>
|
||||
|
||||
<p>In this article I use the somewhat outdated term 'solid state' to differentiate between valve amps, and those built using bipolar transistors, MOSFETs or other non-vacuum tube devices.</p>
|
||||
|
||||
<p>I have also introduced a new (?) test method, which I have called a SIM (Sound Impairment Monitor), the general concept of which is described in the appendix to this article.</p>
|
||||
|
||||
|
||||
<hr /><a id="s31"></a><b>3.1 Clipping Distortion</b>
|
||||
<p>How can one amplifier's clipping distortion sound different from that of another? Most of the hi-fi fraternity will tend to think that clipping is undesirable in any form at any time. While this is undeniably true, many of the amps used in a typical high end setup will be found to be clipping during normal programme sessions. I'm not referring to gross overload - this is quite unmistakable and invariably sounds awful - regardless of the amplifier.</p>
|
||||
|
||||
<p>There are subtle differences between the way amplifiers clip, that can make a great impact on the sound. Valve amps are the most respectable of all, having a 'soft' clipping characteristic which is comparatively unobtrusive. However, this comes at a cost. While distortion can be very low at low levels, with low feedback valve amp designs, the distortion rises as level increases. The change from 'unclipped' to 'clipped' may be less abrupt, but the distortion just before clipping can be surprisingly high. Low feedback Class-A amplifiers are next, with slightly more 'edge', but otherwise are usually free from any really nasty additions to the overall sound.</p>
|
||||
|
||||
<p>Then there are the myriad of Class-AB discrete amps. Most of these (but by no means all) are reasonably well behaved, and while the clipping is 'hard' it does not have significant overhang - this is to say that once the output signal is lower than the supply voltage again it just carries on as normal. This is the ideal case - when any amp clips, it should add no more nastiness to the sound than is absolutely necessary. Clipping refers to the fact that when the instantaneous value of output signal attempts to exceed the amplifier's power supply voltage, it simply stops, because it cannot be greater than the supply. We know it must stop, but what is of interest is how it stops, and what the amplifier does in the brief period during and immediately after the clipping has occurred.</p>
|
||||
|
||||
<p class="t-pic"><img src="as-f01.gif" alt="Figure 1" border="1"><br />Figure 1 - Comparison of Basic Clipping Waveforms</p>
|
||||
|
||||
<p>In Figure 1, you can see the different clipping waveforms I am referring to, with 'A' being representative of typical push-pull valve amps, 'B' is the waveform from a conventional discrete Class-AB solid state amp, and 'C' shows the overhang that is typical of some IC power amps as well as quite a few discrete designs. This is a most insidious behaviour for an amp, because while the supply is 'stuck' to the power rail, any signal that might have been present in the programme material is lost, and a 100Hz (or 120Hz) component is added if the clipping + 'stuck to rail' period lasts long enough. This comes from the power supply, and is only avoidable by using a regulated supply or batteries. Neither of these is cheap to implement, and they are rarely found in amplifier designs.</p>
|
||||
|
||||
<p>Although Figure 1 shows the signal as a sinewave for ease of identification, in a real music signal it will be a sharp transient that will clip, and if the amp behaves itself, this will be (or should be) more or less inaudible. Should it stick to the supply rail, the resulting description of the effect is unlikely to accurately describe the actual problem, but describe what it has done to the sound - from that listener's perspective. A simple clipped transient should not be audible in isolation, but will have an overall effect on the sound quality. Again, the description of this is unlikely to indicate that the amp was clipping, and regrettably few amps have clipping indicators so most of the time we simply don't know it is happening.</p>
|
||||
|
||||
<p>To be able to visualise the real effect of clipping, we need to see a section of 'real' signal waveform, with the lowest and highest signal frequencies present at the same time. If the amp is clipped because of a bass transient (this is the most common), the period of the waveform is long. even if the signal is clipped for only 5 milliseconds, this means that 5 complete cycles of any signal at 1000Hz are removed completely, or 50 complete cycles at 10kHz. This represents a significant loss of intended information, which is replaced by a series of harmonics of the clipped frequency (if clipping lasts for long enough), or more typically a series of harmonics that is not especially related to anything (musically speaking - all harmonics are related to something, but this is not necessarily musical!)</p>
|
||||
|
||||
<p>I think that no review of any amplifier should ever be performed without some method of indicating that the amp is clipping (or is subject to some other form of signal impairment), and this can be added to the reviewer's notes - along the lines of ...</p>
|
||||
|
||||
<blockquote>
|
||||
"This amplifier was flawless when kept below clipping (or as long as the SIM (or other signal integrity monitoring facility) failed to show any noticeable impairment), but even the smallest
|
||||
amount of overload caused the amp to sound very hard. Transparency was completely lost, imaging was ruined, and it created listener fatigue very quickly."
|
||||
</blockquote>
|
||||
|
||||
<p>Now, wouldn't that be cool? Instead of us being unaware (as was the reviewer in many cases) that the amp in review was being overdriven - however slightly - we now (all of us) have that missing piece of information that is not included at the moment. I have never seen a review of an amp where the output was monitored with an accurate clipping indicator to ensure that the reviewer was not listening to a signal that was undistorted. I'm not saying that no-one does this, just none that I have read.</p>
|
||||
|
||||
<p>The next type of overload behaviour is dramatically worse, and I have seen this in various amps over the years. Most commonly associated with overload protection circuits, the sound is gross. I do not know the exact mechanism that allows this to happen, but it can be surmised that the protection system has 'hysteresis', a term that is more commonly associated with thermal controllers, steel transformer laminations and Schmitt trigger devices. Basically, a circuit with hysteresis will operate once a certain trigger point is reached, but will not reset until the input signal has fallen below a threshold that is lower than the trigger point. The typical waveform of an amplifier with this problem is shown in Figure 2, and I believe it IS a problem, and should be checked for as a normal part of the test process. This type of overload characteristic is not desirable in any way, shape or form.</p>
|
||||
|
||||
<p class="t-pic"><img src="as-f02.gif" alt="Figure 2" border="1"><br />Figure 2 - Hysteresis Overload Waveform</p>
|
||||
|
||||
<p>In this case, the additional harmonic components added to the original sound will be more prominent than with 'normal' clipping. As before, I cannot even begin to imagine how the sound might be described - all the more reason to ensure that testing includes informing the reader if the amp was clipping or not during the listening tests. The loss of signal with this type of distortion will generally be much greater than simple clipping, and the added harmonic content will be much more pronounced, especially in the upper frequencies.</p>
|
||||
|
||||
<p><b>Clipping Synopsis</b>
|
||||
<br />Tests conducted as a part of any review would be far more revealing if the clipping waveform were shown as a matter of course. After some learning on our behalf, we would get to know what various of the hi-fi press meant when they described the sound while the amp was clipping, versus not clipping, or what the amp sounded like when its overload protection circuits came into action.</p>
|
||||
|
||||
<p>To this end I have designed a new distortion indicator circuit, which not only indicates clipping, but will show when the amp is producing distortion of any kind beyond an acceptable level. One version has been published as a project, and I have chosen the acronym SIM (Signal/ Sound Impairment Monitor) for this circuit.</p>
|
||||
|
||||
<p>The SIM will react to any form of signal modification, and this includes phase distortion and frequency response distortion. I do not believe that this approach has been used before in this way. It is not an uncommon method for distortion measurement, but has not been seen anywhere as a visual indicator for identifying problem areas that an amp may show in use. This circuit will also show when an amplifier's protection circuit has come into effect.</p>
|
||||
|
||||
<p>Although the detector has no idea what type of problem is indicated, it does indicate when the input and output signals no longer match each other - for whatever reason. Oscilloscope analysis would be very useful using this circuit, as with a little practice we would be able to identify many of the currently unknown effects of various amplifier aberrations. Any amp behaviour that results in the input and output signals being unequal (within a few millivolts at most) indicates that something is wrong.</p>
|
||||
|
||||
|
||||
<hr /><a id="s32"></a><b>3.2 Crossover Distortion</b>
|
||||
<p>Class-A amplifiers have no crossover distortion at all, because they maintain conduction in the output device(s) for the entire waveform cycle and never turn off. Class-A is specifically excluded from this section for that reason.</p>
|
||||
|
||||
<p>For the rest, a similar question as the one before - how can one amplifier's crossover distortion sound different from another? Surely if there is crossover distortion it will sound much the same? Not so at all. Again, valve amplifiers are much better in this area than solid state amps (at least in open loop conditions). When valves cross over from one output device to the next (standard push-pull circuit is assumed), the harmonic structure is comprised of mainly low order odd harmonics. There will be some 3rd harmonics, a smaller amount of 5th, and so on.</p>
|
||||
|
||||
<p>Solid state amps tend to create high order odd harmonics, so there will be the 3rd harmonic, only a tiny bit less of the 5th harmonic, and the harmonics will extend across the full audio bandwidth. Transistor and MOSFET amps have very high open loop gains, and use feedback to reduce distortion. In all cases, the crossover distortion is caused because the power output devices are non-linear. At the low currents at which the changeover occurs, these non-linearities are worse, as well, the devices usually have a lower gain at these currents.</p>
|
||||
|
||||
<p>This has two effects. The open loop gain of the amplifier is reduced because of the lower output device gain, so there is less negative feedback where it is most needed. Secondly, the feedback tries to compensate for the lower gain (and tries to eliminate the crossover distortion), but is limited by the overall speed of the internal circuitry of the amplifier. This results in sharp transitions in the crossover region, and any sharp transition means high order harmonics are produced (however small they might be).</p>
|
||||
|
||||
<p>One method to minimise this is to increase the quiescent (no signal) current in the output transistors. With a linear output stage in a well designed circuit, crossover distortion should be all but non-existent with any current above about 50 to 100mA (but note that if the quiescent current is increased too far, overall distortion may actually get <i>worse</i>). Figure 3 shows the crossover distortion (at the centre of the red trace) and the residue as seen on an oscilloscope (green trace, amplified by 10 for clarity) - this is the typical output from a distortion meter, with an amplifier that has noticeable crossover distortion. If measured properly, the distortion is highly visible, even though it may be barely audible. Note that the waveform below would not qualify for the last statement - this amount of crossover distortion would be very audible indeed.</p>
|
||||
|
||||
<p class="t-pic"><img src="as-f03.gif" alt="Figure 3" border="1"><br />Figure 3 - Crossover Distortion Waveform</p>
|
||||
|
||||
<p>If THD is quoted without reference to its harmonic content, then it is quite possible that two amplifiers may indicate identical distortion figures, but one will sound much worse than the other. Distortion at a level of 1W should always be quoted, and the waveform shown. Once the waveform can be seen, it is easy to determine whether it will sound acceptable or dreadful - before we even listen to the amp. Listening tests will confirm the measured results with great accuracy, although the descriptive terms used will vary, and may not indicate the real problem.</p>
|
||||
|
||||
|
||||
<p><b>Crossover Distortion Synopsis</b>
|
||||
<br />Although this is one area where modern amplifiers rarely perform badly, it is still important, and should be measured and described with more care than is usually the case. While few amplifiers will show up badly in this test now, crossover distortion was one of the main culprits that gave solid state a bad name when transistors were first used in amplifiers.</p>
|
||||
|
||||
<p>I do not believe that we can simply ignore crossover distortion on the basis that "everyone knows how to fix it, and it is not a problem any more". I would suggest that it is still a real problem, only the magnitude has been reduced - the problem is still alive and well. Will you be able to hear it with most good quality amp? Almost certainly not.</p>
|
||||
|
||||
|
||||
<hr /><a id="s33"></a><b>3.3 Frequency And Phase Distortion</b>
|
||||
<p>Distortion of the frequency response should not be an issue with modern amplifiers, but with some (such as single ended triode valve designs), it does pose some problems. The effect is that not all frequencies are amplified equally, and the first to go are the extremes at both ends of the spectrum. It is uncommon for solid state amps to have a frequency response at low powers that extends to anything less than the full bandwidth from 20Hz to 20kHz. This is not the case with some of the simple designs, and single ended triode (SET) Class-A - as well as inductance loaded solid state Class-A amps - will often have a less than ideal response.</p>
|
||||
|
||||
<p>I would expect any amplifier today should be no more than 0.5dB down at 20Hz and 20kHz, referred to the mid-band frequency (usually taken as 1kHz, but is actually about 905Hz). (My preferred test frequency is 440Hz (concert pitch A, below middle C), but none of this is of great consequence.) 0.5dB loss is acceptable in that it is basically inaudible, but most amps will do much better than this, with virtually no droop in the response from 10Hz to over 50kHz.</p>
|
||||
|
||||
<p>For reference, the octaves included for 'normal' sound are:</p>
|
||||
|
||||
<blockquote>
|
||||
20 40 80 160 320 640 1,280 2,560 5,120 10,240 20,480 (all in Hertz)
|
||||
</blockquote>
|
||||
|
||||
<p>To determine the halfway point between two frequencies one octave apart, we multiply the lower frequency by the square root of 2 (1.414). The halfway point is between 650 and 1280Hz, or 904.96Hz. You must be so pleased to have been provided with this piece of completely useless information! Just think yourselves lucky that I didn't tell you how to calculate the distance between the frets on a guitar. <img src="bgrin.gif" alt="" /></p>
|
||||
|
||||
<p>Most amplifiers will manage well beyond the range necessary for accurate reproduction, at all power levels required to cater for the requirements of music. So why are some amps described as having poor rendition of the high frequencies? They may be described as 'veiled' or something similar, but there is no measurement that can be applied to reveal this when an amplifier is tested. Interestingly, some of the simpler amplifiers (again, such as the single ended triode amps) have poorer response than most of the solid state designs, yet will regularly be described as having highs that 'sparkle', and are 'transparent'.</p>
|
||||
|
||||
<p>These terms are not immediately translatable, since they are subjective, and there is no known measurement that reveals this quality. We must try to determine what measurable effect might cause such a phenomenon. There are few real clues, since amplifiers that should not be classified as exceptional in this area are often described as such. Other amps may be similarly described, and these will not have the distortion of a single ended triode and will have a far better response.</p>
|
||||
|
||||
<p>We can (almost) rule out distortion as a factor in this equation, since amps with comparatively high distortion can be comparable to others with negligible distortion. Phase shift is also out of the question, since amps with a lot of phase shift can be favourably compared to others with virtually none. One major difference is that typical SET amplifiers have quite high levels of low order even harmonics. Although these will give the sound a unique character, I doubt that this is the sole reason for the perceived high frequency performance - I could also be wrong.</p>
|
||||
|
||||
<p>Phase distortion occurs in many amplifiers, and is worst in designs using an output transformer or inductor (sometimes called a choke). The effect is that some frequencies are effectively delayed by a small amount. This delay is usually less than that caused by moving one's head closer to the loudspeakers by a few millimetres. It is generally thought to be inaudible, and tests that I (and many others) have conducted seem to bear this out.</p>
|
||||
|
||||
|
||||
<p><b>Frequency And Phase Distortion - Synopsis</b>
|
||||
<br />There must be some mechanism that causes multiple reviewers to describe an amplifier as having a poor high frequency performance, such as (for example) a lack of transparency. There are few real clues that allow us to determine exactly what is happening to cause these reviewers to describe the sound of the amp in such terms, and one may be tempted to put it all down to imagination or 'experimenter expectancy'. This is likely to be a mistake, and regardless of what we might think about reviewers as a species, they do get to listen to many more amplifiers than most of us.</p>
|
||||
|
||||
<p>One of the few variables is a phenomenon called slew rate. This is discussed fully in the next section.</p>
|
||||
|
||||
|
||||
<hr /><a id="s34"></a><b>3.4 Slew Rate Distortion</b>
|
||||
<p>This has always been somewhat controversial, but no-one has ever been able to confirm satisfactorily that slew rate (within certain sensible limits) has any real effect on the sound. Figure 4 is a nomograph that shows the required slew rate for any given power output to allow full power at any frequency. To use it, determine the power and calculate the peak voltage, and place the edge of a ruler at that voltage level. Tilt the ruler until the edge also aligns with the maximum full power frequency on the top scale. The slew rate is indicated on the bottom scale.</p>
|
||||
|
||||
<p>For example, if the peak voltage is 50V (a 150W/8 ohm amp) and you expect full power to 20kHz, the required slew rate is 6V/µs. Bear in mind that no amplifier is <i>ever</i> expected to provide full power at 20kHz, and if it did the tweeters would fail very quickly.</p>
|
||||
|
||||
<p class="t-pic"><img src="slewrate.gif" alt="Figure 4" border="1"><br />Figure 4 - Slew Rate Nomograph</p>
|
||||
|
||||
<p>Slew rate distortion is caused when a signal frequency and amplitude is such that the amplifier is unable to reproduce the signal as a sine wave. Instead, the input sine wave is 'converted' into a triangle wave by the amplifier. This is shown in Figure 5, and is indicative of this behaviour in any amplifier with a limited slew rate. The basic problem is caused by the 'dominant pole' filter included in most amplifiers to maintain stability and prevent high frequency oscillation. While very few amplifiers even come close to slew rate induced distortion (AKA Transient Intermodulation Distortion) with a normal signal, this is one of the very few possibilities left to explain why some amps seem to have a less than enthusiastic response from the reviewers' perspective.</p>
|
||||
|
||||
<p>If you don't like the nomograph, you can calculate the maximum slewrate if a sinewave easily. The formula is ...</p>
|
||||
|
||||
<blockquote>
|
||||
SR = 2<span class="times">π</span> × f × V<small>p</small><br />
|
||||
Where SR is slewrate in V/s and V<small>p</small> is the <i>peak</i> voltage of the sinewave (V<small>RMS</small> × 1.414)
|
||||
</blockquote>
|
||||
|
||||
<p>For example, 20kHz at 28V RMS (100W/ 8 ohms) requires a slewrate of ...</p>
|
||||
|
||||
<blockquote>
|
||||
SR = 2<span class="times">π</span> × 20,000 × 40<br />
|
||||
SR = 5,026,548 V/s = 5.03V/µs
|
||||
</blockquote>
|
||||
|
||||
<p>We already know <i>absolutely</i> that no music source will ever provide a full power signal at 20kHz, but to allow it the amp needs a slewrate of 5V/µs (close enough). Should someone claim that you need 100V/µs or better, that their amp can do just that and you'll miss out on much of your music, then you know that the claims are fallacious. Having a higher slewrate than strictly necessary does no harm, provided that the design's stability hasn't been compromised to achieve the claimed figure. All design is the art of compromise, and some compromises can be a giant leap backwards if the designer concentrates on one issue and ignores others. I happen to think that stability is <i>extremely</i> important - no amp should oscillate when operated normally into any likely speaker load ... ever!</p>
|
||||
|
||||
<p class="t-pic"><img src="as-f05.gif" alt="Figure 5" border="1"><br />Figure 5 - Slew Rate Limiting In An Amplifier</p>
|
||||
|
||||
<p>The red trace shows the amp operating normally, and the green trace shows what happens if the slew rate is deliberately reduced. Is this the answer, then? I wish it were, since we could all sleep soundly knowing exactly what caused one amp to sound the way it did, compared to another, which should have sounded almost identical.</p>
|
||||
|
||||
<p>A further test is to apply a low frequency square wave at about half to 3/4 power, mixed with a low-level high frequency sinewave to the amplifier. At the transitions of the squarewave, the sinewave should simply move up and down - 'riding' the squarewave. If there is any misbehaviour in the amp, the sinewave may be seen to be compressed so its shape will change, or a few cycles may even go missing entirely. Either is unacceptable, and should not occur.</p>
|
||||
|
||||
<p>This is an extremely savage test, but most amplifiers should be able to cope with it quite well. Those that don't will modify the music signal in an unacceptable way in extreme cases (which this test simulates). Again, this is an uncommon test to perform, but may be quite revealing of differences between amps.</p>
|
||||
|
||||
<p><b>Frequency And Slew Rate Distortion - Synopsis</b>
|
||||
<br />We need to delve deeper, and although there seems to be little (if any) useful evidence we can use to explain this particular problem, there is an answer, and it therefore possible to measure the mechanism that causes the problem to exist.</p>
|
||||
|
||||
|
||||
<hr /><a id="s4"></a><b>4.0 Open Loop Response</b>
|
||||
<p>The performance of a feedback amplifier is determined by two primary factors. These are</p>
|
||||
|
||||
<ul>
|
||||
<li>Open loop performance</li>
|
||||
<li>Feedback ratio</li>
|
||||
</ul>
|
||||
|
||||
<p>If the amp has a poor open loop gain and high distortion, then sensible amounts of feedback will not be able to correct the deficiencies, because there is not sufficient gain reserve. By the time the performance is acceptable, it may mean that the amplifier has unity gain, and is now impossible to drive with any normal preamp.</p>
|
||||
|
||||
<p>Many amplifiers have a very high open loop gain, but may have a restricted frequency response. Let's assume an amp that has a gain of 100dB at 20Hz, and 40dB gain at 20kHz. If we want 30dB of overall gain (which is about standard), then there is 70dB of feedback at 20Hz, but only 10dB at 20kHz. As a very rough calculation, distortion and output impedance are reduced by the feedback ratio, so if open loop distortion were 3% (not an unreasonable figure), then at 20Hz, this is reduced to 0.0015%, but will be only just under 1% at 20kHz.</p>
|
||||
|
||||
<p>Because these figures are so rarely quoted (and I must admit, I have not really measured all the characteristics of the 60W amp in Project 03 - open loop measurements are difficult to make accurately), we have no idea if amplifiers with poor open loop responses are responsible for so many of the failings we hear about. It is logical to assume that there must be some correlation, but we don't really know for sure.</p>
|
||||
|
||||
<p>Ideally, an amplifier should have wide bandwidth and low distortion before global feedback is applied, which will just make a good amp better. Or will it? I have read reviews where a very simple amp was deemed one of the best around (this was quite a few years ago), and I was astonished when I finally saw the circuit - it was almost identical to the 'El Cheapo' amplifier (see the projects pages for more info on this amp).</p>
|
||||
|
||||
<p>The only major difference between this amp and most of the others at the time was the comparatively low open loop gain, and a somewhat wider bandwidth than was typical at the time, because it does not need a Miller capacitor for stability. So the amp was better in one respect, worse in another.</p>
|
||||
|
||||
<p>In the end, it doesn't really matter what the open loop response is like, as long as closed loop (i.e. with feedback applied) performance does not degrade the sound. Again, we have the same quandary as before - unless we can monitor the difference between input and output at all levels and with normal signal applied, we really don't know what is going on. The usual tests are useful, but cannot predict how an amp will sound. I have heard countless stories about amps that measure up extremely well, but sound 'hard and dry', and have no 'music' in them.</p>
|
||||
|
||||
<p>Unless these measurements are made (or at least some modified form), we will still be no further in understanding why so many people prefer one brand of amp over another (other than peer pressure or advertising hype).</p>
|
||||
|
||||
<p>One possibility is to measure the amp with a gain of 40dB. This is an easy enough modification to make for testing, and the performance is far easier to measure than if we attempt open loop testing. The difference between measured performance at 30dB gain (about 32) versus 40dB (100) would be an excellent indicator of the amp's performance, and it is not too hard to predict the approximate open loop response from the different measurements. To be able to do this requires that all measurements be very accurate.</p>
|
||||
|
||||
<p>Would these results have any correlation with the review results? We will never know if someone doesn't try it - work the techniques discussed here thoroughly, with a number of different amps. It would be useful to ensure that the reviewer was unaware of the test results before listening, to guard against experimenter expectancy or sub-conscious prejudice.</p>
|
||||
|
||||
<p>It is very hard to do a synopsis of this topic, since I have too little data to work with. Only by adopting new ideas and test methods will we be able to determine if the 'golden-ear' brigade really does have golden ears, or that they actually hear much the same 'stuff' as the rest of us, but have a better vocabulary. That is not intended as a slur, just a comment that we have to find out if there is anything happening that we (the 'engineering' types) don't know about, or not. Unless we can get a match between measured and described performance, we get nowhere (which is to say that we stay where we are, on opposite sides of the fence).</p>
|
||||
|
||||
|
||||
<hr /><a id="s5"></a><b>5.0 Speaker - Amplifier Interface</b>
|
||||
<p>Many is the claim that the ear is one of the most finely tuned and sensitive measuring instrument known. I am not going to dispute this - not so that I will not offend anyone (I seem to have done this many times already), but because in some respects it is true. Having said that, I must also point out that although extremely sensitive, the ear (or to be more correct, the brain) is also easily fooled. We can imagine that we can hear things that absolutely do not exist, and can just as easily imagine that one amplifier sounds better than another, only to discover that the reverse is true under different circumstances. Listeners have even declared one amp to be clearly superior to another when the amp hasn't been changed at all.</p>
|
||||
|
||||
<p>Could it be the influence of speaker cables, or even loudspeakers themselves? This is quite possible, since when amps are reviewed it is generally with the reviewer's favourite speaker and lead combination. This might suit one amplifier perfectly, while the capacitance and inductance of the cable might cause minute instabilities in other otherwise perfectly good amplifiers. Although it a fine theory to suggest that a speaker lead should not affect the performance of a well designed amplifier, there are likely to be some combinations of cable characteristics that simply freak out some amps. Likewise, some amps just might not like the impedance presented by some loudspeakers - this is an area that has been the subject of many studies, and entire amplifiers have been designed specifically to combat these very problems <sup>[<a href="amp-sound.htm#ref"> 1 </a>]</sup>.</p>
|
||||
|
||||
<p>Many published designs never get the chance of a review, at least not in the same sense as a manufactured amplifier, so it can be difficult (if not impossible) to make worthwhile comparisons. In addition, we sometimes have different reviewers making contradictory remarks about the same amp. Some might think it is wonderful, while others are less enthusiastic. Is this because of different speakers, cables, or some other influence? The answer (of course) is that we have no idea.</p>
|
||||
|
||||
<p>We come back to the same problem I described earlier, which is that the standard tests are not necessarily appropriate. A frequency response graph showing that an amp is ruler flat from DC to daylight is of absolutely no use if everyone says that the highs are 'veiled', or that imaging is poor. Compare this with another amp that is also ruler flat, and (nearly) everyone agrees that the highs are detailed, transparent, and that imaging is superb.</p>
|
||||
|
||||
<p>We need to employ different testing methodologies to see if there is a way to determine from bench (i.e. objective) testing, what a listening (i.e. subjective) test might reveal. This is a daunting task, but is one that must be sought vigorously if we are to learn the secrets of amplifier sound. It is there - we just don't know where to look, or what to look for ... yet. Until we have correlation between the two testing methods, we are at the mercy of the purveyors of amplifier snake oil and other magic potions.</p>
|
||||
|
||||
<p>The SIM distortion indicator is one possible method that might help us, but it may also react to the wrong stimulus. Perhaps we need to add the ability to detect small amounts of high frequencies with greater sensitivity, but now a simple idea becomes quite complex, possibly to no avail. It is also important that such a device has zero effect on the incoming signal itself, so some care is needed to ensure that there is negligible loading on the source preamplifier.</p>
|
||||
|
||||
<p>This is not the only avenue open to us to correlate subjective versus objective testing. Both are important, the problem is that one is purely concerned with the way an amplifier behaves on the test bench, and a whole series of more or less identical results can be expected. The other is veiled in 'reviewer speak', and although it might be useful if the reviewer is known and trusted, is not measurable or repeatable. The whole object is to try to determine what physical factors cause amplifiers to sound different, despite that fact that conventional testing indicates that they should sound the same.</p>
|
||||
|
||||
|
||||
<hr /><a id="s6"></a><b>6.0 Impedance</b>
|
||||
<p>The output impedance of any amplifier is finite. There is no such thing as an amplifier with zero output impedance, so all amps are influenced to some degree by the load. An ideal load is perfectly resistive, and has no reactive elements (inductance or capacitance) at all. Just as there is no such thing as a perfect amplifier, there is also no such thing as a perfect load. Speakers are especially gruesome in this respect, having significant reactance, which varies with frequency.</p>
|
||||
|
||||
<p>A genuine zero impedance source is completely unaffected by the load, and it does not matter if it is reactive or not. If such a source were to be connected to a loudspeaker load, the influence of the load will be zero, regardless of frequency, load impedance variations, or anything else. It's worth mentioning that by clever manipulation of feedback, it <i>is</i> (theoretically) possible to achieve zero output impedance (and even negative impedance which I have done in a test amp I use in my workshop). The problem is that doing so involves a small amount of <i>positive</i> feedback, which is inherently unstable. All amps normally have a low but measurable <i>positive</i> (i.e. 'normal') output impedance, but it's possible that internal wiring can be mis-routed such that an amplifier does have a small amount of <i>negative</i> impedance. Poor grounding practices can achieve this, and it's definitely not something to aim for!</p>
|
||||
|
||||
<p>Since true zero impedance is not the case in the real world, the goal is generally to make the amplifier have the lowest output impedance possible (but remaining positive at all times), in the somewhat futile hope that the amp will not be adversely affected by the variable load impedance. In essence, this is futile, since there will always be some output impedance, and therefore the load will always have some influence on the behaviour of the amp.</p>
|
||||
|
||||
<p>Another approach might be to make the output impedance infinite, and again, the load will have zero effect on the amplifier itself (the amplifier will, however, have a great influence on the load!). Alas, this too is impossible. Given that the conventional approaches obviously cannot work, we are faced with the problem that all amplifiers are affected by the load, and therefore all amplifiers must show some degree of sensitivity to the speaker lead and speaker.</p>
|
||||
|
||||
<p>The biggest problem is that no-one really knows what an amplifier will do when a reactive load reflects some of the power back into the amp's output. We can hope (without success) that the effects will be negligible, or we can try to make speakers appear as pure resistance (again, without success).</p>
|
||||
|
||||
<p>A test method already exists for this, and uses one channel of an amp to drive a signal back into the output of another. The passive amplifier is the one under test. It is also possible to use a different source amplifier altogether, since there is no need for it to be identical to the test amp. Use of a 'standard' amplifier whose characteristics are well known is useful, since the source will be a constant in all tests. Differences may then be seen clearly from one test to the next.</p>
|
||||
|
||||
<p>The method is shown in Figure 6, and is a useful test of the behaviour of an amp when a signal is driven into its output. This is exactly what speakers do - the reactive part of the loudspeaker impedance causes some of the power to be 'reflected' back into the amplifier. Since one amplifier in this test is the source, the device under test can be considered a 'sink'.</p>
|
||||
|
||||
<p class="t-pic"><img src="as-f06.gif" alt="Figure 6" border="1" /><br />Figure 6 - Amplifier Power Sink Test</p>
|
||||
|
||||
<p>I have used this test, and although it does show some interesting results, the test is essentially not useful unless used as a comparative test method. The amplifier under test is also subjected to very high dissipation (well above that expected with any loudspeaker load), because the transistors are expected to 'dump' a possibly large current while they have the full rail voltage across them. There is a real risk of damaging the amplifier, and I suggest that you don't try this unless you are very sure of the driven amplifier's abilities.</p>
|
||||
|
||||
<p>We may now ask "Why is this not a standard test for amplifiers, then?" The answer is that no-one has really thought about it enough to decide that this will (or should) be part of the standard set of tests for objective testing of an amplifier. The results might be quite revealing, showing a signal that may be non-linear (i.e. distorted), or perhaps showing a wide variation in measured signal versus frequency. The result of this test with amps having extensive protection circuits will be a lottery - most will react (often very) badly at only moderate current.</p>
|
||||
|
||||
<p>If there is high distortion or a large frequency dependence, then we have some more information about the amplifier that was previously unknown. It might be possible to correlate this with subjective assessments of the amp, and gain further understanding of why some amps supposedly sound better than others. We might discover that amps with certain characteristics using this test are subjectively judged as sounding better than others ... or not.</p>
|
||||
|
||||
<p>If this test became standard, and was routinely allied with the SIM tester described above, we may become aware of many of the problems that currently are (apparently and/or allegedly) audible, but for which there is no known measurement technique.</p>
|
||||
|
||||
|
||||
<hr /><a id="outro"></a><b>Conclusions</b>
|
||||
<p>This article has described some tests that although not new, are possibly the answer to so many questions we have about amplifiers. The tests themselves have been known for some time, but their application is potentially of benefit. We may be able to finally perform an objective test, and be able to predict with a degree of confidence how the amp will sound. It may also happen that these tests are not sufficient to reveal all the subtleties of amplifier sound, but will certainly be more useful than a simple frequency response and distortion test.</p>
|
||||
|
||||
<p>Any change to the testing methods used is not going to happen overnight, and nor are we going to be able to see immediately which problems cause a difference, and which ones have little apparent effect. Time, patience, and careful correlation of the data are essential if this is to succeed. There are laws of physics, and there are ears. Somewhere the two must meet in common ground. We already know that this happens, since there are amplifiers that sound excellent - according to a large number of owners, reviewers, etc. - now we need to know why.</p>
|
||||
|
||||
<p>There is a test method (or a series of methods) that will allow us to obtain a suite of tests that makes sense to designers and listeners alike, so we can get closer to the ideal amplifier, namely the mythical 'straight wire with gain', but from the listener perspective rather than the senseless repetition of tests that seem to have no bearing on the perceived quality of the amp. This is not to say that the standard tests are redundant (far from it), but they do not seem to reveal enough information.</p>
|
||||
|
||||
<p>For this to succeed, the subjectivists must be convinced, as must the 'objectivists'. We are all looking for the same thing - the flawless reproduction of sound - but the two camps have drifted further and further apart over the years. This is not helped by the common practice of reviewers to connect everything up themselves, and rely not just on the sound, but their knowledge of which amplifier they are listening to. Sighted tests are <i>invariably</i> flawed, and the only test methodology that should ever be used is a full blind or double-blind test, with the ability to switch from one amplifier to the other, but <i>without</i> knowing which is which.</p>
|
||||
|
||||
<p>These are my musings, and I am open to suggestions for other testing methods that may reveal the subtle differences that undeniably exist between amplifiers. At the moment we have a chasm between those who can (or think they can) hear the difference between a valve and an opamp, a bipolar junction transistor and a MOSFET, or Brand 'A' versus Brand 'B', and those who claim that there is no difference at all.</p>
|
||||
|
||||
<p>The fact that there are differences is obvious. The degree of difference and why there are differences is not. It would be nice for all lovers of music (and the accurate reproduction of same) if we can arrive at a mutually agreeable explanation for these differences, that is accurate, repeatable, and measurable.</p>
|
||||
|
||||
<p>If these criteria are not met, then the assessment is not useful to either camp, and the chasm will simply widen. This is bad news - it is high time we all get together and stop arguing amongst ourselves whether (for example) it is better to use one brand of capacitor in the signal path or another. The continued use of sighted test procedures does <i>nothing</i> to advance the state of the art.</p>
|
||||
|
||||
<p>These testing methods can also be applied to the measurement of individual components, speaker cables, interconnects and preamps, particularly the SIM tester. Using the amplifier power sink test with different cables and speakers might give us some clues as to why so many people are adamant that one speaker cable sounds better than another, even though there is no measurable difference using conventional means.</p>
|
||||
|
||||
<p>The greatest benefit of these tests is that they will reveal things we have not been looking at (or for) in the past, and may show differences that come as a very great surprise to designers and listeners alike.</p>
|
||||
|
||||
<p>Another very useful test is a 'null test', as proposed by Ethan Winer. For example, a signal is applied simultaneously to two leads. and the tester adjusted until there is nothing left of the original signal. If a complete null can be achieved, the two leads are essentially <i>identical</i>. If it were otherwise, it would not be possible to achieve a null, and the original signal will still be present, either as a distorted version of the original, or a low-level version of the original. This is not new - It was used by Peter Baxandall and Peter Walker (Quad) many years ago, but it's not trivial when measuring active circuits. This is mainly due to tiny phase shifts that are very hard to duplicate perfectly.</p>
|
||||
|
||||
<p>For information on the use of the SIM, and an initial article describing how it works and my results so far, please see '<a href="sim.htm" target="_blank">Sound Impairment Monitor - The Answer?</a>'.</p>
|
||||
|
||||
<hr /><a id="ref"></a><b>References</b>
|
||||
<ol>
|
||||
<li>Douglas Self - Blameless Amplifier, Electronics World (Refer to <a href="http://www.douglas-self.com/" target="_blank">The Self Site</a>)
|
||||
<li><a href="https://www.youtube.com/watch?v=ZyWt3kANA3Q" target="_blank">Ethan Winer's Null test demonstration</a> (YouTube)
|
||||
<li>Towards A Definitive Analysis Of Audio System Errors (AES 91<sup>st</sup> Convention, October 1991)
|
||||
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<tr><td class="t-wht"><a id="copyright"></a><b>Copyright Notice.</b> This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is © 2000. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro- mechanical, is strictly prohibited under International Copyright laws. The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference. Commercial use is prohibited without express written authorisation from Rod Elliott.</td></tr>
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<div class="t-sml">Page created and copyright (c) 26 Feb 2000 Updates - 23 dec - added some more data, and square/sine test info./ 27 Feb, moved SIM description to its own page.</div><br />
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<table style="width:100%" class="tblblue"><tr><td class="hdrl"> Elliott Sound Products</td>
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<td align="right" class="hdrr">Power Amplifier Design Guidelines </td></tr></table>
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<h1>Power Amplifier Design Guidelines</h1>
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<div align="center" class="t_11">© 1999-2006, Rod Elliott (ESP)<br />
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<hr /><a id="contents"></a><b>Contents</b>
|
||||
<ul>
|
||||
<li><a href="amp_design.htm#intro">Introduction</a>
|
||||
<li><a href="amp_design.htm#s1">1 - Input Stages</a>
|
||||
<ul>
|
||||
<li><a href="amp_design.htm#s11">1.1 - Symmetrical Input Stages</a>
|
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<li><a href="amp_design.htm#s12">1.2 - RF Interference Suppression</a>
|
||||
</ul>
|
||||
<li><a href="amp_design.htm#s2">2 - Gain Stages (Class-A Amplifier)</a>
|
||||
<ul>
|
||||
<li><a href="amp_design.htm#s22">2.2 - Active Current Source or Bootstrap?</a>
|
||||
</ul>
|
||||
<li><a href="amp_design.htm#s3">3 - Output Stages</a>
|
||||
<ul>
|
||||
<li><a href="amp_design.htm#s31">3.1 - Thermal Stability</a>
|
||||
<li><a href="amp_design.htm#s32">3.2 - Output Stage Linearity</a>
|
||||
<li><a href="amp_design.htm#s33">3.3 - Bias Servos</a>
|
||||
<li><a href="amp_design.htm#s34">3.4 - Output Stage Stability</a>
|
||||
<li><a href="amp_design.htm#s35">3.5 - Output Current</a>
|
||||
</ul>
|
||||
<li><a href="amp_design.htm#s4">4 - Some Notes on Power Supply Design</a></li>
|
||||
<li><a href="amp_design.htm#s5">5 - Measurements Vs. Subjectivity</a>
|
||||
<ul>
|
||||
<li><a href="amp_design.htm#s51">5.1 - Valves Vs. Transistors Vs. MOSFETs</a>
|
||||
</ul>
|
||||
<li><a href="amp_design.htm#s7">7 - Slew Rate and Intermodulation</a>
|
||||
<ul>
|
||||
<li><a href="amp_design.htm#s71">7.1 - Slew Rate Nomograph</a>
|
||||
</ul>
|
||||
<li><a href="amp_design.htm#s8">8 - Frequency Response Etc.</a>
|
||||
<li><a href="amp_design.htm#s9">9 - Designs to Avoid</a>
|
||||
<li><a href="amp_design.htm#s10">10 - Further Reading</a>
|
||||
<li><a href="amp_design.htm#ref">References</a>
|
||||
</ul>
|
||||
|
||||
<hr /><a id="intro"></a><b>Introduction</b>
|
||||
<p>I am amazed at the number of amplifier designers who have, for one reason or another, failed to take some of the well known basics and pitfalls of amp design into consideration during the design phase. While some of these errors (whether of judgement or through ignorance is uncertain) are of no great consequence, others can lead to the slow but sure or instantaneous destruction of an amplifier's output devices.</p>
|
||||
|
||||
<p>When I say 'of no great consequence', this is possibly contentious, since a dramatic increase in distortion is hardly that, however in this context it will at least not destroy anything - other than the listener's enjoyment.</p>
|
||||
|
||||
<p>Even well known and respected designs can fall foul of some basic errors - this is naturally ignoring the multitude of 'off the wall' designs (e.g. Single-ended MOSFETs without feedback (yecch! - 5% distortion, phtooey), transformer-coupled monstrosities, amplifiers so complex and bizarre that they defy logic or description, etc). This is not including valve amps, these are a 'special' case and in many areas, such as guitar amps, as far as many players are concerned they remain unsurpassed.</p>
|
||||
|
||||
<p>In this article, I have attempted to cover some of the areas which require their own special consideration, and the references quoted at the end are excellent sources of more detailed information on the items where a reference is given.</p>
|
||||
|
||||
|
||||
<p><b>Reference Amplifier</b>
|
||||
<br />My reference amplifier is shown in <a href="project3a.htm" target="_blank">Project 3A</a>, and is a hard act to follow. As I have been refining these pages and experimenting with simulations and real life, I have found that this amp is exemplary. It does need a comparatively high quiescent current to keep the output devices well away from crossover distortion, but this is easily accommodated by using decent heatsinks. Even a Class-A system (Death of Zen) fails to come close at medium power, and is barely better at low power.</p>
|
||||
|
||||
<p>This amp uses the following ...</p>
|
||||
|
||||
<ul>
|
||||
<li>long tailed pair input stage</li>
|
||||
<li>single stage bootstrapped Class-A driver</li>
|
||||
<li>complementary compound pair output stage</li>
|
||||
<li>RC Zobel network (it hates inductors)</li>
|
||||
<li>no current mirrors or sources (other than the bootstrap)</li>
|
||||
</ul>
|
||||
|
||||
<p>It is stable with all conventional loads, capable of 80W into 8 Ohms, and simple to build. Using only commonly available parts, it is also very inexpensive.</p>
|
||||
|
||||
<p><b>Note:</b>
|
||||
<br />This article is not intended to be the 'designers' handbook', but is a collection of notes and ideas showing the influences of the various stages in a typical amplifier. Although I have made suggestions that various topologies are superior to others, this does not mean to imply that they should automatically be used. If one were to combine all the 'best' configurations into a single amp, this is no guarantee that it will perform or sound any better than one using 'lesser' building blocks.</p>
|
||||
|
||||
<p>There is a school of thought that the fewer active devices one uses, the better an amp will sound. I do not believe this to be the case, but my own design philosophy is to make any given design as simple as possible, consistent with the level of performance expected of it.</p>
|
||||
|
||||
<p>Additional schools of thought will make all manner of claims regarding esoteric components, 'unexplained' phenomena, or will imply that most amplifiers as we know them are useless for audio because they do not have predictable performance at DC and/or 10GHz, cannot drive pure inductance or capacitance, etc., etc. Regardless of these claims, most amplifiers actually work just fine, and do not have to do any of the things that the claimants may imply. The vast majority of all the off-the-wall claims you will come across can safely be ignored.</p>
|
||||
|
||||
<p>It's also worth noting that making a design more complex (more parts) doesn't necessarily mean that it will have better performance. More active parts in the signal chain tend to add delays, and this can make it very difficult to keep the final circuit stable. No-one wants (or needs) an amplifier that has marginal stability, meaning that it may be on the verge of oscillation during normal operation. Connecting a speaker lead with above average capacitance may cause spurious (and intermittent) oscillations on parts of the waveform. This is always audible, but might not show up when the amp is on the test bench.</p>
|
||||
|
||||
|
||||
<hr /><a id="s1"></a><b>1 - Input Stages</b>
|
||||
<p>There are two main possibilities for an input stage for a power amplifier. The most common is the long tailed pair, so we shall look at this first. It's not uncommon to see two long-tailed-pairs, one using NPN and the other using PNP transistors. While this makes the circuit <i>appear</i> to be fully symmetrical, it isn't, because the NPN and PNP transistors will never be exact complements of each other.</p>
|
||||
|
||||
|
||||
<p><b>Long Tailed Pair</b>
|
||||
<br />It has been shown <sup><a href="amp_design.htm#ref">[ 1 ]</a></sup> that failing to balance the input Long Tailed Pair properly leads to a large increase in the distortion contributed by the stage. Some designers attempt to remedy the situation by including a resistor in the 'unused' collector circuit, but this is an aesthetic solution - i.e. it looks balanced, but serves no other useful purpose. (See Figure 1a) Note that the 'driver' transistor is simply there to allow us to make comparisons between the circuit topologies, and to provide current to voltage conversion. It is worth noting that even though this resistor serves no purpose electronically, it can make the PCB layout easier.</p>
|
||||
|
||||
<p>Use of the long-tailed (or differential) pair in an amplifier means that the amplifier will operate with what is generally called 'voltage feedback' (VFB). The feedback is introduced as a voltage, since the input impedance of both inputs is high (and approximately equal), and input current is (relatively speaking) negligible.</p>
|
||||
|
||||
<p>The feedback resistor and capacitor are selected to allow the circuit to operate at full open loop gain for the applied AC, but unity gain for DC to allow the circuit to stabilise correctly with a collector voltage at (or near) 0V. The transistors used in the simulations that follow are 'ideal', without internal capacitances etc, and have an h<sub>FE</sub> of 235 in all cases, measured with a base current of 10µA. The simulated circuits were operated at a voltage of ±12V. Different simulators will give different results, but the trends will be the same.</p>
|
||||
|
||||
<p class="t-pic"><img src="ampd-f1a.png" alt="Figure 1" border="1" width="700" height="240" /><br />Figure 1a - Aesthetic Addition Of Resistor To Balance The Collector Load</p>
|
||||
|
||||
<p>As shown, and with a 12mA collector current for Q3, the load imbalance at the LTP collectors is 94µA for Q1, and 1mA for Q2. Simply by reducing the value of R1 it is possible to improve matters, but it is still not going to give the performance of which the circuit is capable. Again, as shown the gain of the LTP is a rather dismal 32 (as measured at the collector of Q2). The inclusion of R3 is purely cosmetic. It does provide a convenient means to measure the gain of the LTP, but otherwise serves no purpose.</p>
|
||||
|
||||
<p>Changing R1 for a current source does not help with gain, but provides a worthwhile improvement in power supply hum rejection, and in particular improves common mode rejection. A common mode signal is one that is applied in the same phase and amplitude to both inputs at once.</p>
|
||||
|
||||
<p>The overall gain of this configuration (measured at the collector of Q3) is 842, but by reducing R2 to 1.8k it can be raised to 1,850. This also improves collector current matching in the LTP, but the value will be device dependent, and is not reliable for production units.</p>
|
||||
|
||||
<p class="t-pic"><img src="ampd-f1b.png" alt="Figure 1a" border="1" width="700" height="240" /><br />Figure 1b - A Current Mirror And Local Feedback Applied To The LTP</p>
|
||||
|
||||
<p>The circuit shown in Figure 1b has improved overall gain to 6,860, a fairly dramatic improvement on the earlier attempt. A further improvement in linearity is to be had by adding resistors (100 Ohm or thereabouts) into the emitter circuits of the current mirror transistors. This will swamp the base-emitter non-linearities, and provide greater tolerance to device gain variations. Overall gain is not affected.</p>
|
||||
|
||||
<p>Proper selection of the operating current will improve matters considerably, and also help to reduce distortion, especially if local negative feedback (as shown in Figure 1b) is applied. This has been discussed at length by various writers <sup><a href="amp_design.htm#ref">[ 1 ]</a></sup>, and a bit of simple logic reveals that benefits are bound to accrue to the designer who takes this seriously.</p>
|
||||
|
||||
<p>Since the value of the transistor's internal emitter resistance (r<sub>e</sub>) is determined by the current flow -</p>
|
||||
|
||||
<blockquote>
|
||||
r<sub>e</sub> = 26 / I<sub>e</sub> (in mA)
|
||||
</blockquote>
|
||||
|
||||
<p>at very low operating currents this value can be quite high. For example, at 0.5 mA, r<sub>e</sub> will be about 52 ohms, increasing further as the current is reduced. Although this will introduce local feedback (and reduce the available gain), it is non-linear, resulting in distortion as the current varies during normal operation. Increasing the current, and using resistors (which are nice and linear) to bring the gain back to where it was before will reduce the distortion, since the resistor value - if properly chosen - will 'swamp' the variations in the internal r<sub>e</sub> due to signal levels.</p>
|
||||
|
||||
<p>At small currents (where the current variation during operation is comparatively high), this internal resistance has a pronounced effect on the performance of the stage. Simple solutions to apparently complex problems abound.</p>
|
||||
|
||||
<p>Use of a current mirror as the load for the long-tailed pair (LTP) again improves linearity and gain, allowing either more local feedback elsewhere, or more global feedback. Either of these will improve the performance of an amplifier, provided precautions are taken to ensure stability - i.e. freedom from oscillation at any frequency or amplitude, regardless of applied load impedance.</p>
|
||||
|
||||
|
||||
<hr /><b>Single Transistor</b>
|
||||
<p>There is another (not often used these days) version of an amplifier input stage. This is a single transistor, with the feedback applied to the emitter. It has been claimed by many that this is a grossly inferior circuit, but it does have some very nice characteristics. Technically, it uses <i>current</i> feedback, rather than the more common voltage feedback.</p>
|
||||
|
||||
<p class="t-pic"><img src="ampd-f2a.png" alt="Fig 2A" border="1" width="700" height="240" /><br />Figure 2a - Single Transistor Input Stage</p>
|
||||
|
||||
<p>So what is so nice about this? In a word, stability. An amplifier using this input stage requires little or no additional stabilisation (the 'Miller' cap, aka 'dominant pole') which is mandatory with amps having LTP input stages.</p>
|
||||
|
||||
<p>An amplifier using this input stage is referred to as a 'current feedback' (CFB) circuit, since the feedback 'node' (the emitter of the input transistor) is a very low impedance. The base circuit is the non-inverting input, and has a relatively high input impedance - but not generally as high as the differential pair. The +ve and -ve inputs are therefore asymmetrical. CFB amplifiers are used extensively in extremely fast linear ICs, and are capable of bandwidths in excess of 300MHz (that is not a misprint!).</p>
|
||||
|
||||
<p>This is the input stage used in the 10W Class-A amp (John Linsley-Hood's design, which is now part of <a href="tcaas/index.htm" target="_blank">TCCAS</a> (The Class-A Audio Site), and also in the 'El-Cheapo' amp described in my Projects Pages. "Well if it is so good, why doesn't anybody use it?" I hear you ask (you must have said it pretty loudly, then, because Australia is a long way from everywhere <img src="grin.gif" alt="" />).</p>
|
||||
|
||||
<p>There is one basic limitation with this circuit, and this was 'created' by the sudden requirement of all power amplifiers to be able to faithfully reproduce DC, lest they be disgraced by reviewers and spurned by buyers.</p>
|
||||
|
||||
<blockquote>
|
||||
(I remain perplexed by this, since I know for a fact that I cannot hear DC, my speakers cannot reproduce it, I know of no musical instrument that creates it,
|
||||
and it would probably sound pretty boring if any of the above did apply. If you don't believe me, connect a 1.5V torch cell to your speaker, and let me know
|
||||
if I'm wrong. I seem to recall something about phase shift being bandied about at the time, but given the acoustics involved in recording in the studio and
|
||||
reproducing in a typical listening room - not to mention the 'interesting' phase shifts generated by loudspeaker enclosures as the speaker approaches resonance
|
||||
- I feel that the effects of a few degrees of low frequency phase shift generated in an amplifier are unlikely to be audible. This is of course assuming that
|
||||
human ears are capable of resolving absolute phase anyway - which they have been categorically proven to be unable to do.)
|
||||
</blockquote>
|
||||
|
||||
<p>This input stage cannot be DC coupled (at least not without using a level shifting circuit), because of the voltage drop in the emitter circuit and between the emitter-base junction of the transistor. Since these cannot be balanced out as they are with an LTP input stage, the input must be capacitively coupled.</p>
|
||||
|
||||
<p>In addition, some form of biasing circuit is needed, and unfortunately this will either have to be made adjustable (which means a trimpot), or an opamp can be used to act as a DC 'servo', comparing the output DC voltage with the zero volt reference and adjusting the input voltage to maintain 0V DC at the output. The use of such techniques will not be examined here, but can provide DC offsets far lower than can be achieved using the amplifier circuit itself. There is no sonic degradation caused by the opamp (assuming for the sake of the discussion that decent opamps cause sonic degradation anyway), since it operates at DC only (it might have some small influence at 0.5Hz or so, but this is unlikely to be audible).</p>
|
||||
|
||||
<p>It has also been claimed that the single transistor has a lower gain than the LTP, but this is simply untrue. Open loop gain of the stage is - if anything - higher than that of a simple LTP for the same device current.</p>
|
||||
|
||||
<p class="t-pic"><img src="ampd-f2b.png" alt="Fig 2B" border="1" width="700" height="240" /><br />Figure 2b - Voltage Gain Comparison Of Input Stages</p>
|
||||
|
||||
<p>I simulated a very simple pair of circuits (shown in Figure 2b) to see the difference between the two. Collector current is approximately 1mA in each, and the output of the LTP shows a voltage gain of 1,770 from the combined circuit (the input stage cannot properly be measured by itself, since it operates as a current amp in both cases). In neither case did I worry about DC offset, since the effects are minimal for the purpose of simply looking at the gain - therefore offset is not shown. (Did you notice that the gains obtained in this simulation are completely different from those obtained earlier for the simple LTP circuit - I used a different voltage (the previous example used ±12V). This in no way invalidates anything, they are just different.)</p>
|
||||
|
||||
<p>By comparison, the open-loop gain of the single transistor stage is 2,000 - this (perhaps unexpectedly) is somewhat higher. Admittedly, the addition of a current mirror would improve the LTP even more dramatically, but do we really need that much more gain? A quick test indicates that we can get a gain of 3,570. This looks very impressive, but is only an increase of a little over 4.2dB compared to the single transistor. By the same logic, the single transistor only has a 1.06dB advantage over the simple LTP, however the difference may be moot ....</p>
|
||||
|
||||
<p>Because the single transistor stage requires no dominant pole Miller capacitor for stability, it will maintain the gain for a much wider frequency range, so in the long run might actually be far superior to the LTP. Further tests were obviously required, so I built them. Real life is never quite like the simulated version, so there was a bit less gain from each circuit than the simulator claimed. The LTP came in with an open loop gain of 1000, while the single transistor managed 1400. The test conditions were a little different from the simulation, in that ±15 volts was used, so the gain difference is about what would be expected, and is very close to the ±12V results obtained in the first set of simulations on the LTP.</p>
|
||||
|
||||
<p>Distortion was interesting, with the LTP producing 0.7% which was predominantly 3rd harmonic. The single transistor was slightly worse for the same output voltage with 0.9%, and this had a dominant 2nd harmonic. This is an open loop test, so it's really an examination of the 'worst-case' performance. If the gain is reduced with feedback, distortion falls dramatically. However, it doesn't necessarily fall in a direct relationship</p>
|
||||
|
||||
<p>As expected, the LTP was unstable without a Miller capacitor, and 56pF managed to tame it down. Quite unexpectedly, the single transistor also required a Miller cap, but only when running open-loop. When it was allowed to have some feedback the oscillation disappeared. The LTP could not be operated without the Miller capacitor at any gain, and as the gain approached unity, more capacitance was needed to prevent oscillation.</p>
|
||||
|
||||
<p>The next step was a test of each circuit providing a gain of about 27, since this is around the 'normal' figure for a 60W power amp. Here, the LTP is clearly superior, with a level of distortion I could not measure. The single transistor circuit had 0.04% distortion, and again this was predominantly 2nd harmonic. In this mode, no Miller capacitor was needed for the single transistor, and it showed a very wide frequency response, with a slight rise in gain at frequencies above 100kHz. This was also noticeable with a 10kHz square wave, which had overshoot, although this was reasonably similar for positive and negative half-cycles. The LTP was well behaved, and showed no overshoot (it had the 56pF Miller cap installed), but it started to run out of gain at about 80kHz, and there was evidence of slew-rate limiting. This effect was not apparent with the single transistor.</p>
|
||||
|
||||
<p>All in all, I thought this was a worthwhile experiment, and the use of a simple resistor for the collector load of the gain stage allowed the final circuit to have a manageable gain. Had a current source or similar been used as the load, I would not have been able to measure the gain accurately, since the input levels would have been too small. As it was, noise pickup proved to be a major problem, and it was difficult to get accurate results without using the signal averaging capability on the oscilloscope.</p>
|
||||
|
||||
|
||||
<hr /><a id="s11"></a><b>1.1 - Symmetrical Input Stages</b>
|
||||
<p>There are many designs that you'll see with what appear to be fully symmetrical input stages. It's implied that the symmetry improves performance, but it may be an illusion. While the schematic <i>looks</i> symmetrical, the fact is that the NPN and PNP devices (or N-Channel and P-Channel FETs) are not perfect mirror images of each other. There are usually easily measured differences between NPN and PNP devices from the same family, and datasheets will quickly disabuse you of the notion that they are the same.</p>
|
||||
|
||||
<p>There is some evidence to show that an apparently symmetrical input stage may be better than a more conventional asymmetrical stage, but there are countless very good amps that don't use the extra circuitry. In some bases, the symmetry is continued throughout the amplifier (the output stages are normally symmetrical anyway, but the Class-A gain stage usually is not). Again, it's easy to run simulations that may show that (apparent) symmetry improves things. However, it requires more parts, and if they don't make a significant (and audible) difference then they are basically wasted.</p>
|
||||
|
||||
<p class="t-pic"><img src="ampd-f2c.png" alt="Figure 2C" border="1" /><br />Figure 2C - Asymmetrical Vs. Symmetrical Input Stage Examples</p>
|
||||
|
||||
<p>The drawing above shows an example, but excluding any caps needed for stability. I included current mirrors, but only used a resistor to bias the two complementary long-tailed pairs. In reality, these would probably be replaced by current sources. While the circuit certainly looks 'nice and symmetrical', that doesn't mean that it really is, electrically speaking. In a simulation, one thing you'd really expect would be lower DC offset with the symmetrical arrangement, but in fact it simulated as being slightly worse. Depending on how the voltage amplifier stages are configured, the distortion can be less, greater, or about the same. My simulation shows lower distortion, but simulators use ideal parts, and real parts may not actually improve matters at all if the devices aren't carefully matched.</p>
|
||||
|
||||
<p>Note that I've only shown the input stage and Class-A amplifier (aka 'VAS' - voltage amplifier stage), and have not included output stage bias networks or the output stage itself. Feedback is normally taken from the output to the speakers, but as shown it works as intended for analysis. One distinct benefit of the symmetrical stage is that the output current is also symmetrical because it's push-pull, and isn't limited by the current available to the Class-A amplifier stage. This means greater drive is possible, but it also makes it easier to destroy the output stage if it doesn't have protection circuits. With no load, the current through the Class-A stages is roughly the same - 5mA.</p>
|
||||
|
||||
<p>None of this means that designs that are symmetrical are worse than asymmetrical designs, but nor does it mean that a symmetrical amp is necessarily 'better'. Many claims are made, but usually with little or no science to substantiate them. There are undoubtedly some very fine amplifiers that use symmetrical input and gain stages, just as there are many very fine amplifier that do <i>not</i> use symmetry as part of the design. It seems that to some people, what the circuit looks like is more important than how it performs. Sighted listening tests will invariably support this bias, and the myths become self-perpetuating.</p>
|
||||
|
||||
<p>A couple of things that help the symmetrical argument is lower noise (gain stages effectively in parallel, so gain is increased by 6dB, noise by 3dB). The gain is also higher, but this is not necessarily a good thing if it leads to instability, or requires much more complex networks to remain stable under all operating conditions. In amplifier design (and indeed virtually all electronics design), everything we do is ultimately a compromise, and it's the designer's job to get performance that meets or exceeds expectations, but not if it requires far greater complexity (unless it can't be avoided).</p>
|
||||
|
||||
<p>To obtain 'true' symmetry, use two amplifiers in BTL (bridge tied load) configuration. If the devices in each amplifier are matched, then the amplifier is <i>completely</i> symmetrical as far as the signal is concerned. Unfortunately, this comes with its own issues, not the least of which is that each amp 'sees' half the actual load impedance. That makes driving 4 ohm loads difficult, because the output current from each amp is double that which would be the case for a single amp driving the same impedance.</p>
|
||||
|
||||
<p>Very high current in BTL amps is always a problem, because the supply has to be able to provide it with minimal ripple, and transistors generally lose linearity at high currents. The entire amp becomes more complex (and expensive), but often with no genuine benefits. I've been asked about symmetrical designs many times, and my answer is the same - feel free to use a design, but don't expect it to measure (or sound) any better than a competently designed 'conventional' amplifier.</p>
|
||||
|
||||
|
||||
<hr /><b>Input Stage Summary</b>
|
||||
<p>Based on the tests, there are pros and cons to all approaches (single transistor, long-tailed pair and symmetrical - and I'll bet that came as a surprise). The LTP in its simple form is a clear loser for gain, but use of a current mirror allows it to 'blow away' the single transistor, which cannot capitalise on this technique since there is nothing to mirror. Symmetrical inputs are considerably more complex, and you may (or may not) actually measure a difference between the simple LTP input and a symmetrical version.</p>
|
||||
|
||||
<p>Stability is very important to me, and I tend towards an amp which absolutely does not oscillate, even at the expense of a little more distortion. My own 60W reference amp is unconditionally stable with normal loads, and it uses an LTP for the input. Although I have experimented with symmetrical input stages, I have not published a design using this technique.</p>
|
||||
|
||||
<p>While there is no doubt at all that a symmetrical input stage can work very well, it does not automatically mean that the amp will sound any better. Adding the extra components makes the PCB more complex and the layout is critical. There's also a lot more to go wrong, especially with a compact input stage with many closely spaced transistors. Whether it's worth the effort depends on what you are trying to achieve, and you need to run tests to verify that what you <i>think</i> is 'better' is <i>actually</i> better. In many cases, a blind test may reveal that there's no audible difference, so the extra effort and parts serve no useful purpose.</p>
|
||||
|
||||
|
||||
<hr /><a id="s12"></a><b>1.2 - Protection From Radio Frequency Interference</b>
|
||||
<p>A favourite pastime of many designers is to connect a small capacitor as shown in Figure 3 directly to the base of the input transistor. This is supposed to prevent detection (rectification) of radio frequency signals picked up by the input leads. Well, to a certain degree this is true, as the Resistor-Capacitor (RC) combination forms a low pass filter, which will reduce the amount of RF applied to the input. As shown this has a 3dB frequency of 159kHz (although this will be affected by the output impedance of the preceding stage).</p>
|
||||
|
||||
<p class="t-pic"><img src="ampd-f3.png" alt="Figure 3" border="1" /><br />Figure 3 - The Traditional Method for Preventing RF Detection</p>
|
||||
|
||||
<p>This approach <i>might</i> work if PCB track lengths in that part of the circuit are very short, ensuring minimal inductance. This is not always the case, and some layouts may include more than enough track length to not only act as an inductor, but as an antenna as well. Then things can get really sneaky, such as when the levels of RF energy are so high that some amount manages to get through anyway. I once had a workshop/lab which was triangulated by three TV transmission towers - very nasty. RF interference was a fact of life there.</p>
|
||||
|
||||
<p>The traditional method not only did not work, but often made matters worse by ensuring that the transistor base was fed from a very low impedance (from an RF perspective) because of C1. A vast number of commercial amplifiers and other equipment which I worked on in that time picked up quite unacceptable amounts of TV frame buzz, caused by the detection of the 50Hz vertical synchronisation pulses in the TV signal. As the picture component of analogue TV is (or was - it's almost completely digital now) amplitude modulated RF, this was readily converted into audio - of the most objectionable kind.</p>
|
||||
|
||||
<p class="t-pic"><img src="ampd-f4.png" alt="Figure 4" border="1" /><br />Figure 4 - Use of a Stopper Resistor to Prevent RF Detection</p>
|
||||
|
||||
<p>Figure 4 shows the remedy - but to be effective the R2 must be as close as possible to the base, or the performance is degraded. How does this work? Simple, the base-emitter junction of a transistor is a diode, and even when conducting it will retain non-linearities. These are often sufficient to enable the input stage to act as a crude AM detector, which will be quite effective with high-level TV or CB radio signals. Adding the external resistance again swamps the internal non-linearities, reducing the diode effect to negligible levels. This is not to say that it will entirely eliminate the problem where strong RF fields are present, but will at least reduce it to 'nuisance' rather than 'intolerable' levels.</p>
|
||||
|
||||
<p><b><small>UPDATE: </small></b>I have been advised by a reader who works in a transmitting station that connecting the capacitor directly between base and emitter (in conjunction with the stopper resistor) is very effective. He too found that the traditional method was useless, but that when high strength fields are encountered, the simple stopper is not enough.</p>
|
||||
|
||||
<p>With opamps, the equivalent solution is to connect the stopper resistor in series with the +ve input, and the capacitor between the +ve and -ve inputs, with no connection to earth.</p>
|
||||
|
||||
<p>In all both cases it is essential to keep all leads and PCB tracks as short as possible, so they cannot act as an antenna for the RF. Needless to say, a shielded (and grounded) equipment case is mandatory in such conditions.</p>
|
||||
|
||||
|
||||
<hr /><a id="s2"></a><b>2 - Gain Stage (Class-A Amplifier Section)</b>
|
||||
<p>The Class-A amp stage is also commonly known as the Voltage Amplification Stage (VAS), but both terms are common, and are generally interchangeable. There are a number of traps here, not the least of which is that it is commonly assumed that the load (from the output stage) is infinite. Oh, sure, every designer knows that the Class-A stage must carry a current of at least 50% more than the output stage will draw, and this is easily calculated ...</p>
|
||||
|
||||
<blockquote>
|
||||
I<sub>A</sub> = Peak_V / Op_R / Op_Gain × 1.5
|
||||
</blockquote>
|
||||
|
||||
<p>where I<sub>A</sub> is the Class-A current, Peak_V is the maximum voltage across the load Op_R, and Op_Gain is the current gain of the output transistor combination.</p>
|
||||
|
||||
<p>For a typical 100W / 8 Ohm amplifier this will be somewhere between 5 and 10mA. Assuming an output transistor combination with a current gain of 1000 (50 for the driver, and 20 for the power transistor), with an 8 Ohm load, the impedance presented to the Class-A stage will be about 2k Ohms, which is a little shy of infinity.</p>
|
||||
|
||||
<p>Added to this is the fact that the impedance reflected back is non-linear, since the driver and output transistors change their gain with current - as do all real-life semiconductors. There are some devices available today which are far better than the average, but they are still not perfect in this respect.</p>
|
||||
|
||||
<p>The voltage gain is typically about 0.95 to 0.97 with the compound pair configuration. It must be noted that this figure will only be true for mid-range currents, and will be reduced at lower and higher values. Figure 5 shows the basic stage type - the same basic amplifier we used before, with the addition of a current source as the collector load. Also common is the bootstrapped circuit (not shown here, but evident on many ESP designs).</p>
|
||||
|
||||
<p>There is not a lot of difference between current source and bootstrap circuits, but the current source gives <i>slightly</i> higher gain. With either type, there are some fairly simple additions which will improve linearity quite dramatically. Figure 5 shows the typical arrangement, including the 100pF dominant pole stabilisation capacitor connected between the Class-A transistor's collector and base.</p>
|
||||
|
||||
<p class="t-pic"><img src="ampd-f5.png" alt="Figure 5" border="1" /><br />Figure 5 - Typical Class-A Driver Configuration</p>
|
||||
|
||||
<p>It is important to try to make the Class-A stage capable of high gain, even when loaded by the output stage. There have been many different methods used to achieve this, but none is completely successful. The output stage is not a simple impedance, and it varies as the load impedance changes. Bipolar transistors reflect the load impedance back to the base, adjusted according to the device's gain. A potential problem is that some designers seem completely oblivious to this problem area, or create such amazingly complex 'solutions' as to make stabilisation (against oscillation) very difficult.</p>
|
||||
|
||||
<p>This is one area where MOSFETs may be found superior to BJTs. The gate capacitance is not affected by the load impedance, and nothing is reflected back to the Class-A driver. This will typically allow it to have higher gain - especially when low load impedances are involved. The Class-A driver needs only to be able to charge and discharge the gate capacitance of the MOSFETs, and this is not influenced by the output current or load.</p>
|
||||
|
||||
<p class="t-pic"><img src="ampd-f6.png" alt="Figure 6" border="1" /><br />Figure 6 - Improving Open Loop Output impedance of Class-A Driver</p>
|
||||
|
||||
<p>The above is simple and very effective. This straightforward addition of an emitter follower to the Class-A driver (with the 1k 'bootstrap' resistor) has increased the combined LTP and Class-A driver gain to 1,800,000 (yes, 1.8 million!) or 125dB (open loop and without the dominant pole capacitor connected). Open loop output impedance is about 10k, again without the cap. Once the latter is in circuit, gain is reduced to a slightly more sensible 37,000 at 1kHz with the 100pF value shown. Output impedance at 1kHz is now (comparatively) very low, at about 150 Ohms.</p>
|
||||
|
||||
<p>Note that in the above, I have used a 5k resistor instead of the more usual current source to bias the long-tailed pair. This is for clarity of the drawing, and not a suggestion that the current source should be forsaken in this position.</p>
|
||||
|
||||
<p>A special note for the unwary - If one is to use a single current control transistor for both the LTP and Class-A driver, do not use the Class-A (aka VAS - voltage amplifier stage) current as the reference, but rather the LTP. If not, the varying current in the Class-A circuit will cause modulation of the LTP emitter current, with results that are sure to be as unwelcome as they are unpredictable <sup><a href="amp_design.htm#ref">[ 4 ]</a></sup>. Where the current source reference is based on the VAS (Class-A driver), it's advisable to decouple the voltage reference for the LTP source to minimise interactions.</p>
|
||||
|
||||
<p>I have often seen amplifier designs where the circuit is of such complexity that one must wonder how they ever managed to stop them from becoming high power radio frequency oscillators. The maze of low value capacitors sometimes used - some with series resistance - some without, truly makes one wonder what the open loop frequency and phase response must look like. Couple this with the fact that many of these amps do not have wonderful specifications anyway, and one is forced to ponder what the designer was actually trying to accomplish (being 'different' is not a valid reason to publish or promote a circuit in my view, unless it offers some benefit otherwise unattainable).</p>
|
||||
|
||||
<p>Having carried out quite a few experiments, I am not convinced that vast amounts of gain from the input stage and Class-A amplifier stage are necessary or desirable. As long as the circuit is linear (i.e. has low distortion levels before the addition of feedback), the final result is likely to be satisfactory. I have seen many circuits with far more open loop gain than my reference amp (<a href="project3a.htm" target="_blank">Project 3A</a>), that in theory should be vastly superior - yet they apparently are not.</p>
|
||||
|
||||
|
||||
<hr /><a id="s22"></a><b>2.2 - Active Current Source or Bootstrap?</b>
|
||||
<p>There are essentially two ways to create a constant current feed to the Class-A driver stage. The active current source is one method, and this is very common. It does introduce additional active devices, but it is possible to make a current source that has an impedance so close to infinity that it will be almost impossible to measure it without affecting the result just by attaching measurement equipment. For more detailed information on current sources, see the article <a href="ism.htm" target="_blank">Current Sources Sinks and Mirrors</a>. Figure 6A shows an active current source for reference.</p>
|
||||
|
||||
<p>A simpler way is to use the bootstrap circuit, where a capacitor is used from the output to maintain a relatively constant voltage across a resistor. If the voltage across a resistor is constant, then it follows that the current flowing through it must also be constant. Figure 6a shows the circuit of a bootstrap constant current source. Unlike a true current source, the current through the bootstrap circuit will change with the supply voltage. This is a gradual change, and is outside the audio spectrum - or at least it should be if the circuit is designed correctly.</p>
|
||||
|
||||
<p class="t-pic"><img src="ampd-f6a.png" alt="Figure 6a" border="1" /><br />Figure 6A - Active And Bootstrapped Current Source</p>
|
||||
|
||||
<p>The bootstrap circuit works as follows. Under quiescent conditions, the output is at zero volts, and the positive supply is divided by Rb1 and Rb2. The base of the upper transistor will be at about +0.7V - just sufficient to bias the transistor. As the output swings positive or negative, the voltage swing is coupled via Cb, so the voltage across Rb2 remains constant. The current through Rb2 is therefore constant, since it maintains an essentially constant voltage across it. Note that this applies only for AC voltages, as the capacitor cannot retain an indefinite charge if there is a DC variation.</p>
|
||||
|
||||
<p>The overall difference is not great in a complete design. Although the current source is theoretically better, a bootstrap circuit is simpler and cheaper, and introduces no additional active devices. The capacitor needs to be large enough to ensure that the AC across it remains small (less than a few hundred millivolts) at the lowest frequency of interest. Assuming Rb1 and Rb2 are equal, the cap's voltage rating needs to be a minimum of ½ the positive supply voltage, but preferably greater.</p>
|
||||
|
||||
|
||||
<hr /><a id="s3"></a><b>3 - Output Stage</b>
|
||||
<p>There are countless amplifiers which still use the Darlington type configuration, even though this was shown by many <sup><a href="amp_design.htm#ref">[ 2 ]</a></sup> to be inferior to the Sziklai/ complementary pair. Both configurations (in basic form, since there are many variations) are shown in Figure 7. There are two main areas where the Darlington configuration is inferior, and we shall look at each. In the following, bias networks and Class-A driver(s) are not included, only the output and driver transistors ...</p>
|
||||
|
||||
<p class="t-pic"><img src="ampd-f7.png" alt="Figure 7" border="1" /><br />Figure 7 - The Basic Configurations Of Output Stages In Common Use</p>
|
||||
|
||||
<p>Of the two shown, it will be apparent that I have not included MOSFET output stages - this is because MOSFETs require no driver transistor as such - they are normally driven directly from the Class-A amplifier (or a modified version - often an additional long-tailed pair. As can be seen, the component count is the same for those shown, but instead of using two same polarity (both PNP or both NPN), the compound pair (also called a Sziklai pair) uses one device of each polarity. The final compound device assumes the characteristics of the <i>driver</i> in terms of polarity, and the Emitter, Base and Collector connections for each are shown. The 220 ohm resistor (or other value determined by the design) is added to prevent output transistor collector to base leakage current from allowing the device to turn itself on, and also speeds up the turn-off time. Omission of this resistor is not a common mistake to make, but it has been done. In some cases, you'll see a comparatively high value used. The results are degraded distortion figures, especially at high frequency, and poor thermal stability.</p>
|
||||
|
||||
<p>The value must be selected with reasonable care, if it is too low, the output transistor will not turn on under quiescent (no signal) conditions, the driver transistor(s) will be subject to excessive dissipation, and crossover distortion will result. If too high, turn-off performance of output devices will be impaired and thermal stability will not be as good. The final value depends (to some extent) on the current in the Class-A driver stage and the gain of the driver transistor, but the final arbiter of quiescent is the Vbe multiplier stage. These comments apply equally to the Darlington and compound pairs.</p>
|
||||
|
||||
<p>Values of between 100 Ohms up to a maximum of perhaps 1k should be fine for most amplifiers, with lower values used as power increases. High power creates higher currents throughout the output stage and makes the transistors harder to turn off again, especially at high frequencies. This can lead to a phenomenon called 'cross-conduction', which occurs because the transistors cannot switch off quickly enough, so there is a period where both power transistors are conducting simultaneously. It won't happen at normal audio frequencies, although you may get slightly higher than normal current drawn from the power supply even at 20kHz.</p>
|
||||
|
||||
<p>If an amp is driven to any reasonable power at higher frequencies, it can spontaneously self-destruct if there is sufficient cross conduction happening. The easiest way to reduce it is to use smaller resistors between base and emitter of the power transistors, but be aware that this will increase the demands on the drivers. For example, with 220 ohm resistors as shown above, the resistors will only pass around 3-5mA, but if they are reduced to (say) 47 ohms, that increases to perhaps 16mA or more. The drivers have to supply this current, even at idle, and their quiescent power dissipation increases from 120mW to over 550mW with ±35V supplies. A heatsink for the drivers becomes a necessity.</p>
|
||||
|
||||
<p>Normally, there should be little or no need to use resistors less than ~100 ohms. If you want to get full power at 100kHz or more (why? it serves no purpose for an audio amplifier), then you'll need to make these resistors even lower in value and ensure proper heatsinks for the drivers. You will also need to increase the power rating for the Zobel network resistor, or it will overheat at high frequencies.</p>
|
||||
|
||||
|
||||
<hr /><a id="s31"></a><b>3.1 - Thermal Stability</b>
|
||||
<p>It can be seen that in the Darlington configuration, there are two emitter-base junctions for each output device. Since each has its own thermal characteristic (a fall of about 2mV per degree C), the combination can be difficult to make thermally stable. In addition, the gain of transistors often increases as they get hotter, thus compounding the problem. The bias 'servo', typically a transistor Vbe multiplier, must be mounted on the heatsink to ensure good thermal equilibrium with the output devices, and in some cases can still barely manage to maintain thermal stability.</p>
|
||||
|
||||
<p>If stability is not maintained, the amplifier may be subject to thermal runaway, where after a certain output device temperature is reached, the continued fall of Vbe causes even more quiescent current to flow, causing the temperature to rise further, and so on. A point is reached where the power dissipated is so high that the output transistors fail - often with catastrophic results to the remainder of the circuit and/or the attached loudspeakers.</p>
|
||||
|
||||
<p>The Sziklai/ compound pair has only one controlling Vbe, and is thus far easier to stabilise. Since the single Vbe is that of the driver (which should <i>not</i> be mounted on the main heatsink, and in some will have no heatsink at all), the requirements for the Vbe multiplier are less stringent, mounting is far simpler and thermal stability is generally very good to excellent.</p>
|
||||
|
||||
<p>I have used the compound pair since the early 1970s, and when I saw it for the first time, it made too much sense in all respects to ignore. Thermal stability in a fairly basic 100W/4 Ohm amplifier of my design (of which many hundreds were built - it was the predecessor of the P3A design in the projects section) was assured with a simple 2-diode string - no adjustment was ever needed. (However, there <i>were</i> a couple of other tricks used at the time to guarantee stable operation.)</p>
|
||||
|
||||
|
||||
<hr /><a id="s32"></a><b>3.2 - Design of Bias Servo</b>
|
||||
<p>It would seem (at first glance at least) that there is nothing to this piece of circuitry. It is a very basic Vbe multiplier circuit, and seemingly, nothing can go wrong. This is almost true, except for the following points.</p>
|
||||
|
||||
<p class="t-pic"><img src="ampd-f9.png" alt="Figure 9" border="1" /><br />Figure 9 - The Basic Bias Servo Circuit</p>
|
||||
|
||||
<p>The design of many amps (especially those using a Darlington output stage) requires that the bias servo be made adjustable, to account for the differing characteristics of the transistors. If resistor R1 (in Fig 9) is instead a trimpot (i.e. variable resistor), what happens when (if) the wiper decides (through age, contamination or rough handling) to go open-circuit?</p>
|
||||
|
||||
<p>The answer is simple - the voltage across the bias servo is now the full supply voltage (less a transistor drop or two), causing both the positive and negative output devices to turn on as hard as they possibly can. The result of this is the instantaneous destruction of the output devices - this will happen so fast that fuses cannot possibly prevent it, and even the inclusion of sophisticated Load-Line output protection circuitry is unlikely to be able to save the day.</p>
|
||||
|
||||
<p>The answer of course is so simple that it should be immediately obvious to all, but sadly this is not always the case. By making R2 the variable component, should it happen to become open-circuited the bias servo simply removes the bias. This will introduce crossover distortion, but the devices are saved. To prevent the possibility of reducing the pot value to 0 ohms (which will have the same effect as described above!), there is often a series resistor, whose value is selected to allow adequate adjustment while retaining a respectable safety margin. It's not essential, provided that the setup instructions are followed carefully.</p>
|
||||
|
||||
<p>An additional precaution must be taken here, in that if the resistor values are too low, the offset voltage seen by the output transistors is simply the voltage drop across the resistors, with the transistor having little or no control over the result. This is easily avoided by ensuring that the resistor current is 1/10 (or thereabouts) of the total Class-A bias current.</p>
|
||||
|
||||
<p>It is also possible to make the resistance too large, and the bias servo will be less stable with varying current. This may also cause the bias servo to have too much gain, which can cause the amplifier's quiescent current to fall as it gets hotter. While this is a good thing from the reliability point of view, if it causes crossover distortion to appear when the amp is hot, the audible effect will obviously be disappointing. It will generally be necessary to experiment with the values to ensure that stability is maintained - there is no way to calculate this that comes to mind, although I am sure it is possible. The base-emitter voltage falls at 2mV /°C, but the variation in gain with temperature is not as readily calculated.</p>
|
||||
|
||||
<p>As a secondary safeguard, using a suitable diode string in parallel with the servo may be useful. These should be chosen to prevent destructive current, but some method of over temperature protection will be needed. This can be a fan blowing onto the heatsink, or a thermal cutout to switch off the power if the amp gets too hot.</p>
|
||||
|
||||
<p>Note that if the output stage uses the Darlington arrangement, the bias servo transistor must be located on the main heatsink. If you use a compound (Sziklai) pair, it is <i>imperative</i> that the bias servo senses the <i>driver transistor(s)</i> (which should <i>not</i> be on the main heatsink). Failure to locate the bias servo properly is inviting output stage failure due to thermal runaway.</p>
|
||||
|
||||
|
||||
<hr /><a id="s33"></a><b>3.3 - Linearity</b>
|
||||
<p>Numerous articles have been written on the superior linearity of the compound (Sziklai) stage (Otala <sup><a href="amp_design.htm#ref">[ 3 ]</a></sup>, Self, Linsley Hood among others) and I cannot help but be astonished when I see a new design in a magazine, still using the Darlington arrangement. The use of the compound pair requires no more components - the same components are simply arranged in a different manner. It was with great gusto that an Australian electronics magazine proudly announced (in 1998) that "this is the first time we have used this arrangement in a published design" (or words to that effect). I don't know the reason(s) they may have had for not using the complementary pair in <i>every</i> design they published (this magazine is a lot younger than I). Words fail me. The magazine in question is not the only one, and the Web abounds with designs old and new - all using the Darlington emitter-follower.</p>
|
||||
|
||||
<p>This is not to say that the Darlington stage shouldn't be used - there are many fine amplifiers that use it, and with a bit of extra effort to get the bias servo right, such amps will give many years of reliable service. It is particularly suited to very high power amps, because of its simplicity - especially with multiple paralleled output devices. Parallel operation is more irksome with the Sziklai configuration. An example of paralleled Sziklai pairs is seen in <a href="project27.htm" target="_blank">Project 27</a>. Having to use additional emitter resistors for each output transistor (in series with its <i>physical</i> emitter) is a nuisance, but the arrangement works very well indeed.</p>
|
||||
|
||||
<center>
|
||||
<table border="1" style="width:700px">
|
||||
<tr class="tbldark"><td colspan="3"><b>Darlington </b></td></tr>
|
||||
<tr><td width="33%">Driver</td><td width="33%">O/P Transistor</td><td>Total Gain</td></tr>
|
||||
<tr><td>50</td><td>25</td><td>1310</td></tr>
|
||||
<tr class="tbldark"><td colspan="3"><b>Compound (Sziklai)</b></td></tr>
|
||||
<tr><td>Driver</td><td>O/P Transistor</td><td>Total Gain</td></tr>
|
||||
<tr><td>50</td><td>25</td><td>1290</td></tr></table>
|
||||
<span class="t-pic">Table 1 - Relative Forward Current Gain of Compound Pair vs. Darlington Emitter Follower</span>
|
||||
</center>
|
||||
|
||||
<p>The lower gain of the compound pair indicates that there is internal local negative feedback inherent in the configuration, and all tests that have been performed indicate that this is indeed true. Although the gain difference is not great, much of the improved linearity can be assumed to result from the fact that only one emitter-base junction is directly involved in the signal path rather than two, so only one set of direct non-linearities is brought into the equation. The second (output) device effectively acts as a buffer for the driver.</p>
|
||||
|
||||
<p>Having said that, there are some very well respected amplifiers using Darlington emitter-follower output stages. There are no hard and fast rules that can be applied to make the perfect amplifier (especially since it does not yet exist), and with careful design it is quite possible to make a very fine amplifier using almost any topology.</p>
|
||||
|
||||
<p>One thing that can (and does) cause problems is the output stage gain. If it's biased to a lower than optimum current, the gain falls dramatically. If the output stage's current gain falls too far, the entire amplifier effectively has no gain, so negative feedback can do nothing to reduce crossover distortion. This is why an amplifier should be as linear as possible before the application of feedback, but claims that feedback "ruins the sound" are divorced from reality. While it's possible to design an amplifier with no feedback, there's really very little point. It won't perform as well as a more conventional design, regardless of 'reviews' that may extol it's alleged virtues.</p>
|
||||
|
||||
|
||||
<hr /><a id="s34"></a><b>3.4 - Output Stage Stability</b>
|
||||
<p>It is a simple fact of life that an emitter follower (whether Darlington or compound) is perfectly happy to become an oscillator - generally at very high frequencies. This is especially true when the output lead looks like a tuned circuit. A length of speaker cable, while quite innocuous at audio frequencies, is a transmission line at some frequency determined by its length, conductor diameter and conductor spacing. A copy of the ARRL handbook (from any year) will provide all the formulae needed to calculate this, if you really want to go that far.</p>
|
||||
|
||||
<p>All power amplifiers (well, nearly all) use emitter follower type output stages, and when a speaker lead and speaker (or even a non-inductive dummy load) are connected, oscillation often results. This is nearly always when the amp is driven, and is more likely when current is being drawn from the circuit. It is a little sad that the compound pair is actually more prone to this errant behaviour than a Darlington, possibly because the driver is the controlling element (and its emitter is connected to the load), and has a higher bandwidth.</p>
|
||||
|
||||
<p>Some of the 'super' cables - much beloved by audiophiles - are often worse in this respect for their ability to act as RF transmission lines than ordinary Figure-8, zip cord or 3-core mains flex, and are therefore more likely to cause this problem.</p>
|
||||
|
||||
<p class="t-pic"><img src="ampd-f10.png" alt="Figure 10" border="1" /><br />Figure 10 - The Standard Output Arrangement For Power Amp Stability</p>
|
||||
|
||||
<p>The conventional Zobel network (consisting of the 10 Ohm resistor and 100nF capacitor) generally swamps the external transmission line effect of the speaker cables and loudspeaker internal wiring, and provides stability under most normal operating conditions.</p>
|
||||
|
||||
<p>In a great many amplifiers, the amp may oscillate with no load or speaker cables attached, and a Zobel network as shown stops this, too. The reasons are a little difficult to see at first, but can be traced to small amounts of stray inductance and capacitance around the output stage in particular. At very high frequencies, these strays can easily form a tuned circuit, causing phase shift between the amp's output and inverting input. At these high frequencies, few amplifiers have a great deal of phase margin (the difference between the amplifier's phase shift and 180°). Any stray inductance and/or capacitance may only need to create a few additional degrees of phase shift to cause oscillation. Because there is very little feedback at such high frequencies, the overall impedance can be much higher than expected.</p>
|
||||
|
||||
<p>At these frequencies, the Zobel capacitor is essentially a short circuit, so there is now a 10 ohm resistor in parallel with a high impedance tuned circuit. The 10 ohm resistor ruins the Q of the tuned circuit(s), and applies heavy damping, thus negating the phase shift to a large degree and restoring stability. Personally, I don't recommend that this network be omitted from any amplifier, even if it appears to be stable without it.</p>
|
||||
|
||||
<p>With capacitive loading (as may be the case when a loudspeaker and passive crossover are connected), the Zobel network has very little additional effect - may have no effect whatsoever. The only sure way to prevent oscillation or severe ringing with highly capacitive cables is to include an inductor in the output of the amplifier. This should be bypassed with a suitable resistor to reduce the Q of the inductor, and the typical arrangement is shown in Fig 10. For readers wishing to explore this in greater depth, read 'The Audio Power Interface' <sup><a href="amp_design.htm#ref">[ 2 ]</a></sup>. In many cases it might be better to use a far lower resistance than the 10 Ohms normally specified - I am thinking around 1 Ohm or so. Some National Semiconductor power opamps specify 2.7 ohms as the optimum. Ideally, cables with low inductance and high capacitance should <i>always</i> have an additional 100nF/10 ohm Zobel network at the loudspeaker end. When this is done, the cable no longer appears as a capacitor at high frequencies. Regrettably, few (if any) loudspeaker manufacturers see fit to include this at the input terminals.</p>
|
||||
|
||||
<p>Another alternative is to include a resistor in series with the output of the amplifier, but this will naturally have the dual effect of reducing power output and reducing damping factor. At resistor values sufficient to prevent oscillation, the above losses become excessive - and all wasted power must be converted into heat in the resistor.</p>
|
||||
|
||||
<p>The choice of inductor size is not difficult - for an 8 Ohm load it will be typically a maximum of 20µH, any larger than this will cause unacceptable attenuation of high frequencies. A 6µH inductor as shown in Figure 10 will introduce a low frequency loss (assuming 0.03 Ohm resistance) of 0.03dB and will be about 0.2dB down at 20kHz. These losses are insignificant, and will not be audible. In contrast, ringing (or in extreme cases, oscillation) of the output devices will be audible (even at very low levels) as increased distortion, and in extreme cases may destroy the transistors.</p>
|
||||
|
||||
|
||||
<hr /><b>3.4.1 - Transistor Oscillation ...</b>
|
||||
<p>It is not realised by everyone, but a single unity gain transistor stage can oscillate. Opamps and power amps commonly use emitter followers for their outputs, and failure to isolate the transistor stage from cable effects can (and regularly does) cause the stage to oscillate. All opamps that connect to the outside world (via connectors on the front or rear panel for example) <i>must</i> use a series resistor. Values from 47 ohm up to 220 ohms are usually enough. I use 100 ohms as a matter of course, but lower (or higher) values may be needed, depending on what you are trying to achieve.</p>
|
||||
|
||||
<p class="t-pic"><img src="ampd-f11.png" alt="Figure 11" border="1" /><br />Figure 11 - Lumped Component Transmission Line Causes Emitter Follower To Oscillate</p>
|
||||
|
||||
<p>In simulations and on the lab bench, I have been able to make a single transistor emitter follower circuit oscillate quite happily, with a real transmission line (such as a length of co-axial cable), or a lumped component equivalent of a transmission line, consisting of a 500µH inductor and 100pF as a series tuned circuit. This is shown in Figure 11.</p>
|
||||
|
||||
<p class="t-pic"><img src="ampd-f12.png" alt="Figure 12" border="1" /><br />Figure 12 - Simulator Oscilloscope Display Of Oscillation In Emitter Follower</p>
|
||||
|
||||
<p>This effect is made worse as the source impedance is lowered, but even a base stopper resistor will not prevent oscillation - only the swamping effect of the transmission line by a Zobel network or a series resistance succeeds. In case you were wondering why the oscilloscope take-off point is at the junction of the L and C components, this allows series resonance to amplify the HF component, making it more readily seen.</p>
|
||||
|
||||
<p>For power amplifiers, this problem is solved by using a Zobel network, optionally with a series inductor. For low-level stages, it is more sensible to use a resistor in series with the output. The resistor is normally between 22 and 100 ohms, and this will be seen in <i>all</i> ESP designs where an opamp connects to the outside world (or even an internal cable). A resistor can be used with power amps too, but at the expense of power loss, heat, and loss of damping factor. For a power amp, the output inductor can be replaced by a 1 ohm resistor (sometimes less), but this is extremely rare.</p>
|
||||
|
||||
<hr />
|
||||
|
||||
<p>In my own amp (P3A being the latest incarnation), I did not use an output inductor, but instead made the dominant pole (the capacitance from the collector to base of the Class-A driver) a little larger than you mat see in other designs. This keeps the amp stable under all operating conditions, but at the expense of slew rate (and consequent slew rate limited power at high frequencies). This was initially largely an economic decision, since a couple of ceramic capacitors are much cheaper than an inductor, and the amp was used in large numbers at the time largely for musical instrument amplification, so an extended high frequency response was actually undesirable. Full power bandwidth - the ability of an amp to supply full power over its entire operating frequency range - is a sure way to destroy hearing, HF horn drivers (etc) in a live music situation, so the compromise was not a limitation. P3A does allow the value to be changed, provided you have an oscilloscope and can check for (sometimes parasitic) oscillation. However ...</p>
|
||||
|
||||
<p>There is another reason that a series output inductor may be helpful. It has been suggested (but by whom I cannot remember) that radio frequencies picked up by the speaker leads may be injected back into the input stage via the negative feedback path. When one looks at a typical circuit, this seems plausible, but I have not tested the theory too deeply.</p>
|
||||
|
||||
<p>The basics behind it are not too difficult to work out. Since it is known that there must be a dominant pole in the amplifier's open-loop frequency response (the capacitor shown in all figures including a Class-A amp stage) if it is to remain stable when feedback is applied, it follows that as internal gain decreases with increasing frequency then the output impedance must rise (due to less global feedback). Indeed this is the case, and by the time the frequency is into the MHz regions, there will be negligible loading of any such frequencies by the output stage.</p>
|
||||
|
||||
<p>If appropriate precautions are not taken (as in Figure 4) for the negative feedback return path, then it is entirely likely that RF detection could occur. In my own bi-amped system (which uses the predecessor of the P3A amplifier described above, still without an output inductor), I recently had problems with detection of a local AM radio station. Fitting of RF 'EMI' suppression chokes (basically, loop the speaker cable through a ferrite ring 3 or 4 times) completely eliminated the problem, so I must conclude that it is indeed possible or even probable.</p>
|
||||
|
||||
<p>If an amplifier is ever likely to be connected to 'exotic' (expensive 'audiophile') cables then it is essential that an output inductor is used. As noted above, the inductance has to be limited to prevent high frequency rolloff, and for load impedances down to 4 ohms, the inductance should not exceed about 10µH. In most cases, as many turns as will fit onto a 10 ohm 1W resistor will be sufficient, and the wire used must be thick enough to carry the full speaker current.</p>
|
||||
|
||||
|
||||
<hr /><a id="s35"></a><b>3.5 - Output Current</b>
|
||||
<p>The maximum output current of a power amplifier is often thought to be something that affects the output transistors only, and that adding more transistors will automatically provide more current to drive lower impedances. This is only partially true, because bipolar transistors need base current, and this must come from the driver stage.</p>
|
||||
|
||||
<p>It is common to bias the Class-A driver stage so that it can provide between 1.5 to 5 times the expected base current needed by the output transistors and their drivers. As the current in this stage is lowered, there is likely to be a substantial increase in the distortion, since the current will change by a larger percentage. If the Class-A driver current is too high, there will be too much heat to get rid of, and it is possible to exceed the transistor's maximum ratings. I normally work to a figure of about double the expected output device base current, but in some cases it will be more or less than this. We also have to design around the lowest expected current gain for all transistors used.</p>
|
||||
|
||||
<p>As an example, let's look at a typical power amplifier output stage. Assuming a power supply of ±35V, the maximum output current will be 35 / 8 = 4.375 Amps (an 8 ohm load is assumed). Since we know that there will be some losses in the driver / power transistor combination, we can safely assume a maximum (peak) current of 4A. A suitable power transistor may be specified for a minimum gain (h<sub>FE</sub>) of 25, with a collector current of 4A. The driver transistors will generally have a higher gain - perhaps 50 at a collector current of 250mA. The product of the two current gains is accurate enough for what we need, and this gives a combined h<sub>FE</sub> of 1,000. The peak base current will therefore be 4mA.</p>
|
||||
|
||||
<p>If we choose to use a Class-A driver current of double the expected output device base current, this means that the driver will operate at about 8mA. This could be achieved with a current source, or a bootstrapped circuit using a pair of 2.2k resistors in series. At the maximum voltage swing (close to ±35V), the driver current will be increased to 12mA or decreased to 4mA, depending on the polarity. The current source or bootstrap circuit will maintain a constant current, but the driver has to deal with a current that varies by ±4mA as the current into the load changes.</p>
|
||||
|
||||
<p>If the load impedance is dropped to 4 ohms, the current source will still only be able to provide 8mA, so output current will be limited to 8A - the driver at this point in the cycle has zero current. At the opposite extreme, the driver will have to cope with 16mA when it is turned on fully. At lower impedances, the driver will be able to supply more current, but the current source will steadfastly refuse to provide more than the 8mA it was designed for, so the peak output current will be limited to 8A in one direction (when the current source provides the drive signal and the Class-A driver is turned off), or some other (possibly destructive) maximum current in the opposite polarity.</p>
|
||||
|
||||
<p>But hang on! A Class-A driver is called a Class-A driver because it never turns off - we now have a Class-AB driver, which is not the desired objective and doesn't even work for a single-ended amplifier stage! The power amplifier will clip asymmetrically, and is no longer operating in the linear range - it is distorting.</p>
|
||||
|
||||
<p>Adding more power transistors will provide a very limited benefit, since the maximum base current is still limited by the current source supplying the Class-A driver. In order to obtain maximum power at lower impedances requires that either the gain of the output stage is increased, or the Class-A driver current must be increased. Increasing the gain of the output stage devices is not trivial - you must either use a different topology or higher gain power and driver transistors.</p>
|
||||
|
||||
<p>The design phase of an amplifier follows similar guidelines, regardless of topology. From <a href="amp-basics.htm">Amplifier Basics</a> ...</p>
|
||||
|
||||
<blockquote>
|
||||
<b>Power Output vs. Impedance</b>
|
||||
<blockquote>
|
||||
The power output is determined by the load impedance and the available voltage and current of the amplifier. An amplifier that is capable of a maximum of 2A output current will be
|
||||
unable to provide more just because you want it to. Such an amp will be limited to 16W 'RMS' into 8 ohms, regardless of the supply voltage. Likewise, an amp with a supply voltage
|
||||
of ±16V will be unable to provide more than 16W into 8 ohms, regardless of the available current. Having more current available will allow the amp to provide (for example)
|
||||
32W into 4 ohms (4A peak current) or 64W into 2 ohms (8A peak current), but will give no more power into 8 ohms than the supply voltage will allow.
|
||||
</blockquote>
|
||||
|
||||
<b>Driver Current</b>
|
||||
<blockquote>
|
||||
Especially in the case of bipolar transistors, the driver stage must be able to supply enough current to the output transistors - with MOSFETs, the driver must be able to charge and
|
||||
discharge the gate-source capacitance quickly enough to allow you to get the needed power at the highest frequencies of interest.<br /><br />
|
||||
For the sake of simplicity, if bipolar output transistors have a gain of 20 at the maximum current into the load, the drivers must be able to supply enough base current to allow this.
|
||||
If the maximum current is 4A, then the drivers must be able to supply at least 200mA of base current to the output devices.
|
||||
</blockquote>
|
||||
|
||||
<b>Class-A Driver Stage</b>
|
||||
<blockquote>
|
||||
The stages that come before the drivers must be able to supply sufficient current for the load imposed. The Class-A driver of a bipolar or MOSFET amp must be able to supply enough
|
||||
current to satisfy the base current needs of bipolar drivers, or the gate capacitance of MOSFETs.
|
||||
|
||||
<br /><br />Again, using the bipolar example from above, the maximum base current for the output transistors was 200mA. If the drivers have a minimum specified gain of 50, then
|
||||
their base current will be ...<br />
|
||||
|
||||
<blockquote>
|
||||
200 / 50 = 4mA.
|
||||
</blockquote>
|
||||
|
||||
Since the Class-A driver must operate in Class-A (what a surprise), it will need to operate with a current of 1.5 to 5 times the expected maximum driver transistor base current, to
|
||||
ensure that it never turns off. The same applies with a MOSFET amp that will expect (for example) a maximum gate capacitance charge (or discharge) current of 4mA at the highest
|
||||
amplitudes and frequencies. For the sake of the exercise, we shall assume a Class-A driver (VAS) current of double the base current needs of the drivers ... 8mA.
|
||||
</blockquote>
|
||||
|
||||
<b>Input Stages</b>
|
||||
<blockquote>
|
||||
The input stages of all transistor amps must be able to supply the base current of the Class-A driver. This time, a margin of between 2 and 5 times the expected maximum base current
|
||||
is needed. If the Class-A driver operates with a quiescent current of 8mA, the maximum current will be 12mA (quiescent + driver base current). Assuming a gain of 50 (again), this means
|
||||
that the input stage has to be able to supply 12 / 50 = 240µA, so it must operate at a minimum current of
|
||||
240µA × 2 = 480µA to preserve linearity.
|
||||
</blockquote>
|
||||
|
||||
<b>Input Current</b>
|
||||
<blockquote>
|
||||
The input current of the first stage determines the input impedance of the amplifier. Using the above figures, with a collector current of 480µA, the base current will be
|
||||
4.8µA for input devices with a gain of 100. If maximum power is developed with an input voltage of 1V, then the impedance is 208k ( R = V / I ).
|
||||
|
||||
<br /><br />Since the stage must be biased, we apply the same rules as before - a margin of between 2 and 5, so the maximum value of the bias resistors should be 208 / 2 = 104k.
|
||||
A lower value is preferred, and I suggest that a factor of 5 is more appropriate, giving 208 / 5 = 42k (47k can be used without a problem).
|
||||
</blockquote>
|
||||
</blockquote>
|
||||
|
||||
<p>These are only guidelines (of course), and there are many cases where currents are greater (or smaller) than suggested. The end result is in the performance of the amp, and the textbook approach is not always going to give the result you are after. Remember that higher value resistors mean greater thermal noise, although this is rarely a problem with power amps.</p>
|
||||
|
||||
<p>Be careful if you decide to use a lower than normal feedback resistor, as it may run quite hot. A 100W (8 ohm) amp will have about 28V across the feedback resistance, so a 22k resistor will dissipate 35mW. Reduce that to 1k (which would be silly for a variety of reasons), and dissipation is nearly 800mW. Of course, increasing the amplitude increases dissipation by the square of the voltage, so even a 22k resistor will dissipate over 220mW in a 600W amplifier.</p>
|
||||
|
||||
<p>Reality is different of course - we generally don't listen to full power sinewaves, and normal music keeps the feedback resistor cool enough not to cause problems in the majority of designs. Resistors that are run close to their maximum power (or voltage) ratings have a much shorter life than those that run cool and/or well within voltage ratings. And yes, resistors <i>do</i> have voltage ratings that are independent of their power rating.</p>
|
||||
|
||||
|
||||
<hr /><a id="s4"></a><b>4 - Some Notes on Power Supply Design</b>
|
||||
<p>When specified, transformer regulation is based upon a resistive load over the full cycle, but when used in a capacitor input filter (99.9% of all amplifier power supplies), the quoted and measured figures will never match.</p>
|
||||
|
||||
<p>Since the applied AC from the transformer secondary spends so much of its time at a voltage lower than that of the capacitor, there is no diode conduction. During the brief periods when the diode conducts, the transformer has to replace all energy drained from the capacitor in the intervening period between diode conductions, as well as provide instantaneous output current.</p>
|
||||
|
||||
<p>Consider a power supply as shown in Figure 13. This is a completely conventional full-wave capacitor input filter (it is shown as single polarity for convenience). The circuit is assumed to have a total effective series resistance of 1 Ohm - this is made up by the transformer winding resistances (primary and secondary). The capacitor C1 has a value of 4,700µF. The transformer has a secondary voltage of 28V. Diodes will lose around 760mV at full power.</p>
|
||||
|
||||
<p class="t-pic"><img src="ampd-f13.png" alt="Figure 13" border="1" /><br />Figure 13 - Full Wave, Capacitor Input Filter Rectifier</p>
|
||||
|
||||
<p>The transformer is rated at 60VA and has a primary resistance of 4.3 Ohms, and a secondary resistance of 0.5 Ohms. This calculates to an internal copper loss resistance of 1.0 Ohm.</p>
|
||||
|
||||
<p>With a 20 Ohm load as shown and at an output current of 1.57A, diode conduction is about 3.5ms, and the peak value of the current flowing into the capacitor is 4.8A - 100 times per second (10ms interval). Diode conduction is therefore 35% of the cycle. RMS current in the transformer secondary is 2.84A.</p>
|
||||
|
||||
<blockquote>
|
||||
<table style="width:700px">
|
||||
<tr><td>Secondary AC Amps</td><td>2.84A RMS</td><td>6.4A Peak</td></tr>
|
||||
<tr><td>Secondary AC Volts (loaded)</td><td>25.9V RMS</td><td>34.1V Peak</td></tr>
|
||||
<tr><td>Secondary AC Volts (unloaded)</td><td>28.0V RMS</td><td>39.6V Peak</td></tr>
|
||||
<tr><td>DC Current</td><td>1.57A</td><td></td></tr>
|
||||
<tr><td>Capacitor Ripple Current<td>2.36A
|
||||
<tr><td>DC Voltage (loaded)</td><td>31.6V</td><td></td></tr>
|
||||
<tr><td>DC Voltage (unloaded)</td><td>38.3V</td><td></td></tr>
|
||||
<tr><td>DC Ripple Voltage</td><td>692mV RMS</td><td>2.2V Peak-Peak</td></tr>
|
||||
</table>
|
||||
</blockquote>
|
||||
|
||||
<p>Ripple across the load is 2.2V peak-peak (692mV RMS), and is the expected sawtooth waveform. Average DC loaded voltage is 31.6V. The no-load voltage of this supply is 38.3V, so at a load current of 1.57A, the regulation is ...</p>
|
||||
|
||||
<blockquote>
|
||||
Reg (%) = (( V<sub>n</sub> - V<sub>l</sub> ) / Vn ) × 100
|
||||
</blockquote>
|
||||
|
||||
<p>Where V<sub>n</sub> is the no-load voltage, and V<sub>l</sub> is the loaded voltage
|
||||
|
||||
<p>For this example, this works out to close enough to 17% which is hardly a good result. By comparison, the actual transformer regulation would be in the order of 8% for a load current of 2.14A at 28V. Note that the RMS current in the secondary of the transformer is 2.84A AC (approximately the DC current multiplied by 1.8) for a load current of 1.57A DC - this must be so, since otherwise we would be getting something for nothing - a practice frowned upon by physics and the taxman.</p>
|
||||
|
||||
<p>Output power is 31.6V × 1.57A = 49.6W, and the input is 28V × 2.84A = 79 VA.</p>
|
||||
|
||||
<p>The input <i>power</i> to the transformer is 60W, so power factor is ...</p>
|
||||
|
||||
<blockquote>
|
||||
PF (Power Factor) = Actual Power / Apparent Power = 60 / 79 = 0.76
|
||||
</blockquote>
|
||||
|
||||
<p>There are many losses to account for, with most being caused by the diode voltage drop (600mW each diode - 2.4W total) and winding resistance of the transformer (8W at full load). Even the capacitors ESR (equivalent series resistance) adds a small loss, as does external wiring. There is an additional loss as well - the transformer core's 'iron loss' - being a combination of the current needed to maintain the transformer's flux level, plus eddy current losses which heat the core itself. Iron loss is most significant at no load and can generally be ignored at full load.</p>
|
||||
|
||||
<p>Even though the transformer is overloaded for this example, provided the overload is short-term no damage will be caused. Transformers are typically rated for average power (VA), and can sustain large overloads as long as the average long-term rating is not exceeded. The duration of an acceptable overload is largely determined by the thermal mass of the transformer itself.</p>
|
||||
|
||||
|
||||
<table>
|
||||
<tr><td valign="top"><img src="note.gif" alt="NOTE!"></td>
|
||||
<td class="t_12"><b>Capacitor Ripple Current</b> - It is well known that bigger transformers have better efficiency that small ones, so it is a common practice to use a
|
||||
transformer that is over-rated for the application. This can improve the regulation considerably, but also places greater stresses on the filter capacitor due to higher
|
||||
ripple current. This is quoted in manufacturer data for capacitors intended for use in power supplies, and must not be exceeded. Excessive ripple current will cause
|
||||
overheating and eventual failure of the capacitor.
|
||||
|
||||
<p>Large capacitors usually have a higher ripple current rating than small ones (both physical size and capacitance). It is useful to know that two 4,700µF caps will
|
||||
usually have a higher combined ripple current than a single 10,000µF cap, and will also show a lower ESR (equivalent series resistance). The combination will generally
|
||||
be cheaper as well - one of the very few instances where you really can get something for nothing.</td></tr>
|
||||
</table>
|
||||
|
||||
<p>For further reading on this topic, see the <a href="power-supplies.htm" target="_blank">Linear Power Supply Design</a> article.</p>
|
||||
|
||||
|
||||
<hr /><a id="s5"></a><b>5 - Measurements Versus Subjectivity</b>
|
||||
<p>If I <i>never</i> hear someone complaining that "distortion measurements are invalid, and a waste of time" again, it will be too soon. I am so fed up with self-proclaimed experts (where 'x' is an unknown quantity, and a 'spurt' is a drip under pressure) claiming that 'real world' signals are so much more complicated than a sinewave, and that static distortion measurements are completely meaningless. Likewise, some complain that sinewaves are 'too simple', and that somehow they fail to stress an amplifier as much as music will.</p>
|
||||
|
||||
<p>Measurements <b><i>are not</i></b> meaningless, and real world signals <b><i>are</i></b> sinewaves! The only difference is that with music, there is usually a large number of sinewaves, all added together. There is not a myriad of simultaneous signals passing through an amp, just one (for a single channel, naturally).</p>
|
||||
|
||||
<p>Since physics tells us that no two masses can occupy the same physical space at the same time, so it is with voltages and currents. There can only ever be one value of voltage and one value of current flowing through a single circuit element at any instant of time - if it were any different, the concept of digital recording could never exist, since in a digital recording the instantaneous voltage is sampled and digitised at the sampling rate. This would clearly be impossible if there were say 3 different voltages all present simultaneously.</p>
|
||||
|
||||
<p>So, how do these x-spurts determine if an amplifier has a tiny bit of crossover distortion (for example). I can see it as the residual from my distortion meter, and it is instantly recognisable for what it really is, and I can see the difference when I make a change to a circuit to correct the problem. If I had to rely on my ears (which although getting older, still work quite well), It would take me much longer to identify the problem, and even longer to be certain that it was gone. I'm not talking about the really gross crossover distortion that one gets from an under-biased amp, I am referring to <i>vestiges</i> - miniscule amounts that will barely register on the meter - I use my oscilloscope to see the exact distortion waveform. I suspect that this dilemma is 'solved' by some by simply not using the push-pull arrangement at all, thereby ensuring that power is severely limited, and other distortion is so high that they would not dare to publish the results.</p>
|
||||
|
||||
<p>These same x-spurts may wax lyrical about some really grotty single ended triode amp, with almost no power and a highly questionable output transformer, limited frequency response and a damping factor of unity if it is lucky.</p>
|
||||
|
||||
<p>Don't get me wrong - I'm not saying that this is a definition of single-ended triode amps (for example), there are some which I am sure sound very nice - not my cup of tea, but 'nice'. I have seen circuits published on the web that I would not use to drive a clock radio speaker (no names, so don't ask), and 'testimonials' from people who have purchased this rubbish, but there are undoubtedly some that do use quality components and probably sound ok at low volume levels.</p>
|
||||
|
||||
<p>Sorry if I sound vehement (vitriolic, even), but quite frankly this p****s me off badly. There are so many people waving their 'knowledge' about, and many of them are either pandering to the Magic Market, or talking through their hats.</p>
|
||||
|
||||
<p>The whole idea of taking measurements is to ensure that the product meets some quality standard. Once this standard is removed and we are expected to let our ears be the judge, how are we supposed to know if we got what we paid for? If the product turns out to sound 'bad', should we accept this, or perhaps we should listen to it for long enough that we get used to the sound (this will happen - eventually - it's called 'burn-in' by the subjectivists). I am not willing to accept this, and I know that many others feel the same.</p>
|
||||
|
||||
<p>Please don't think that I am advocating specsmanship, because I'm not. I just happen to think that consumers are entitled to some minimum performance standard that the equipment should meet (or exceed). I have yet to hear <b><i>any</i></b> amplifier with high distortion levels and/or limited bandwidth sound better than a similar amplifier with lower distortion and wider bandwidth. This implies that we compare like with like - a comparison between a nice valve amp and a nasty transistor amp will still show the transistor amp as having better specs, but we can be assured that it will sound worse. In similar vein, a nice transistor amp compared against a rather poor valve amp may cause some confusion, often due to low damping from the valve amp which makes it easy to imagine that it sounds 'better'.</p>
|
||||
|
||||
<p>We need measurements, because they tell us about the things that we often either can't hear, or that may be audible in a way that confuses our senses. Listening tests are also necessary, but they <i>must</i> be properly conducted as a true blind A-B test or the results are meaningless. Sighted tests (where you know exactly which piece of gear you are listening to) are fatally flawed and will almost always provide the expected outcome.</p>
|
||||
|
||||
|
||||
<hr /><a id="s51"></a><b>5.1 - Valves Vs. Transistors Vs. MOSFETs</b>
|
||||
<p>This is an argument that has been going for years, and it seems we are no closer to resolving the dilemma than we ever were. I have worked with all three, and each has its own sonic quality. Briefly, we shall have a look at the differences - this is not an exhaustive list, nor is it meant to be - these are the main points, influenced by my own experiences (and I must admit, prejudices). Please excuse the somewhat random order of the comparisons ...</p>
|
||||
|
||||
<hr />
|
||||
<blockquote>
|
||||
<p><b>Valves:</b>
|
||||
<br />Valves are Voltage to Current Converters, so the output current is controlled by an input voltage. It is necessary to apply the varying output current to a load (the anode resistor or transformer) to derive an output voltage.
|
||||
|
||||
<ul>
|
||||
<li>Valves themselves are inherently passably linear, and can operate with no feedback at all within a restricted range, and still provide a high quality
|
||||
signal. The range is usually more than sufficient for preamps, but is pushed to its limits in power amplifiers.<br /><br /></li>
|
||||
|
||||
<li>Relatively low gain per device, meaning that more are needed, or less feedback can be used.<br /><br /></li>
|
||||
|
||||
<li>'Soft' distortion characteristics, meaning that most of the distortion is low order (including crossover distortion and clipping) - this is not (quite) as
|
||||
obtrusive or fatiguing as 'hard' distortion.<br /><br /></li>
|
||||
|
||||
<li>Distortion onset is gradual, and effectively warns the listener that the limits are being approached by losing clarity, but in a manner that is not
|
||||
too obtrusive.<br /><br /></li>
|
||||
|
||||
<li>Distortion is usually measurable at nearly any power level, but is low order (mainly 2nd and 3rd harmonics - small amounts of additional harmonics
|
||||
are usually also present).<br /><br /></li>
|
||||
|
||||
<li>Limited feedback, mainly due to the fact that the output transformer introduces low and high frequency phase shift, so large amounts of global feedback
|
||||
are generally not possible without oscillation. This results in a (comparatively) limited bandwidth.<br /><br /></li>
|
||||
|
||||
<li>High output impedance, meaning that damping factor in power amps is generally rather poor. Extremely low values of output impedance are very difficult
|
||||
to achieve (although it can be done at considerable extra expense).<br /><br /></li>
|
||||
|
||||
<li>Valves have a perfect dielectric (mainly a vacuum, with some mica), leading to a highly linear Miller capacitance - it is unknown if this contributes
|
||||
any audible benefit.<br /><br /></li>
|
||||
|
||||
<li>Inefficient output stage, allowing the amp to sound louder than it really is on a watt for watt basis. This may sound like a contradiction, but a valve
|
||||
amp has a 'compliant' output, that allows it to provide a larger voltage swing to high impedance loads (such as a loudspeaker driver at resonance).<br /><br /></li>
|
||||
|
||||
<li>Fairly rugged, and can withstand short circuits without damage - BUT open circuits can cause the output transformer to create high flyback voltages
|
||||
that can cause insulation breakdown in the transformer windings or the valve sockets (short circuits are OK, open circuits are bad)<br /><br /></li>
|
||||
|
||||
<li>Usually quite tolerant of difficult loads, such as electrostatic loudspeakers.<br /><br /></li>
|
||||
|
||||
<li>A wonderful nostalgia value, which allows people to accept the shortcomings, and truly believe that the amp really does sound better than a really
|
||||
good solid-state unit. Proper double-blind testing will usually reveal the truth - provided that the solid-state equivalent is modified to match the
|
||||
output impedance of the valve unit!</li>
|
||||
</ul>
|
||||
</blockquote>
|
||||
|
||||
<hr />
|
||||
<blockquote>
|
||||
<p><b>Transistors (BJTs):</b>
|
||||
<br />By default, bipolar transistors are Current to Current Converters. This means that they use an input current change to derive an output current change that is greater than the input (therefore amplification occurs). Again, it is necessary to use a resistor or other load to allow an output voltage to be developed. It's worth noting that in some texts you will see that the author <i>insists</i> that transistors are voltage controlled, but I find this to be at odds with reality. I have always worked with them as current controlled devices, and will continue to do so.</p>
|
||||
|
||||
<ul>
|
||||
<li>Transistors are also quite linear within a restricted range, but due to the lower operating voltages usually cannot successfully be used without
|
||||
feedback if a very high quality signal is desired, even in preamp stages.<br /><br /></li>
|
||||
|
||||
<li>High to very high gain per device, allowing local feedback to linearise the circuit before the application of global feedback.<br /><br /></li>
|
||||
|
||||
<li>Onset of distortion is sudden and without warning in most feedback topologies.<br /><br /></li>
|
||||
|
||||
<li>Low to very low distortion, provided clipping is not introduced. This creates both the low order harmonics of the valve amp, plus high order harmonics
|
||||
which may be very fatiguing.<br /><br /></li>
|
||||
|
||||
<li>Wide to very wide bandwidth, and low phase shift, largely due to the elimination of the output transformer. The wide bandwidth is obviously an advantage,
|
||||
the phase response is highly debatable as to its overall value to the listener.<br /><br /></li>
|
||||
|
||||
<li>Usually large amounts of global feedback, which is needed to linearise the output stage, especially at the crossover point between output devices
|
||||
(0 Volts) for power amplifiers.<br /><br /></li>
|
||||
|
||||
<li>Completely oblivious to open circuit loads, but must be protected against instant damage with short circuited outputs (open circuits are OK, short
|
||||
circuits are bad - i.e. the opposite of valves)<br /><br /></li>
|
||||
|
||||
<li>The Miller capacitance of transistors has an imperfect dielectric, and varies with applied voltage. This might be the reason that some transistor amps
|
||||
can be seen to oscillate at a specific voltage level (small bursts of oscillation on the waveform, but only above a certain voltage across the device).
|
||||
Tricky.<br /><br /></li>
|
||||
|
||||
<li>Intolerant of difficult loads, unless extensive measures are taken to ensure stability. This can increase complexity quite dramatically.</li>
|
||||
</ul>
|
||||
</blockquote>
|
||||
|
||||
<hr />
|
||||
<blockquote>
|
||||
<p><b>MOSFETs:</b>
|
||||
<br />Like valves, MOSFETs are voltage to current converters, and rely on a voltage on the gate to control the output current. As before, a resistor or other load converts the varying current into a voltage. Here I discuss lateral (designed for audio) MOSFETs, not switching types. HEXFETs and similar switching MOSFETs (vertical MOSFETs) are not really suited to linear operation, and have some interesting failure mechanisms just waiting to bite you. So, for lateral MOSFETs ...</p>
|
||||
|
||||
<ul>
|
||||
<li>Similar to most of the comments about bipolar transistors, with the following differences:<br /><br /></li>
|
||||
|
||||
<li>Onset of (clipping) distortion is (usually) not quite as savage as transistors, but is much more sudden than valves. This is a very minor difference,
|
||||
and can safely be ignored.<br /><br /></li>
|
||||
|
||||
<li>May not be as linear as valves or bipolar transistors, especially near the cutoff region. Big differences between different types (lateral/ vertical)<br /><br /></li>
|
||||
|
||||
<li>More efficient than valves, but not as efficient as bipolar transistors. There will always be less output voltage swing available from a MOSFET amp than a
|
||||
bipolar transistor amp (for the same supply voltage), unless an auxiliary power supply is used.<br /><br /></li>
|
||||
|
||||
<li>Gain is (usually) higher than valves, but lower than bipolar transistors - limiting the ability to apply local feedback, and even overall (global) feedback may
|
||||
not produce distortion figures as good as bipolar transistors - especially with vertical MOSFETs.<br /><br /></li>
|
||||
|
||||
<li>Low distortion (lateral types), but may require more gain in the preceding stages to allow sufficient feedback to eliminate crossover distortion.<br /><br /></li>
|
||||
|
||||
<li>Very wide bandwidth (better than bipolar transistors), allowing less compensation and full power operation up to 100 kHz in some amps - the value of this
|
||||
is debatable.<br /><br /></li>
|
||||
|
||||
<li>More rugged than bipolar transistors, and do not suffer from second breakdown effects - fuses can be used for short circuit protection, and no open circuit
|
||||
protection is needed.<br /><br /></li>
|
||||
|
||||
<li>Reasonably tolerant of difficult loads without excessive circuit complexity.</li>
|
||||
</ul>
|
||||
</blockquote>
|
||||
|
||||
<p>To complicate matters, there are two main types of MOSFET as stated at above - lateral and vertical. This applies to the internal construction. Lateral MOSFETs are well suited to audio (see Project 101), while vertical (e.g. HEXFETs) are designed for high speed switching, and are not really suitable for audio. Despite this, it is possible to make an amplifier using HEXFETs that performs well, and this has been achieved by many hobbyists and manufacturers.</p>
|
||||
|
||||
<p>Thermal stability is critical with vertical MOSFETs, and a very good bias servo is essential. Because of their high transconductance (and wide parameter spread), when used in parallel for audio they need to be matched for gate threshold voltage. If this isn't done, the device with the lowest gate threshold will take most of the current, causing it to get hotter, meaning that it will take even more of the current. This will lead to output stage failure.</p>
|
||||
|
||||
<p>Lateral MOSFETs do not have this problem, because they have a relatively high R<sub>DS-On</sub> (on resistance), and they share current well despite gate threshold voltage differences.</p>
|
||||
|
||||
<hr />
|
||||
|
||||
<p>Because of the differences outlined above it is very important to compare like with like, since each has its own strengths and problems. Also, each of the solid state amp types has its niche area, where it will tend to outperform the other, regardless of specifications. The valve amp is the odd man out here, as it is far more likely to have devoted fans who would use nothing else - most solid state amp users are (or should be) a pragmatic lot, using the most appropriate configuration for the intended application.</p>
|
||||
|
||||
<p>There is no such thing at the time of writing as the much sought after (but elusive) 'straight wire with gain'. But wait - there's more ....</p>
|
||||
|
||||
|
||||
<hr /><a id="s7"></a><b>7 - Slew Rate and Intermodulation</b>
|
||||
<p>Another aspect of amplifier design is slew rate. Slew-rate simply refers to the rate of change of voltage in a given time. It's typically quoted in volts per microsecond (V/µs). This term and (more to the point) its effects are not well understood, and the possible effects are often taken to extremes to 'prove' a point. In reality, no competent amplifier will show any sign of 'slew induced distortion' or undesirable behaviour with any normal music signal. Virtually <i>any</i> amplifier can be forced into slew-rate limiting if pushed to a high enough frequency at full power.</p>
|
||||
|
||||
|
||||
<hr /><a id="s71"></a><b>7.1 - Slew Rate Nomograph</b>
|
||||
<p>It has been claimed by many writers on the subject that a slew-rate limited amplifier will introduce transient intermodulation distortion, or TIM (aka TID - transient induced distortion). In theory, this is perfectly true, provided that the slew rate is sufficiently low as to be within the audible spectrum (i.e. below 20 kHz), and the program material has sufficient output voltage at high frequencies to cause the amplifier to limit in this fashion.</p>
|
||||
|
||||
<p>The following nomograph is helpful in allowing you to determine the required slew rate of any amplifier, so that it can reproduce the required audio bandwidth without introducing distortion components as a result of not being fast enough.</p>
|
||||
|
||||
<p class="t-pic"><img src="ampd-f14.png" alt="Figure 14" border="1" /><br />Figure 14 - Slew Rate Nomograph</p>
|
||||
|
||||
<p>To use this nomograph, first select the maximum frequency on the top row. Let's assume 30kHz as an example. Next, select the actual output voltage (peak, which is RMS × 1.414) that the amplifier must be able to reproduce. For a 100W 8 Ohms amp, this is 28V RMS, or 40V peak. Now draw a line through these two points as shown, and read the slew rate off the bottom row. For the example, this is 8V/µs. This is in fact far in excess of what is really needed, since it is not possible for an amp reproducing music to have anywhere near full power at 30kHz.</p>
|
||||
|
||||
<p>By 20kHz, our 100W amp will need an output of perhaps 10W (typically much less), and this is only about 12V peak. Using the nomograph with this data reveals that a slew rate of about 2V/µs is quite sufficient. Such an amp will go into what is known as slew-rate limiting at full power with frequencies above 10kHz or so, converting the input sinewave into a triangular wave whose amplitude decreases with increasing frequency.</p>
|
||||
|
||||
<p>Some claim that this is audible, and although this is largely subjective it can be measured by a variety of means. That a typical audio signal is a complex mixture of signals is of no real consequence, because an amp has no inherent concept of 'complex' any more than it has an opinion about today's date or the colour of your knickers. At any given point in time, there is an instantaneous value of input voltage that must be increased in amplitude and provide the current needed to drive the loudspeaker. As long as this input voltage does not change so fast that the amplifier cannot keep up with the change then little or no degradation should occur, other than (hopefully) minor non-linearities that represent distortion.</p>
|
||||
|
||||
<p>Although this is a fine theory, there seems to be much entrenched prejudice against 'slow' amplifiers. Whether they sound different from another that is not constrained by slew rate limiting within the full audible range remains debatable. These differences are easily measured, but may be irrelevant when the system is used for music, which simply does not have very fast rise or fall times.</p>
|
||||
|
||||
<p>As shown above, the slew rate of an amplifier is usually measured in Volts / microsecond, and is a measure of how fast the amplifier's output can respond to a rapidly changing input signal. Few manufacturers specify slew rate these days (mainly because few buyers understand what it is), but it is an important aspect of an amplifier's design. It's also important to understand that music <i>never</i> contains any signals that produce full power at 10 or 20kHz. It's generally accepted that the amplitude falls at ~6dB/octave above 1-2kHz, so a 100W amp with a peak output of 40V won't be called upon to provide much above 5V (peak) at 20kHz. There will always be exceptions, and it's safer to assume and plan for at least 10V peak at 20kHz. More doesn't hurt anything, but usually doesn't make an audible difference (assuming a proper double blind test of course).</p>
|
||||
|
||||
<p>As can be seen from the above, for an amplifier (of any configuration) to reproduce 28V RMS at 20 kHz (about 100W / 8 Ohms) requires a slew rate of 4.4 V / µs. This is to say that the output voltage can change (in either direction) at the rate of 8 Volts in one microsecond. This is not especially fast, and as should be obvious, is dependent upon output voltage. A low power amp need not slew as fast as a higher powered amp. There is no real requirement for any amp to be able to slew faster than this, as there is a significantly large margin provided already. This can be calculated or measured.</p>
|
||||
|
||||
<p>Doubling the amplifier's output voltage (four times the power) requires that the slew rate doubles, and vice versa, so a 400W amp needs a slew rate of 8.8 V / µs, while a 25W amp only needs 2.2 V /µs. This is a very good reason to use a smaller amplifier for tweeters in a triamped system, since it is much easier to achieve a respectable slew rate when vast numbers of output devices are not required.</p>
|
||||
|
||||
<p>Essentially, if the amplifier's output cannot respond to the rapidly changing input signal, an error voltage is developed at the long-tailed pair stage, which tries to correct the error. The LTP is an amplifier, but more importantly, an error amplifier, whose sole purpose is to keep both of its inputs at the same voltage. This is critical to the operation of a solid state amp, and the LTP output will generally be a very distorted voltage and current waveform, producing a signal that is the exact opposite of all the accumulated distortions within the remainder of the amp (this also applies to opamps).</p>
|
||||
|
||||
<p>The result is (or is supposed to be) that the signal applied to the inverting input is an inverted <i>exact</i> replica of the input signal. Were this to be achieved in practice, the amp would have no distortion at all. In reality, there is always some small difference, and if the Class-A driver or some other stage enters (or approaches) the slew rate limited region of operation, the error amp (LTP) can no longer compensate for the error.</p>
|
||||
|
||||
<p>Once this happens distortion rises, but more importantly, the input signal is exceeding the capabilities of the amplifier, and the intermodulation products rise dramatically. Intermodulation distortion is characterised by the fact that a low frequency signal modulates the amplitude (and / or shape) of a higher frequency signal, generating additional frequencies that were not present in the original signal. This also occurs when an amplifier clips, or if it has measurable crossover distortion.</p>
|
||||
|
||||
<p>Sounds like ordinary distortion, doesn't it? That too creates frequencies that were not in the original, but the difference is that harmonic distortion creates harmonics (hence the name), whereas intermodulation distortion creates frequencies that have no harmonic relationship to either of the original frequencies. Rather, the new frequencies are the sum and difference of the original two frequencies. (This effect is used extensively in radio, to create the intermediate frequency from which the audio, video or other wanted signal can be extracted.) The term 'harmonic' basically can be translated to 'musical', and 'non-harmonic' is mathematically derived, but not musically related .... if you see what I mean.</p>
|
||||
|
||||
<p>Whenever the LTP (error amplifier) loses control of the signal, intermodulation products will be generated, so the bandwidth of an amplifier must be wide enough to ensure that this cannot happen with any normal audio input signal. There is nothing wrong or difficult about this approach, and it is quite realisable in any modern design. Although unrealistic from a musical point of view, it is better if an amplifier is capable of reproducing full power at the maximum audible frequency (20 kHz) than if it starts to go into slew rate limiting at some lower frequency.</p>
|
||||
|
||||
<p>The reason I say it is unrealistic musically is simply because there is no known instrument - other than a badly set up synthesiser - that is capable of producing any full power harmonic at 20 kHz, so in theory, the amp does not have to be able to reproduce this. In reality, inability to reproduce full power at 20 kHz means that the amp <i>might</i> suffer from some degree of transient intermodulation distortion with <i>some</i> program material. Or it might not.</p>
|
||||
|
||||
<p>This is not a problem that affects simple amps with little or no feedback - they generate enough harmonic distortion to more than make up for the failings of more complex circuits with lots of global feedback. This fact tends to annoy the minimalists, who are often great believers in no feedback under any circumstance, which relegates them to listening to equipment that would have been considered inferior in the 1950s.</p>
|
||||
|
||||
<p>If preferred, you can calculate the slew rate of any signal at any amplitude. Use the formula ...</p>
|
||||
|
||||
<blockquote>
|
||||
ΔV / Δt = 2<span class="times">π</span> × f × V<sub>Peak</sub>
|
||||
</blockquote>
|
||||
|
||||
<p>ΔV / Δt is the slew rate (change of voltage vs change of time). V<sub>Peak</sub> is the peak voltage of the sinewave. For example, if you use a voltage of 40V (peak) and a frequency of 20kHz, you get 5,026,548V/s, which is (close enough to) 5.03V/µs.</p>
|
||||
|
||||
|
||||
<hr /><a id="s8"></a><b>8 - Frequency Response, etc.</b>
|
||||
<p>Few sensible people would argue that measurements of frequency response are unimportant or irrelevant, and this is one of the simplest measurements to take on an amplifier. Again, the subjectivists would have it that these fail to take into account some mysterious area of our brain that will compensate for a restricted response, and allow us to just enjoy the experience of the sound system. This is true - we will compensate for diminished (or deranged) frequency response, but it need not be so.</p>
|
||||
|
||||
<p>If you listen to a clock radio for long enough, your brain will think that this is normal, and will adjust itself accordingly. Imagine your surprise when you hear something that actually has real low and high frequencies to offer - the first reaction is that there is too much of everything, but again, the brain will make the required allowances and this will sound normal after a time.</p>
|
||||
|
||||
<p>There are so many standard measurements on amplifiers that are essential to allow us to make an informed judgement (is this amp even worth listening to?). I really object to the attitude that "it does not matter what the measurements say, it sounds great". In reality this is rarely the case - if it measures as disgusting, then it will almost invariably sound disgusting. There is no place for hi-fi equipment that simply does not meet some basic standards - and I have <i>never</i> heard an amp that looked awful on the oscilloscope, measured as awful on my distortion meter, but sounded good - period. I have heard some amps that fall into that category that sound 'interesting' - not necessarily bad, but definitely not hi-fi by any stretch of the imagination. To the dyed-in-the-wool subjectivist, it seems that 'different' means 'better', regardless of any evidence one way or another.</p>
|
||||
|
||||
|
||||
<hr /><a id="s9"></a><b>9 - Designs to Avoid</b>
|
||||
<p>There are some designs that should simply be avoided. Two in particular are shown here, but this doesn't mean that there aren't others as well. The two shown below suffer from a number of problems, with the biggest issue being thermal stability. This is by no means all though - the first to avoid is shown in Figure 14, and includes a compound (Sziklai) pair for comparison. As you can see, the 'output stage with gain' (output 1) simply breaks the feedback loop within the compound pair and adds resistors. The gain is directly proportional to the resistor divider ratio, so the gain is 3.2 with 220 ohms and 100 ohms as shown. The problem is that this applies to DC as well as AC, so the stage amplifies its own thermal instability. Because of the relatively high output impedance, the actual gain will be less than calculated.</p>
|
||||
|
||||
<p class="t-pic"><img src="ampd-f15.png" alt="Figure 15" border="1" /><br />Figure 15 - Output Stage with Gain (Sziklai Pair for Comparison)</p>
|
||||
|
||||
<p>Why would anyone bother? The stage has the advantage that having gain, so it can be driven directly by an opamp whose output level would normally be too low to be useful. Several amplifiers have been built using this circuit over the years, and all those I have seen have been thermally unstable, and some also have high frequency instability issues. Because the output stage local feedback is reduced by the amount of gain used, distortion is significantly higher than with a conventional compound pair (for example). In the above circuits, the stage with gain has an open loop distortion of 4%, while the compound pair stage has distortion less than 0.1%. This was simulated using an 8 ohm load - in reality, the distortion difference is usually greater than this, with the gain version showing even higher distortion. A vast amount of negative feedback is needed to make the circuit linear enough to be usable. As noted above, output impedance is also much higher than the compound pair.</p>
|
||||
|
||||
<p>If the circuit is driven by an opamp, the opamp's high gain helps to linearise the output stage, but high frequency instability remains an issue. It can be solved, but usually requires several HF stability networks. Such arrangements are usually easy to coax into oscillation because they tend to have a poor phase margin (the difference between the actual phase shift and 180°, where the amp will oscillate).</p>
|
||||
|
||||
<p>There is no simple cure for the thermal instability though. A single transistor cannot compensate for the quiescent current shift, and a Darlington pair overcompensates. While it is certainly possible to come up with a composite circuit that will work, the complexity is not warranted for an output stage that doesn't perform well at the best of times.</p>
|
||||
|
||||
<p>Another travesty was unleashed many years ago, and fortunately I've not seen it re-surface for many years. I am almost unwilling to post the circuit, lest someone think it's a good idea. It isn't a good idea, and never was. Again, thermal instability was a major problem, and HF instability was also common. The idea was to use an opamp's supply pins to drive output transistors. By loading the opamp, the supply current varies from a couple of milliamps at idle, up to perhaps 20-30mA (depending on the opamp). An example circuit of this disaster is shown below.</p>
|
||||
|
||||
<p class="t-pic"><img src="ampd-f16.png" alt="Figure 16" border="1" /><br />Figure 16 - Opamp Based Power Amplifier</p>
|
||||
|
||||
<p>If you happen to see this circuit (or any of its variations) anywhere, avert your eyes immediately <img src="grin.gif" alt=":-)">. I recall messing around with it some time in the 1970s, and while it was (barely) possible to make it reasonably stable (against HF instability), the only way to achieve thermal stability is to use relatively large resistors in the output transistor emitters. This limits the output power, but it is capable of driving headphones, provided you can live with it's other failings which are many and varied. Since there are so many circuits that outperform it (including cheap and cheerful 'power opamps' - IC power amps), there is no reason to consider it for anything other than your own amusement.</p>
|
||||
|
||||
<p>Note that the values shown on these circuits are for example only. I cannot guarantee that the opamp based amp will even work as shown - the circuit is there only so you can see the general arrangement. Since I strongly suggest that you stay well clear of this topology, I do not propose to waste time to ensure that the circuit will function as shown.</p>
|
||||
|
||||
|
||||
<hr /><a id="s10"></a><b>10 - Further Reading</b>
|
||||
<p>For further reading, I can recommend <a href="http://www.douglas-self.com/" target="_blank">The Self Site</a>, and in particular 'Science and Subjectivism in Audio' and also my own article on the subject <a href="cables.htm" target="_blank">Cables, Interconnects & Other Stuff - The Truth</a>. There is also an article called <a href="amp-sound.htm" target="_blank">Amplifier Sound - What Are The Influences?</a> that goes a little deeper into the measured and subjective performance of amplifiers, and suggests a couple of new tests that might be applied.</p>
|
||||
|
||||
|
||||
<hr /><a id="ref"></a><b>References</b>
|
||||
<ol>
|
||||
<li class="t_11">Refer to the Douglas Self Pages</li>
|
||||
<li class="t_11">The Audio Power Interface, Douglas Self, Electronics World September 1997, p717</li>
|
||||
<li class="t_11">Intermodulation at the amplifier-loudspeaker interface, Matti Otala and Jorma Lammasneimi, Wireless World, December 1980, p42</li>
|
||||
<li class="t_11">Douglas Self - actual source unknown (but I did read it in one of his papers!)</li>
|
||||
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<tr><td class="t-wht"><a id="copyright"></a><b>Copyright Notice.</b> This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is Copyright (c) 1999, 2000, 2001. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro- mechanical, is strictly prohibited under International Copyright laws. The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference. Commercial use is prohibited without express written authorisation from Rod Elliott.</td>
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<div class="t-sml">Last Update: 09 Apr - added VFB and CFB info./ 18 Feb - added some minor details (various)./ 09 Apr-Added a few small extras and a correction./ 26 Feb-Added reference to Amp Sound article./ 29 Jan 2000-Fixed a couple of confused statements and typos, added missing component references to Figures 1a and 1b./ 17 Dec-added slew rate nomograph./ 15 Dec 99-added extra info about reference amp, use of inductor in O/P stage, bias servo, and RF stoppers./ 16 Jan 06 - additional comments on MOSFETs, minor reformatting + spelling corrections./ 27 Dec 06 - added 'designs to avoid'.</div><br />
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<h1>Precision Rectifiers</h1>
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<div class="t_11" align="center">Copyright © Rod Elliott (ESP) Jun 2005<br />Updated Jan 2021</div>
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<span class="imgswap"><a href="index.html" style="display:block;"><img src="a1.gif" alt="app notes"/><b class="bb">App. Notes Index</b></a></span><br />
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||||
<hr /><b>First and Second Rules of Opamps</b>
|
||||
<p>To be able to understand much of the following, the basic rules of opamps need to be firmly embedded in the skull of the reader. I came up with these many years ago, and - ignoring small errors caused by finite gain, input and output impedances - all opamp circuits make sense once these rules are understood. They are also discussed in the article <a href="../dwopa.htm" target="_blank">Designing With Opamps</a> in somewhat greater detail. Highly recommended if you are in the least bit unsure.</p>
|
||||
|
||||
<blockquote>The two rules are as follows ...<br />
|
||||
|
||||
<ol>
|
||||
<li>An opamp will attempt to make both inputs exactly the same voltage (via the feedback path)
|
||||
<li>If it cannot achieve #1, the output will assume the polarity of the most positive input
|
||||
</ol></blockquote>
|
||||
|
||||
<p>These two rules describe <i>everything</i> an opamp does in <i>any</i> circuit, with no exceptions ... <i>provided that the opamp is operating within its normal parameters</i>. This means power supply voltage(s) must be within specifications, signal voltage is within the allowable range, and load impedance is equal to or greater than the minimum specified. The signal frequency must also be low enough to ensure that the opamp can perform normally for the chosen gain. For most cheap opamps, a gain of 100 with a frequency of 1kHz should be considered the maximum allowable, since the opamp's open loop gain may not be high enough to accommodate higher gain or frequency.</p>
|
||||
|
||||
<p>Armed with these rules and a basic understanding of Ohm's Law and analogue circuitry, it is possible to figure out what any opamp circuit will do under all normal operating conditions. Needless to say, the rules no longer apply if the opamp itself is faulty, or is operating outside its normal parameters (as discussed briefly above).</p>
|
||||
|
||||
|
||||
<hr /><b>Opamp Selection</b>
|
||||
<p>The choice of opamp determines the highest frequency that can be accommodated. While many of the circuits are shown using a TL072 or similar, these are very limiting. The highest frequency will be well above the audio band, but if you need to rectify higher frequencies you will need something faster. A TL072 will get to about 150kHz, but if you need to rectify (say) 500kHz or so, you need an opamp that has a much higher upper frequency. The LM318 is a reasonable candidate and is fairly cheap. These are rated for a unity gain frequency (gain bandwidth product or GBW) of 15MHz with a 50V/µs slew rate (the TL07x is 3MHz and 13V/µs).</p>
|
||||
|
||||
<p>It's worthwhile reading the article <a href="../articles/frequency-vs-gain.htm" target="_blank">Opamp Bandwidth Vs. Gain And Slew Rate</a>, which goes into detail as to how these factors influence the frequency response of a circuit. If you need to go even higher in frequency, consider using an LM4562 (GBW of 55MHz and 20V/µs). Selection of a suitable candidate isn't always easy, but you don't need to be concerned if you're only interested in the audio range (20Hz - 20kHz).</p>
|
||||
|
||||
|
||||
<hr /><b>Half Wave Precision Rectifier</b>
|
||||
<p>There are many applications for precision rectifiers, and most are suitable for use in audio frequency circuits, so I thought it best to make this the first ESP Application Note. While some of the existing projects in the audio section have a rather tenuous link to audio, this information is more likely to be used for instrumentation purposes than pure audio applications. There are exceptions of course.</p>
|
||||
|
||||
<p>Typically, the precision rectifier is not commonly used to drive analogue meter movements, as there are usually much simpler methods to drive floating loads such as meters. Precision rectifiers are more common where there is some degree of post processing needed, feeding the side chain of compressors and limiters, or to drive digital meters.</p>
|
||||
|
||||
<p>There are several different types of precision rectifier, but before we look any further, it is necessary to explain what a precision rectifier actually is. In its simplest form, a half wave precision rectifier is implemented using an opamp, and includes the diode in the feedback loop. This effectively cancels the forward voltage drop of the diode, so very low level signals (well below the diode's forward voltage) can still be rectified with minimal error.</p>
|
||||
|
||||
<p>The most basic form is shown in Figure 1, and while it does work, it has some serious limitations. The main one is speed - it will not work well with high frequency signals. To understand the reason, we need to examine the circuit closely. This knowledge applies to all subsequent circuits, and explains the reason for the apparent complexity.</p>
|
||||
|
||||
<p class="t-pic"><img src="an001-f1.gif" alt="Figure 1" border="1"><br />Figure 1 - Basic Precision Half Wave Rectifier</p>
|
||||
|
||||
<p>For a low frequency positive input signal, 100% negative feedback is applied when the diode conducts. The forward voltage is effectively removed by the feedback, and the inverting input follows the positive half of the input signal almost perfectly. When the input signal becomes negative, the opamp has no feedback at all, so the output pin of the opamp swings negative as far as it can. Assuming 15V supplies, that means perhaps -14V on the opamp output.</p>
|
||||
|
||||
<p>When the input signal becomes positive again, the opamp's output voltage will take a finite time to swing back to zero, then to forward bias the diode and produce an output. This time is determined by the opamp's slew rate, and even a very fast opamp will be limited to low frequencies - especially for low input levels. The test voltage for the waveforms shown was 20mV at 1kHz. Although the circuit does work very well, it is limited to relatively low frequencies (less than 10kHz) and only becomes acceptably linear above 10mV or so (opamp dependent).</p>
|
||||
|
||||
<p>Note the oscillation at the rectified output. This is (more or less) real, and was confirmed with an actual (as opposed to simulated) circuit. This is the result of the opamp becoming open-loop with negative inputs. In most cases it is not actually a problem. The large voltage swing <i>is</i> a problem though.</p>
|
||||
|
||||
<p class="t-pic"><img src="an001-f2.gif" alt="Figure 2" border="1"><br />Figure 2 - Rectified Output and Opamp Output</p>
|
||||
|
||||
<p>Figure 2 shows the output waveform (left) and the waveform at the opamp output (right). The recovery time is obvious on the rectified signal, but the real source of the problem is quite apparent from the huge voltage swing before the diode. While this is of little consequence for high level signals, it causes considerable non-linearity for low levels, such as the 20mV signal used in these examples.</p>
|
||||
|
||||
<p>The circuit is improved by reconfiguration, as shown in Figure 3. The additional diode prevents the opamp's output from swinging to the negative supply rail, and low level linearity is improved dramatically. A 2mV (peak) signal is rectified with reasonably good accuracy. Although the opamp still operates open-loop at the point where the input swings from positive to negative or vice versa, the range is limited by the diode and resistor. Recovery time is therefore a great deal shorter.</p>
|
||||
|
||||
<p class="t-pic"><img src="an001-f3.gif" alt="Figure 3" border="1"><br />Figure 3 - Improved Precision Half Wave Rectifier</p>
|
||||
|
||||
<p>This circuit also has its limitations. The input impedance is now determined by the input resistor, and of course it is more complicated than the basic version. It must be driven from a low impedance source. Not quite as apparent, the Figure 3 circuit also has a defined output load resistance (equal to R2), so if this circuit were to be used for charging a capacitor, the cap will also <i>discharge</i> through R2. Although it would seem that the same problem exists with the simple version as well, R2 (in Figure 1) can actually be omitted, thus preventing capacitor discharge. Likewise, the input resistor (R1) shown in Figure 1 is also optional, and is needed only if there is no DC path to ground.</p>
|
||||
|
||||
|
||||
<hr /><b>Full Wave Precision Rectifiers</b>
|
||||
<p>Figure 4 shows the standard full wave version of the precision rectifier. This circuit is very common, and is pretty much the textbook version. It has been around for a very long time now, and I would include a reference to it if I knew where it originated. The tolerance of R1, 2, 3, 4 and 5 are critical for good performance, and all five resistors should be 1% or better. Note that the diodes are connected to obtain a positive rectified signal. The second stage inverts the signal polarity. To obtain improved high frequency response, the resistor values should be reduced.
|
||||
|
||||
<p class="t-pic"><img src="an001-f4.gif" alt="Figure 4" border="1"><br />Figure 4 - Precision Full Wave Rectifier</p>
|
||||
|
||||
<p>This circuit is sensitive to source impedance, so it is important to ensure that it is driven from a low impedance, such as an opamp buffer stage. Input impedance as shown is 6.66k, and any additional series resistance at the input will cause errors in the output signal. The input impedance is linear. As shown, and using TL072 opamps, the circuit of Figure 4 has good linearity down to a couple of mV at low frequencies, but has a limited high frequency response. Use of high speed diodes, lower resistance values and faster opamps is recommended if you need greater sensitivity and/ or higher frequencies.</p>
|
||||
|
||||
|
||||
<hr /><b>The Alternative (Analog Devices)</b>
|
||||
<p>A little known variation of the full wave rectifier was published by Analog Devices, in Application Brief AB-109 <sup>[<a href="an001.htm#ref"> 1 </a>]</sup>. In the original, a JFET was used as the rectifier for D2, although this is not necessary if a small amount of low level non-linearity is acceptable. The resistors marked with an asterisk (*) should be matched, although for normal use 1% tolerance will be acceptable. C1 may be needed to prevent oscillation.</p>
|
||||
|
||||
<p class="t-pic"><img src="an001-f5.gif" alt="Figure 5" border="1"><br />Figure 5 - Original Analog Devices Circuit</p>
|
||||
|
||||
<p>It was pointed out in the original application note that the forward voltage drop for D2 (the FET) must be less than that for D1, although no reason was given. As it turns out, this may make a difference for very low level signals, but appears to make little or no difference for sensible levels (above 20mV or so).</p>
|
||||
|
||||
|
||||
<hr /><b>Simplified Alternative</b><br />
|
||||
<p>For most applications, the circuit shown in Figure 6 will be more than acceptable. Linearity is very good at 20mV, but speed is still limited by the opamp. To obtain the best high frequency performance use a very fast opamp and reduce the resistor values.</p>
|
||||
|
||||
<p class="t-pic"><img src="an001-f6.gif" alt="Figure 6" border="1"><br />Figure 6 - Simplified Version of the AD Circuit</p>
|
||||
|
||||
<p>It is virtually impossible to make a full wave precision rectifier any simpler, and the circuit shown will satisfy the majority of low frequency applications. Where very low levels are to be rectified, it is recommended that the signal be amplified first. While the use of Schottky (or germanium) diodes will improve low level and/or high frequency performance, it is unreasonable to expect perfect linearity from any rectifier circuit at extremely low levels. Operation up to 100kHz or more is possible by using fast opamps and diodes. R1 is optional, and is only needed if the source is AC coupled, so extremely high input impedance (with no non-linearity) is possible. C1 may be needed to prevent oscillation.</p>
|
||||
|
||||
<p>The simplified version shown above (Figure 6) is also found in a Burr-Brown application note <sup>[<a href="an001.htm#ref"> 3 </a>]</sup>.</p>
|
||||
|
||||
|
||||
<hr /><b>Another Version</b>
|
||||
<p>Purely by chance, I found the following variant in a phase meter circuit. This version is used in older SSL (Solid Stage Logic) mixers, as part of the phase correlation meter. This circuit exists on the Net in a few forum posts and a site where several SSL schematics are re-published. The original drawing I found is dated 1984. It's also referenced in a Burr-Brown paper from 1973 and an electronics engineering textbook <sup>[<a href="an001.htm#ref"> 5, 6 </a>]</sup>.</p>
|
||||
|
||||
<p class="t-pic"><img src="an001-f6a.gif" alt="Figure 6A" border="1"><br />Figure 6A - Another Version of the AD Circuit</p>
|
||||
|
||||
<p>While it initially looks completely different, that's simply because of the way it's drawn (I copied the drawing layout of the original). This version is interesting, in that the input is not only inverting, but provides the opportunity for the rectifier to have gain. The inverting input is of no consequence (it is a full wave rectifier after all), but it does mean that the input impedance is lower than normal ... although you could make all resistor values higher of course. Input impedance is equal to the value of R1, and is linear as long as the opamp is working well within its limits.</p>
|
||||
|
||||
<p>R6 isn't used in the SSL circuit I have, and while the circuit works without it, there can be a significant difference between the rectified positive and negative parts of the input waveform. Without R6, the loading on D2 is less than that of D1, causing asymmetrical rectification. This resistor is included in the Figure 6 version, and the need for it was found as I was researching precision rectifiers for a project. It's not a problem with normal silicon small-signal diodes (e.g. 1N4148), but it becomes very important if you use germanium or Schottky diodes due to their higher leakage.</p>
|
||||
|
||||
<p>If R1 is made lower than R2-R5, the circuit has gain. If R1 is <i>higher</i> than R2-R5, the circuit can accept higher input voltages because it acts as an attenuator. For example, if R1 is 1k, the circuit has a gain of 10, and if 100k, the gain is 0.1 (an attenuation of 10). All normal opamp restrictions apply, so if a high gain is used frequency response will be affected. C1 is optional - you may need to include it if the circuit oscillates. The value will normally be between 10pF and 100pF, depending on the speed you need and circuit layout.</p>
|
||||
|
||||
<p>One interesting result of using the inverting topology is that the input node is a 'virtual earth' and it enables the circuit to sum multiple inputs. R1 can be duplicated to give another input, and this can be extended. The original SSL circuit used two of these rectifiers with four inputs each. Remember that this is the same as operating the first opamp with a gain of four, so high frequency response may be affected without you realising it.</p>
|
||||
|
||||
<p>The circuits shown in Figures 6 and 6A are the simplest high performance full wave rectifiers I've come across, and are the most suitable for general work with audio frequencies. In most applications, you'll see the Figure 4 circuit, because it's been around for a long time, and most designers know it well. However, it is definitely <i>not</i> the best performer, and has no advantages over the Figure 6 and 6A simpler alternatives, but it uses more parts and has a comparatively low input impedance.</p>
|
||||
|
||||
|
||||
<hr /><p>I've been advised by a reader that Neve also used a similar circuit in their BA374 PPM drive circuit. In the interests of consistency I've shown the resistors (R1-R5 & R8) as 10k, where 51k was used in the original circuit. This doesn't change the way the circuit works, but it reduces resistive loading on the opamps (which doesn't affect low-frequency operation). The amended schematic is shown below.</p>
|
||||
|
||||
<p class="t-pic"><img src="an001-f6b.gif" alt="Figure 6B" border="1"><br />Figure 6B - Neve PPM Rectifier Circuit</p>
|
||||
|
||||
<p>The R/C network (R6, R7 and C1) sets the ballistics of the meter, which is determined by the attack and release times. The output of the rectifier is processed further in the BA374 circuit to provide a logarithmic response which allows the meter scale to be linear. This isn't shown because it's not relevant here. Unfortunately, it's extremely difficult to determine who came up with the idea first. The Neve schematic I was sent is dated 1981 if that helps.</p>
|
||||
|
||||
|
||||
<hr /><b>Another Precision Rectifier (Intersil)</b>
|
||||
<p>A simple precision rectifier circuit was published by Intersil <sup>[<a href="an001.htm#ref"> 2 </a>]</sup>. This is an interesting variation, because it uses a single supply opamp but still gives full-wave rectification, with both input and output earth (ground) referenced. Unfortunately, the specified opamp is not especially common, although other devices could be used. The CA3140 is a reasonably fast opamp, having a slew rate of 7V/µs. I will leave it to the reader to determine suitable types (other than that suggested below). The essential features are that the two inputs must be able to operate at below zero volts (typically -0.5V), and the output must also include close to zero volts.</p>
|
||||
|
||||
<p class="t-pic"><img src="an001-f7.gif" alt="Figure 7" border="1"><br />Figure 7 - Original Intersil Precision Rectifier Circuit</p>
|
||||
|
||||
<p>During the positive cycle of the input, the signal is directly fed through the feedback network to the output. R3 actually consists of R3 itself, plus the set value of VR2. The nominal value of the pair is 15k, and VR2 can be usually be dispensed with if precision resistors are used (R3 and VR2 are replaced by a single 15k resistor).</p>
|
||||
|
||||
<p>This gives a transfer function of ...</p>
|
||||
|
||||
<blockquote>
|
||||
Gain = 1 / ( 1 + (( R1 + R2 ) / R3 )) ... 0.5 with the values shown above
|
||||
</blockquote>
|
||||
|
||||
<p>1V input will therefore give an output voltage of 0.5V. During this positive half-cycle of the input, the diode disconnects the op-amp output, which is at (or near) zero volts. Note that the application note shows a different gain equation which is incorrect. The equation shown above works.</p>
|
||||
|
||||
<p>During a negative half-cycle of the input signal, the CA3140 functions as a normal inverting amplifier with a gain equal to -( R2 / R1 ) ... 0.5 as shown. Since the inverting input is a virtual earth point, during a negative input it remains at or very near to zero volts. When the two gain equations are equal, the full wave output is symmetrical. Note that the output is not buffered, so the output should be connected only to high impedance stage, with an impedance much higher than R3.</p>
|
||||
|
||||
<p class="t-pic"><img src="an001-f8.gif" alt="Figure 8" border="1"><br />Figure 8 - Modified Intersil Circuit Using Common Opamp</p>
|
||||
|
||||
<p>Where a simple, low output impedance precision rectifier is needed for low frequency signals (up to perhaps 10kHz as an upper limit), the simplified version above will do the job nicely. It does require an input voltage of at least 100mV because there is no DC offset compensation. Expect around 30mV DC at the output with no signal. Because the LM358 is a dual opamp, the second half can be used as a buffer, providing a low output impedance. The second half of the opamp can be used as an amplifier if you need more signal level. Minimum suggested input voltage is around 100mV peak (71mV RMS), which will give an average output voltage of 73mV. Higher input voltages will provide greater accuracy, but the maximum is a little under 10V RMS with a 15V DC supply as shown. The LM358 is not especially fast, but is readily available at low cost.</p>
|
||||
|
||||
<p><b>Limitations: </b>Note that the input impedance of this rectifier topology is non-linear. The impedance presented to the driving circuit is very high for positive half cycles, but only 10k for negative half-cycles. This means that it must be driven from a low impedance source - typically another opamp. This increases the overall complexity of the final circuit. Note that symmetry can be improved by changing the value of R3. It can be made adjustable by using a 20k trimpot (preferably multi-turn). This isn't necessary unless your input voltage is less than 100mV, and the optimum setting depends on the signal voltage.</p>
|
||||
|
||||
|
||||
<hr /><b>Single Supply Precision Rectifier (B-B/ TI)</b>
|
||||
<p>An interesting variation was shown in a Burr-Brown application note <sup>[<a href="an001.htm#ref"> 3 </a>]</sup>. This rectifier operates from a single supply, but accepts a normal earth (ground) referenced AC input. The only restriction is that the incoming peak AC signal must be below the supply voltage (typically +5V for the OPA2337 or OPA2340). The opamps used must be rail-to-rail, and the inputs must also accept a zero volt signal without causing the opamp to lose control.</p>
|
||||
|
||||
<p>The circuit is interesting for a number of reasons, not the least being that it uses a completely different approach from most of the others shown. The rectifier is not in the main feedback loop like all the others shown, but uses an ideal diode (created by U1B and D1) at the non-inverting input, and this is outside the feedback loop.</p>
|
||||
|
||||
<p class="t-pic"><img src="an001-f9.gif" alt="Figure 9" border="1"><br />Figure 9 - Burr-Brown Circuit Using Suggested Opamp</p>
|
||||
|
||||
<p>For a positive-going input signal, the opamp (U1A) can only function as a unity gain buffer, since both inputs are driven positive. Both the non-inverting and inverting inputs have an identical signal, a condition that can only be achieved if the output is also identical. If the output signal attempted to differ, that would cause an offset at the inverting input which the opamp will correct. It is worth remembering my opamp rules described at the beginning of this app. note.</p>
|
||||
|
||||
<p>For a negative-going input signal, The ideal diode (D1 and U2B) prevents the non-inverting input from being pulled below zero volts. Should this happen, the opamp can no longer function normally, because input voltages are outside normal operating conditions. The opamp (U1A) now functions as a unity gain <i>inverting</i> buffer, with the inverting input maintained at zero volts by the feedback loop. If -10µA flows in R1, the opamp will ensure that +10uA flows through R2, thereby maintaining the inverting input at 0V as required.</p>
|
||||
|
||||
<p><b>Limitations: </b>Input impedance is non-linear, having an almost infinite impedance for positive half-cycles, and a 5k input impedance for negative half-cycles. The input must be driven from an earth (ground) referenced low impedance source. Capacitor coupled sources are especially problematical, because of the widely differing impedances for positive and negative going signals. The <i>maximum</i> source resistance for a capacitor-coupled signal input is 100 ohms for the circuit as shown (one hundredth of the resistor values used for the circuit), and preferably less. The capacitance is selected for the lowest frequency of interest.</p>
|
||||
|
||||
|
||||
<hr /><b>Simple Full Wave Rectifier</b>
|
||||
<p>This rectifier is something of an oddity, in that it is not really a precision rectifier, but it <i>is</i> full wave. It is an interesting circuit - sufficiently so that it warranted inclusion even if no-one ever uses it. This rectifier was used as part of an oscillator <sup>[<a href="an001.htm#ref"> 4 </a>]</sup> and is interesting because of its apparent simplicity and wide bandwidth even with rather pedestrian opamps.</p>
|
||||
|
||||
<p>A simulation using TL072 opamps indicates that even with a tiny 5mV peak input signal (3.5mV RMS) the frequency response extends well past 10kHz but for low level signals serious amplitude non-linearity can be seen. The original article didn't even mention the rectifier, and no details were given at all. However, I have been able to determine the strengths and weaknesses by simulation. Additional weaknesses may show up in use of course. A reader has since pointed out something I should have seen (but obviously did not) - R3 should <i>not</i> be installed. Without R3, linearity is far better than expected.</p>
|
||||
|
||||
<p>It's not known why R3 was included in the original JLH design, but in the case of an oscillator stabilisation circuit it's a moot point. The circuit will always have more or less the same input voltage, and voltage non-linearity isn't a problem.</p>
|
||||
|
||||
<p class="t-pic"><img src="an001-f10.gif" alt="Figure 10" border="1"><br />Figure 10 - Simple Precision Full Wave Rectifier</p>
|
||||
|
||||
<p>One thing that is absolutely critical to the sensible operation of the circuit at low signal levels is that all diodes <i>must</i> be matched, and in excellent thermal contact with each other. The actual forward voltage of the diodes doesn't matter, but all must be identical. The lower signal level limit is determined by how well you match the diodes and how well they track each other with temperature changes.</p>
|
||||
|
||||
<p>The first stage allows the rectifier to have a high input impedance (R1 is 10k as an example only). Nominal gain as shown is 1 (with R3 shorted). R3 was included in the original circuit, but is actually a really bad idea, as it ruins the circuit's linearity. Without it, the circuit is <i>very</i> linear over a 60dB range. This is more than enough for any analogue measurement system.</p>
|
||||
|
||||
<p><b>Limitations: </b>Linearity is very good, but the circuit requires closely matched diodes for low level use because the diode voltage drops in the first stage (D1 & D2) are used to offset the voltage drops of D3 & D4. At input voltages of more than a volt or so, the non-linearities are unlikely to cause a problem, but diode matching is still essential (IMO). Low level performance <i>will</i> be woeful if accurate diode forward voltage and temperature matching aren't up to scratch. A forward voltage difference of only 10mV between any two diodes will create an unacceptable error. The overall linearity is considerably worse if R3 is included.</p>
|
||||
|
||||
<p>Simple capacitor smoothing cannot be used at the output because the output is direct from an opamp, so a separate integrator is needed to get a smooth DC output. This applies to most of the other circuits shown here as well and isn't a serious limitation.</p>
|
||||
|
||||
|
||||
<hr /><b>Simple Full Wave Meter Amplifier</b>
|
||||
<p>The final circuit is a precision full-wave rectifier, but unlike the others shown it is specifically designed to drive a moving coil meter movement. There is no output voltage as such, but the circuit rectifies the incoming signal and converts it to a current to drive the meter. This general arrangement is (or was) extremely common, and could be found in audio millivoltmeters, distortion analysers, VU meters, and anywhere else where an AC voltage needed to be displayed on a moving coil meter. Digital meters have replaced it in most cases, but it's still useful, and there are some places where a moving coil meter is the best display for the purpose. This type of rectifier circuit is discussed in greater detail in <a href="an002.htm" target="_blank">AN002</a>.</p>
|
||||
|
||||
<p class="t-pic"><img src="an001-f11.gif" alt="Figure 11" border="1"><br />Figure 11 - Moving Coil Meter Amplifier</p>
|
||||
|
||||
<p>The circuit is a voltage to current converter, and with R2 as 1k as shown, the current is 1mA/V. If a 1V RMS sinewave is applied to the input, the meter will read the <i>average</i>, which is 900µA. Adjusting R2 varies the sensitivity, and changing R2 to 900 ohms means the meter will show 1mA for each volt (RMS) at the input. This assumes a meter with a reasonably low resistance coil, although in theory the circuit will compensate for any series resistance.</p>
|
||||
|
||||
<p>This type of circuit almost always has R2 made up from a fixed value and a trimpot, so the meter can be calibrated. Although shown with an opamp IC, the amplifying circuit will often be discrete so that it can drive as much current as needed, as well as having a wide enough bandwidth for the purpose. Millivoltmeters and distortion analysers in particular often need an extended response (100kHz or more is common), and few opamp ICs are able to provide a wide enough bandwidth to work well with anything much over 15kHz. The problem is worse at low levels because the opamp output has to swing very quickly to overcome the diode forward voltage drop. It's common to use a capacitor in parallel with the movement to provide damping, but that also changes the calibration.</p>
|
||||
|
||||
<p><b>Limitations: </b>The output is very high impedance, so the meter movement is not damped unless a capacitor is used in parallel. The meter will then show the peak value which might not be desirable, depending on the application.</p>
|
||||
|
||||
<p>As already noted, the opamp needs to be very fast. Linearity is good provided the amplifier used has high bandwidth. The circuit works better with low-threshold diodes (Schottky or germanium for example), which do not need to be matched because the circuit relies on <i>current</i>, and not voltage. It also <i>only</i> works as intended with a moving coil meter and is not suited to driving digital panel meters or other electronic circuits. It <i>can</i> be done, but there's no point as the circuit would be far more complex than others shown here.</p>
|
||||
|
||||
|
||||
<hr /><b>Conclusions</b>
|
||||
<p>Although the waveforms and tests described above were simulated, the Figure 6 circuit was built on my opamp test board. This board uses LM1458s - very slow and extremely ordinary opamps, but the circuit operated with very good linearity from below 20mV up to 2V RMS, and at all levels worked flawlessly up to 35kHz using 1k resistors throughout. Variations of Figure 11 have been used in several published projects and in test equipment I've built over the years. While most of the circuits show standard signal-level diodes (e.g. 1N4148 or similar), most circuits perform better with Schottky diodes, and even germanium diodes can be used with some of the circuits. These both have the advantage of a lower forward voltage drop, but they have higher reverse leakage current which may cause problems in some cases.</p>
|
||||
|
||||
<p>One thing that became very apparent is that the Figure 6 circuit is very intolerant of stray capacitance, including capacitive loading at the output. Construction is therefore fairly critical, although adding a small cap (as shown in Figures 5 & 6) will help to some extent. I don't know why this circuit has not overtaken the 'standard' version in Figure 4, but that standard implementation still seems to be the default, despite its many limitations. Chief among these are the number of parts and the requirement for a low impedance source, which typically means another opamp. The impedance limitation does not exist in the alternative version, and it is far simpler.</p>
|
||||
|
||||
<p>The Intersil and Burr-Brown alternatives are useful, but both have low (and non-linear) input impedance. They do have the advantage of using a single supply, making both more suitable for battery operated equipment or along with logic circuitry. Remember that all versions (Figures 7, 8 & 9) <i>must</i> be driven from a low impedance source, and the Figure 7 circuit must also be followed by a buffer because it has a high output impedance.</p>
|
||||
|
||||
<p>In all, the Figure 6 circuit is the most useful. It is simple, has a very high (and linear) input impedance, low output impedance, and good linearity within the frequency limits of the opamps. The Figure 6A version is also useful, but has a lower input impedance and requires 2 additional resistors (R1 in Figure 6 is not needed if the signal is earth referenced).</p>
|
||||
|
||||
<p>The above circuits show just how many different circuits can be applied to perform (essentially) the same task. Each has advantages and limitations, and it is the responsibility of the designer to choose the topology that best suits the application. Not shown here, but just as real and important, is a software version. Digital signal processors (DSPs) are capable of rectification, conversion to RMS and almost anything else you may want to achieve, but are only applicable in a predominantly digital system.</p>
|
||||
|
||||
<p>With all of these circuits, it's unrealistic to expect more than 50dB of dynamic range with good linearity. This gives a range from 10mV up to 3.2V (peak or RMS) with supplies of ±12-15V. Use of precision high speed opamps may increase that, but if displayed on an analogue (moving coil) meter, you can't read that much range anyway - even reading 40dB is difficult. 100:1 (full scale to minimum) is not easily read on most analogue movements - even assuming that the movement itself is linear at 100th of its nominal FSD current.</p>
|
||||
|
||||
<p>Many of the circuits shown have low impedance outputs, so the output waveform can be averaged using a resistor and capacitor filter. The value appearing across the filter cap is the average of the rectified signal - for a sinewave, the average is calculated by ...</p>
|
||||
|
||||
<blockquote>
|
||||
V<sub>AVG</sub> = ( 2 × V<sub>Peak</sub> ) / π or ...<br />
|
||||
V<sub>AVG</sub> = V<sub>Peak</sub> × 0.637
|
||||
</blockquote>
|
||||
|
||||
<p>It turns out that the RMS value of a sinewave is (close enough to) the average value times 1.11 (the inverse is 0.9) and this makes it easy enough to convert one to another. However, it <i>only</i> gives an accurate reading with a sinewave, and will show serious errors with more complex waveforms. To see just how much error is involved, see <a href="an012.htm" target="_blank">AN012</a> which covers true RMS conversion techniques and includes a table showing the error with non-sinusoidal waveforms.</p>
|
||||
|
||||
|
||||
<a name="ref"></a><hr /><b>References</b>
|
||||
<ol>
|
||||
<li>Analog Devices, Application Briefs, <a href="http://www.analog.com/UploadedFiles/Application_Notes/130445851AB109.pdf">AB-109</a>, James Wong.
|
||||
<li>Intersil CA3140/CA3140A Data Sheet (Datasheet Application Note, 11 July 2005, Page 18), <a href="http://www.intersil.com/data/fn/fn957.pdf"
|
||||
target="_blank">Intersil CA3140</a>
|
||||
<li>SBOA068 - Precision Absolute Value Circuits - By David Jones and Mark Stitt, Burr-Brown (now Texas Instruments)
|
||||
<li>Wien-Bridge Oscillator With Low Harmonic Distortion, J.L. Linsley-Hood, Wireless World, May 1981
|
||||
<li>Applications of Operational Amplifiers, Third Generation Techniques - Jerald Graeme, Burr-Brown, 1973, pp. 123-124
|
||||
<li>Microelectronics: Digital and Analog Circuits and Systems (International Student Edition), Author: Jacob Millman, Publisher: McGraw Hill, 1979 (Chapter 16.8, Fig. 16-27)
|
||||
</ol>
|
||||
|
||||
|
||||
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|
||||
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<hr /><span class="imgswap"><a href="../index.html" style="display:block;"><img src="a1.gif" alt="Home"/><b class="bb">Main Index</b></a></span>
|
||||
<span class="imgswap"><a href="index.html" style="display:block;"><img src="a1.gif" alt="app notes"/><b class="bb">App. Notes Index</b></a></span><br />
|
||||
|
||||
<table class="tblblk" cellpadding="5">
|
||||
<tr><td class="t-wht"><a id="copyright"></a><b>Copyright Notice.</b>This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is © 2004 - 2009. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro-mechanical, is strictly prohibited under International Copyright laws. The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference while constructing the project. Commercial use is prohibited without express written authorisation from Rod Elliott. Referenced material is Copyright - see original material for details.</td></tr>
|
||||
</table>
|
||||
<div class="t-sml">Change Log: Page Created and Copyright © Rod Elliott 02 Jun 2005./ Updated 23 July 2009 - added Intersil version and alternative./ 27 Feb 2010 - included opamp rules and BB version./ Jan 2011 - added figure 10, text and reference./ Mar 2011 - added Fig 6A and text./ Aug 2017 - extra info on Figure 10 circuit, and added peak-average formula./ Dec 2020 - Added Neve circuit.</div><br />
|
||||
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<center><h1>Analogue Meter Amplifiers</h1>
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<small>Rod Elliott (ESP)</small></center>
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<hr /><b>Discrete Meter Amplifier #1</b>
|
||||
<p>This app. note is adapted from the AC millivoltmeter described in the project pages, as well as some additional ideas. while none of the information here is original, it is offered as a potentially useful collection of different metering amplifiers. Meter amplifiers are a special variation of the precision rectifiers described in AN-001, and typically they need extended frequency response. Very high linearity is nice to have, but in reality few analogue meter movements will match the accuracy of any of the circuits shown here.</p>
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||||
<p>Two of the circuits shown here are <i>peak</i> reading (Figures 2 and 3), but calibrated for RMS. If you need an average reading meter (but still usually calibrated for RMS), see Figures 1 and 4, or you can use diodes in place of the voltage-doubler caps in Figure 2 (C4 and C5). This has been tested in the simulator, and it functions as expected. The disadvantage is that there are <i>two</i> diode voltage drops that the amplifier circuit has to overcome, and this reduces high frequency performance.</p>
|
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||||
<p>The discrete version is shown in Figure 1. This is almost identical to that shown in <a href="../project16.htm" target="_blank">Project 16</a>, with the addition of an input resistor to ground, and a higher value cap between the FET preamp and the discrete opamp formed by Q2-Q4. This circuit has the advantage of wide frequency response, and the gain is high enough to enable full scale deflection with signals as low as 3mV RMS. Making the circuit less sensitive is quite simple, and more information is given below.</p>
|
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|
||||
<p>Recommended supply voltage is ±15V, although the circuit works well with ±9V as shown in Project 16.</p>
|
||||
|
||||
<div class="t-pic"><img src="an002-f1.gif" alt="Figure 1" border="1"><br />Figure 1 - Discrete Meter Amplifier</div>
|
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|
||||
<p>If you cannot obtain the 2N5459 JFET, you can substitute a BF244. Almost any other JFET can be used, provided the source resistor (R2) is changed to suit. Because this resistor sets the bias conditions for the JFET, you may need to experiment a little to get best performance. Ideally, the voltage on the drain should be half the voltage between the source and the positive supply. If the JFET has 2V on the source and you use a 15V supply, the optimum voltage is therefore ...</p>
|
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|
||||
<blockquote>
|
||||
Vdrain = ((+V - Vsource) / 2) + Vsource = ((15 - 2) / 2) +2 = (13 / 2) + 2 = 8.5V
|
||||
</blockquote>
|
||||
|
||||
<p>Although it is possible to improve the circuit in terms of linearity, this is not necessary for metering applications. A small amount of signal distortion will cause a very small overall error - usually better than the accuracy of the meter movement itself.</p>
|
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|
||||
<p>As shown, the circuit has a -3dB frequency of 1.17Hz, and according to the simulator the upper -3dB frequency is about 1MHz. I tend to think this is rather optimistic, however it is certainly possible with careful layout. It should be possible to get up to 100kHz with a reasonable error margin (about 5%).</p>
|
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|
||||
<p>The meter movement should ideally be either 50µA or 100µA, however it is possible to use less sensitive movements. I would not recommend anything above 100µA though, because the drive circuit has limited current capability. The maximum with the circuit as shown is capable of an absolute maximum of 300µA output, but this is just below the amplifier's clipping level.</p>
|
||||
|
||||
<p>The maximum input level is limited to around 75mV RMS, although you can increase that by removing C1 (which reduces the gain of the JFET amplifier), or if you need even higher input levels the JFET stage can be removed altogether. The absolute maximum recommended input voltage is 2V RMS - if you need it to be less sensitive, it's easy to add a simple attenuator in front of the circuit.</p>
|
||||
|
||||
<p>To obtain better performance than the standard circuit, replace D1-D4 with OA91 (or OA95, 1N60, 1N34A etc.) or similar germanium diodes, but BAT43 or similar Schottky diodes are almost as good. These are all faster than 1N4148 silicon diodes, and they also have a lower forward voltage drop. It may appear that this would be of no importance, but the low voltage drop is beneficial. Any speed limitation of the amplifier circuit causes a measurable time delay as the signal goes from one polarity to the other. Lower forward voltage means there is less 'dead time', where no diodes are conducting.</p>
|
||||
|
||||
|
||||
<hr /><b>Discrete Meter Amplifier #2</b>
|
||||
<p>The next circuit is based on one that has been used by Hewlett-Packard (which became Agilent and is now Keysight) in some of their older instruments. It has been modified to use standard E12 value resistors and more readily available transistors. One feature of the circuit is the use of R9, and it is used to partially overcome the forward voltage drop of the diodes to get better linearity with low input voltages, for example at 10% of full-scale. The circuit was designed with germanium diodes, because they are fast, and have a very low forward voltage drop. While it is <i>possible</i> to adjust the value of R9 to enable the use of silicon diodes, this is not really recommended. Schottky diodes are a suitable alternative, however germanium diodes (such as the OA91) are still available if you look around.</p>
|
||||
|
||||
<div class="t-pic"><img src="an002-f2.gif" alt="Figure 2" border="1"><br />Figure 2 - Discrete Meter Amplifier (after Hewlett-Packard)</div>
|
||||
|
||||
<p>The sensitivity of the meter amp can be such as to obtain FSD (full scale deflection) with an input of 5mV RMS, and as shown the maximum is around 28mV with the sensitivity pot at maximum resistance. This can be changed, but frequency response will almost certainly suffer because of limited open loop gain and insufficient feedback. As shown, if adjusted for 5mV, the response is flat (within 5%) up to around 300kHz.</p>
|
||||
|
||||
<p>This circuit doesn't have any particular vices, but it is completely unsuitable for DC operation because it uses only AC feedback. While circuit #1 (above) can be modified to allow DC operation, the DC stability almost certainly will not be good enough for precision work. These two discrete meter amps can be expected to perform well up to at least 100kHz, and with some tweaking can probably exceed that quite easily. The suggested power supply is +15V, although they should work with lower (or higher) voltages if needs be. Some modifications may be required.</p>
|
||||
|
||||
|
||||
<hr /><b>Opamp Meter Amplifiers</b>
|
||||
<p>Opamps are very convenient, but unfortunately are not always suitable as meter amps. One thing they do offer is simplicity and great flexibility, with potentially much wider input range and the ability to drive less sensitive meter movements. Very fast opamps should be able to give good frequency response, and up to 100kHz is possible with some care, or if the circuit is designed to have a relatively low sensitivity.</p>
|
||||
|
||||
<p>While it may appear that the circuit shown below cannot work properly because there is virtually no DC feedback path, it actually functions fine even without R4. The electrolytic capacitors have a very small leakage, and the circuit topology generally means that the bias point of the opamp's output will be well within limits. It may make you feel better if you include R4, and although it really doesn't do a great deal it's preferable to include it.</p>
|
||||
|
||||
<div class="t-pic"><img src="an002-f3.gif" alt="Figure 3" border="1"><br />Figure 3 - Opamp Meter Amplifier</div>
|
||||
|
||||
<p>By using low forward voltage diodes (germanium or Schottky), this circuit is capable of very good results, and will be around 0.6dB down at 50kHz (opamp dependent). With small signal silicon diodes (e.g. 1N4148), it is almost worthless if set for high sensitivity. To maintain flat response, it is necessary to keep the gain fairly low, otherwise the internal Miller cap in the opamp will cause premature roll-off of high frequencies. 100mV input sensitivity is about the best you can hope for, and response should extend to 20kHz (-1dB or so). By using an uncompensated opamp, the necessary stability cap becomes an external component, so it can be selected to give the required bandwidth. A TL071 opamp (for example) has 13V/us slew rate, and this is fairly fast ... despite this, it is much too slow to be useful at higher frequencies. Consider the NE5534 with an external compensation cap, as it <i>should</i> be possible to obtain flat response to 100kHz fairly easily.</p>
|
||||
|
||||
<p>The issue with opamps in this role is simply one of slew rate - the amplifier must be able to overcome the diode voltage drop as quickly as possible. Ideally, the opamp would offer an infinite slew rate, but such opamps do not exist, so it is necessary to make do with what we have. Interestingly, it <i>used</i> to be possible to get an opamp that would work with 30mV input, driving a 1mA meter movement, at frequencies up to at least 500kHz. The HA2625 (Harris Technology) opamp had 100MHz unity gain bandwidth, 600kHz full-power bandwidth, with very low input current. With extremely low input offset current and 'respectable' offset voltage as well, there's very little available today that can match the (now ancient) HA2625.</p>
|
||||
|
||||
<p>The gain of the opamp version is much lower than the discrete versions - about 80mV RMS input for 50µA meter current. This can be varied by changing the value of the sensitivity pot and associated series resistor, but I do not recommend using anything less than about 600 Ohms.</p>
|
||||
|
||||
<p>Any additional gain needed may be supplied with a preamplifier circuit, to lift the typical 3mV signal to 100mV. Consider using a JFET in front of any BJT input opamp if high input impedance is needed, otherwise noise will become a problem.</p>
|
||||
|
||||
<p>To get a real-life idea of performance, the opamp circuit was built on an opamp test board. This uses LM1458 dual opamps (equivalent to the µA741), and as predicted it was useless with 1N4148 silicon signal diodes. However, using Schottky diodes and set to have FSD at around 1V (using a 250µA meter that I had handy), response was 0.2dB down at about 35kHz - not a bad effort for a <i>very</i> slow opamp. Performance was degraded significantly if sensitivity was increased. Virtually no opamp circuit is likely to be quite as good as discrete for sensitivity, but given the low cost and great simplicity of the opamp approach it is certainly worth considering.</p>
|
||||
|
||||
<div class="t-pic"><img src="an002-f4.gif" alt="Figure 4" border="1"><br />Figure 4 - Alternative Opamp Meter Amplifier</div>
|
||||
|
||||
<p>The version shown above is suitable for most 'general purpose' applications, and can drive a meter of up to 1mA coil current. It's suitable for voltages of 100mV or more, and has an upper frequency response of about 10kHz (-0.1dB), depending on the opamp used. Even with 1N4148 diodes, response is respectable, but if higher sensitivity is needed you'll need an amplifier circuit in front to boost the level and/ or a more sensitive meter movement. Unlike the version in Figure 3, there is no capacitance in parallel with the meter, so it is <i>average</i> reading. A cap can be added of course, and if large enough it will convert the meter to peak reading. A lower value can be used to damp the meter if necessary.</p>
|
||||
|
||||
|
||||
<hr /><b>Bridge Vs. Voltage Doubler Rectifiers</b>
|
||||
<p>There are many examples of meter amps that use a voltage doubler (e.g. Fig. 2 and Fig.3) rather than a bridge rectifier (e.g. Fig. 1 and Fig. 4). There are good reasons for using a doubler, in particular because there's only one diode voltage drop to be overcome rather than two with a bridge. As always with electronic circuitry there's a trade-off (a compromise). The doubler demands twice the output current from the driver circuit, which means the metering amplifier has to provide twice as much gain as a circuit using a bridge rectifier. These are non-linear circuits, and the effort needed to present enough voltage (quickly enough) to overcome the diode forward voltage drop is the biggest limiting factor (this applies to <i>all</i> metering amplifiers).</p>
|
||||
|
||||
<p>The current is rarely a problem, because it's usually no more than ±3mA (assuming a 1mA meter movement), but when the gain is doubled, the amplifier's full-power bandwidth is reduced. The bridge rectifier demands a higher slew rate than a doubler for a given maximum frequency. A wide bandwidth opamp with moderate slew rate will work best with a doubler, while a moderate bandwidth opamp with <i>high</i> slew rate is probably better with a bridge. Opamps that use external compensation provide greater flexibility than those that are internally compensated.</p>
|
||||
|
||||
<p>To test this hypothesis, I simulated two circuits, with the same sensitivity and the same meter resistance, one using a bridge and the other a doubler. The simulation was based on 4558 opamps (not bad, but far from 'top shelf'). The input was 1V RMS, and both meters were calibrated for ~1mA (there's some variance, as they are tricky to get exact in the simulator). Calibration normally relies on a trimpot. Normally one would choose a very fast opamp, with a full power bandwidth of at least 10MHz (preferably closer to 100MHz) and a slew rate of no less than 20V/µs. There are suitable opamps available, but they aren't cheap.</p>
|
||||
|
||||
<div class="t-pic"><img src="an002-f5.gif" alt="Figure 5" border="1"><br />Figure 5 - Bridge Vs. Voltage Doubler Rectifiers</div>
|
||||
|
||||
<p>The bridge has lower gain (R2A is 820Ω), but there's more output voltage because there are two diode drops for each polarity. The doubler has to provide twice the current, so R1B is 410Ω. There's no difference between the two at 1kHz, but at 100kHz the bridge will read low by over 10%, vs. about 5% low for the doubler. The opamp has enough <i>bandwidth</i>, but the slew rate is too low to allow the output to overcome the diode forward voltage. With two diodes for each polarity, the bridge rectifier is never quite as good as the doubler, which has only one diode for each polarity.</p>
|
||||
|
||||
<p>With a very fast opamp, the difference is academic. Some designers prefer a doubler because the capacitors (C2B, C3B) damp the meter movement so the deflection is smoother than a bridge. A well-damped meter movement won't care either way. While you might think that the doubler must be peak-reading (rather than average-reading), it's not. Both circuits show the average value of the rectified input waveform.</p>
|
||||
|
||||
|
||||
<hr /><b>Conclusion</b>
|
||||
<p>Instrumentation meter amplifiers are a special case of rectifier, and present the designer with a great many sometimes conflicting requirements. Because measurement instruments are expected to perform well below and above the audio frequency range, it becomes a challenge to design a circuit that has sufficient gain and wide enough bandwidth to cover the required frequencies accurately.</p>
|
||||
|
||||
<p>Sensitive meters make the design easier, and in nearly all cases the lowest diode voltage drop possible is highly desirable. Metering amplifiers such as those shown in this article are used in a wide variety of test instruments, including AC millivoltmeters, distortion analysers, impedance meters, etc. They are my no means limited to audio usage, and are used in almost every area of electronics and engineering where analogue metering is required.</p>
|
||||
|
||||
<p>While most meters are now digital, analogue meters have a special place in test equipment. They are generally easier to read, and you can visually gauge the average when the pointer is fluctuating. This isn't possible with a digital meter unless it's designed to be slow, averaging the waveform before it's displayed.</p>
|
||||
|
||||
|
||||
<a name="ref"></a><hr /><b>References</b>
|
||||
<blockquote>
|
||||
1 - Hewlett Packard instrumentation manuals (various).
|
||||
2 - Opamp Datasheets (for the devices mentioned)
|
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<tr><td class="t-wht"><a id="copyright"></a><b>Copyright Notice.</b>This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is © 2005-2022. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro-mechanical, is strictly prohibited under International Copyright laws. The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference while constructing the project. Commercial use is prohibited without express written authorisation from Rod Elliott.</td></tr>
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<div class="t-sml">Page Created and Copyright Rod Elliott, 02 Jun 2005./ Updated Nov 2018 - added Figure 4./ Sep 2022 - added HA2625 info and bridge vs. doubler section.</div><br />
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<center><h1>Simple High-Power LED Regulator</h1>
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<small>Rod Elliott (ESP)</small></center>
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<hr /><b>High Power LEDs</b>
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<p>There are quite a few high power light-emitting diodes now available, but the standard is still the Luxeon Star. Available in a variety of power ratings, colours and light patterns, these LEDs are causing something of a revolution in many areas. They have relatively low heat dissipation compared to light output, long life and there is great flexibility of use - they can be used safely where an incandescent lamp could not.</p>
|
||||
|
||||
<p>Being a LED, they do have the rather annoying trait of being a current driven device, having a relatively low forward voltage. The current must not be allowed to exceed the design maximum, or the LED will be damaged. This requires that a current regulator must be used between the voltage source and the LED itself, so complexity is increased compared to using a normal lamp.</p>
|
||||
|
||||
<hr /><b>LED Regulators</b>
|
||||
<p>Although there are many ICs available that can be adapted to drive the Star LEDs (or their cheaper generic equivalents), not all are easy to obtain, many are available only in surface mount packages, and they can be rather expensive. Most also require external support components as well, increasing the price even further.</p>
|
||||
|
||||
<p>An alternative is to use a linear regulator, but these are very inefficient. The full current (typically around 300mA) is drawn at all supply voltages, so with 12V input, the total circuit dissipation is 3.6W. Admittedly, this is not a great deal, but where efficiency is paramount such as with battery operation, this is not a good solution. The circuit shown in Figure 1 was the result of a sudden brainwave on my part - it may have been triggered by something I saw somewhere, but if so that reference was well gone by the time I decided to simulate it to see if it would work.</p>
|
||||
|
||||
<p class="t-pic"><img src="an003-f1.gif" alt="Figure 1" border="1"><br>Figure 1 - Ultra-Simple LED Switchmode Supply</p>
|
||||
|
||||
<p>Using only three cheap transistors, the circuit works remarkably well. It is not as efficient as some of the dedicated ICs, but is far more efficient than a linear regulator. It has the great advantage that you can actually see what it does and how it does it. From the experimenters' perspective, this is probably one of its major benefits.</p>
|
||||
|
||||
<p>One of the features of this circuit is that it will change from switchmode to linear as the input voltage falls. It still remains a current supply, and the design current (set by R1) does not change appreciably as the operation changes from linear to switchmode or vice versa.</p>
|
||||
|
||||
<hr /><b>How Does It Work?</b>
|
||||
<p>Operation is quite simple - Q1 monitors the voltage across R1, and turns on as soon as it reaches about 0.7V. This turns off Q2, which then turns off Q3 by removing base current. If the voltage is low, a state of equilibrium is reached where the voltage across R1 remains constant, and so therefore does the current through it (and likewise through the LED). The value of R1 can be changed to suit the maximum LED current ...
|
||||
|
||||
<blockquote>
|
||||
<b> I = 0.7 / R1</b> (approx.)
|
||||
</blockquote>
|
||||
|
||||
<p>At higher input voltages, the circuit will over-react. Because of the delay caused by the inductor, the voltage across R1 will manage to get above the threshold voltage by a small amount. Q3 will get to turn on hard, current flows through the inductor and into C1 and the LED. By this time, the transistors will have reacted to the high voltage across R1, so Q1 turns on, turning off Q2 and Q3. The magnetic field in L1 collapses, and the reverse voltage created causes current to flow through D1 and into C2. The cap now discharges through the LED and R1, until the voltage across R1 is such that Q1 turns off again. Q2 and Q3 then turn back on.</p>
|
||||
|
||||
<p>This cycle repeats for as long as power is applied at above the threshold needed for oscillation (a bit over 5V). As shown in the table below, the circuit changes its operating frequency as its method of changing the pulse width. This is not uncommon with self-oscillating switchmode supplies.</p>
|
||||
|
||||
<center><table style="width:700px" border="1">
|
||||
<colgroup span="4" width="25%">
|
||||
<tr class="tbldark"><td><b>Voltage</b></td><td><b>Current</b></td><td><b>Frequency</b></td><td><b>Input Power</b></td></tr>
|
||||
<tr><td>4.5</td><td>260mA</td><td>Not Oscillating</td><td>1.17W</td></tr>
|
||||
<tr><td>6.0</td><td>202mA</td><td>230kHz</td><td>1.21W</td></tr>
|
||||
<tr><td>8.0</td><td>164mA</td><td>172kHz</td><td>1.31W</td></tr>
|
||||
<tr><td>12</td><td>123mA</td><td>123kHz</td><td>1.48W</td></tr>
|
||||
<tr><td>16</td><td>104mA</td><td>100kHz</td><td>1.66W</td></tr>
|
||||
</table><b class="t-pic">Table 1 - Operating Characteristics</b></center>
|
||||
|
||||
<p>The table above shows the operating characteristic of the prototype. I also checked the performance with an ultrafast silicon diode, and the input operating current was increased by almost 10%. The suggested Schottky diode is well worth the effort. LED current remains fairly steady at 260mA, since I used a 2.7 ohm current sensing resistor as shown in the circuit diagram.</p>
|
||||
|
||||
<hr><b>Construction</b><br>
|
||||
Construction is not critical, but a compact layout is recommended. L1 needs to be rated for the continuous LED current, Q1 does not need a heatsink, but one will do no harm. The ripple current rating for C2 needs to be at least equal to the LED current, so a higher voltage cap than you think you need should be used. I recommend that a minimum voltage rating of 25V be used for both C1 and C2.
|
||||
|
||||
<p>Q1 and Q2 can be any low power NPN transistor. BC549s are shown in the circuit, but most are quite fast enough in this application. Q3 needs to be a medium power device, and the BD140 as shown works well in practice. D1 should be a high speed diode, and a Schottky device will improve efficiency over a standard high speed silicon diode. D1 needs to be rated at a minimum of 1A. L1 is a 100µH choke, and will typically be either a small 'drum' core or a powdered iron toroid. An air cored coil can be used, but will be rather large (at least as big as the rest of the circuit).</p>
|
||||
|
||||
<p>The efficiency is not as high as you would get from a dedicated IC, because the switching losses are higher due to relatively slow transitions. At best, I measured around 60%, which isn't bad for such a simple circuit. Input voltage can range from the minimum to turn on the LED up to about 16V or so. Higher voltages may be acceptable, but that has not been tried at the time of writing.</p>
|
||||
|
||||
<p>All resistors can be 0.25 or 0.5W except R1 - this needs to be rated at 0.5W. Paralleled low value resistors may be used to get the exact current you need, but always make sure that you start with a higher resistance than you think you will need. If resistance is too low, the LED may be damaged by excess current.</p>
|
||||
|
||||
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<tr><td class="t-wht"><a id="copyright"></a><b>Copyright Notice.</b>This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is © 2004. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro-mechanical, is strictly prohibited under International Copyright laws. The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference while constructing the project. Commercial use is prohibited without express written authorisation from Rod Elliott.</td></tr>
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<table style="width:100%" class="tblblue"><tr><td class="hdrl"> Elliott Sound Products</td>
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<td align="right" class="hdrr">AN-004 </td></tr></table>
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||||
|
||||
<center><h1>Car Dome Light Extender</h1>
|
||||
<small>Rod Elliott (ESP)</small></center>
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||||
|
||||
<hr /><b>Introduction</b>
|
||||
<p>There are countless dome light extenders on the Net and in magazines, but most of them suffer from one problem ... complexity. Ok, they are not actually complex, but most are far more complex than they need to be. Some are completely over the top, and require additional car wiring, a PCB, ICs, trimpots and lots of other stuff, while others seem to be someone's untested idea or maybe just a brain fart - some circuits I saw will never work. My goal was extreme simplicity, and I think that has been achieved. It is helpful if it works too - there's not much point making it otherwise. Efficiency is not an issue, since the dimming phase is relatively short lived anyway. Worst case dissipation in Q2 should not exceed about 2W or so (momentary) with a standard 6W dome lamp.</p>
|
||||
|
||||
<p>As with so many projects on the ESP site, this came of necessity (or is that desire?). My car has most of the bells and whistles that one expects these days, but the dome light switched off as soon as the door was closed. I figured that about 15 seconds was a reasonable time delay, and I only had a very small space in which to locate the unit - namely in the dome light housing itself.</p>
|
||||
|
||||
<hr><b>Design</b>
|
||||
<p>Not much to it, really. It is obviously important that the existing car wiring be used - the last thing one wants to do is have to run additional wires in a car. Standard dome lights use the door switch to make the negative connection to the lamp, with the positive being permanently connected to the car's positive battery terminal (via the obligatory fuse).</p>
|
||||
|
||||
<p class="t-pic"><img src="an004-f1.gif" alt="Figure 1" border="1"><br>Figure 1 - Dome Light Extender Schematic</p>
|
||||
|
||||
<p>As you can see, it is very simple. Cheap (mainly 'junk box') transistors are used throughout, and the resistors can be very ordinary carbon film types. The cap only needs to have a voltage rating of 16V, but higher voltage caps can be used if you have them to hand.</p>
|
||||
|
||||
<p>When the car door is opened, the 'Trigger' terminal is connected to chassis. This turns on Q1, which promptly charges C1, thus turning on MOSFET Q2. Provided there is enough gate voltage for Q2, the lamp will remain on, but as the cap discharges the gate voltage gets to the point where Q2 is no longer saturated and the lamp starts to dim. As the cap discharges further, the lamp dims more, eventually going out altogether. Full brightness remained in my circuit for about 20 seconds, and the lamp was extinguished within 22 seconds.</p>
|
||||
|
||||
<p>Because a switching MOSFET has a fairly rapid transition from conducting to non-conducting states with a relatively small voltage range between fully on and fully off, that makes the ideal switch. The transition period is quite narrow, so no heatsink is needed. Timing is also reasonably predictable, since it is determined by the resistor and cap. A low value cap can be used, minimising size. The zener is essential to protect the gate against transients (all too common in a car's electrics). The resistor (R3) provides a high impedance for any transients so they don't just blow the zener and the MOSFET gate.</p>
|
||||
|
||||
<p class="t-pic"><img src="an004-f2.gif" alt="Figure 2" border="1"><br>Figure 2 - Dome Light Extender With Voltage Detector</p>
|
||||
|
||||
<p>Figure 2 shows an enhanced version, that uses Q3 as a battery voltage detector. When the engine is off, Q3 remains off too, because the zener (D2) doesn't have enough voltage to conduct. C1 therefore discharges through R4 normally, and the full timeout period applies. When the engine is running, the battery voltage quickly rises to ~13.8V (the normal float charge voltage for a lead-acid car battery. This allows D2 to conduct, turning on Q3, and discharging C1 via R7, so the cap is discharged much faster.</p>
|
||||
|
||||
<p>This addition was made to my unit after I fitted a (home made) LED light to replace the silly incandescent bulb. Because the new LED lamp is so bright (yet only draws about 200mA), it became annoying at night because it was too bright inside the car. By adding the extra bits, it now extinguishes in about 4 seconds when the engine is started or is running. There is now plenty of time to get organised having opened the door and clambered in, but when the engine is started the lamp goes out much more quickly. R7 can be reduced in value if faster operation is required. It can be reduced to about 22k to get a really fast turn off. If the value is too low, the lamp will not turn on at all if the engine is running.</p>
|
||||
|
||||
<hr /><b>Construction</b>
|
||||
<p>Nothing is critical, except that all the usual precautions against short circuits must be taken. If the time delay is too long (or short), simply reduce (or increase) the value of C1 or R4 as appropriate. Because of the design, the existing wiring in the dome light is retained except that the door switch lead needs to connect to the trigger input, rather than directly to the lamp. R7 can be reduced as well to get a faster turn-off when the engine is running.</p>
|
||||
|
||||
<p>The resistor values shown are a guide only, and the circuit will work fine with a fairly wide range of values. Those shown are not bad though, so feel free to use them. Likewise, almost any small signal PNP transistor can be used, the MOSFET can be almost any N-Channel switching device capable of at least a couple of amps.</p>
|
||||
|
||||
<p>Since the typical dome light is only rated at about 6W (0.5A at 12V), high current wiring is not necessary. Just make sure that everything is properly insulated so that nothing can short to chassis.</p>
|
||||
|
||||
<p>I suggest that the dome light switch is wired directly to the lamp as normal - if possible (not all switches will allow this). This prevents the delay from operating should you turn on the interior light, so it goes off immediately when switched. While it is possible to add an extra transistor to reduce the on time if the engine is running (as suggested in at least one circuit I saw), this would normally require running an extra wire - an exercise in futility with most cars.</p>
|
||||
|
||||
<p>The method shown in Figure 2 does not require any additional wiring, and is probably the easiest way to modify the timing to make the lamp turn off faster when the engine is running.</p>
|
||||
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||||
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<tr><td class="t-wht"><a id="copyright"></a><b>Copyright Notice.</b>This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is Copyright © 2004. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro-mechanical, is strictly prohibited under International Copyright laws. The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference while constructing the project. Commercial use is prohibited without express written authorisation from Rod Elliott.</td></tr>
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<div class="t-sml">Page Created and Copyright © Rod Elliott 02 Jun 2005./ Updated 18 Jun 09 - added Figure 2 and details for voltage sensor.</div><br />
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