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Old 13th November 2004, 09:29 PM   #1
Mr Evil is offline Mr Evil  United Kingdom
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Default Unnamed feedback method explored.

This idea is quite obvious so I'm sure someone has done it before, but I've never seen it mentioned anywhere so I'll post it anyway. I'm also not sure what it should be called; "active feedback" would be a fitting name, but searching on both this forum and Google reveals that name already being used to describe a variety of disparate techniques which are not this.

On to the heart of the matter: Negative feedback subtracts the output from the input, reducing nonlinearity and increasing bandwidth at the expense of gain. If instead of feeding back the output, the difference between the output and input is fed back, then linearity and bandwidth can be improved without sacrificing gain.

The attached schematic shows a practical implementation of the concept, with X1 being the main amplifier and X2 being the error amplifier. The error amplifier should be faster than the main amplifier to avoid ringing, or even instability. It should also be as linear as possible, which goes without saying, but it doesn't have to supply any current (ideally).

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Old 13th November 2004, 09:31 PM   #2
Mr Evil is offline Mr Evil  United Kingdom
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Distortion could theoretically be made arbitrarily low by increasing the gain of the error amplifier, but the main amplifier will need to be increasingly slower or the corrections of the error amplifier will cause ringing at frequencies where both the phase shift of the error amplifier is significant and the gain of the main amplifier still high enough. If my thinking is correct, then the reduction in distortion is ideally equal to the gain of the error amplifier (20dB as shown). Simulations show about this much improvement.

In reality I can't measure low distortion, so to see how it works in practice I added an antiparallel pair of diode in series with R1 to simulate massive odd harmonic distortion easily visible on an oscilloscope. The attached picture shows what this looks like before error correction.

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Old 13th November 2004, 09:32 PM   #3
Mr Evil is offline Mr Evil  United Kingdom
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After error correction it looks like the picture attached to this post.

Doing a quick test on a breadboard with a dual op-amp shows that in reality it does exactly what the simulations say, surprise surprise. I haven't bothered showing anything about the increased bandwidth because I don't have a signal generator capable of testing an op-amp to the limit, but simulations show an improvement of similar magnitude to the distortion.

So has anyone made anything like this before, or read about it? Any drawbacks I haven't thought of (increased complexity and care needed for stability is all I can think of)?
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Old 13th November 2004, 10:18 PM   #4
Tom2 is offline Tom2  United States
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Default re: Unnamed feedback method explored

Mr. Evil,

Here is a circuit which looks similar, although it does not incorporate feed back from the main opamp output.

The link is:
http://www.lsionline.co.uk/lsi/featu...asp?ID=-I9LOCH

If you go to electronicsweekly.com and put "sandman" in the search tool, it displays three related articles.

Maybe it helps.

Tom
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Old 13th November 2004, 10:38 PM   #5
Mr Evil is offline Mr Evil  United Kingdom
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Interesting. Sandman's topology does look similar in principle. However, it doesn't improve bandwidth and I think it doesn't decrease distortion by as much as the one I posted, although it would take further analysis to determine that.

The responses to his circuit mentioned such techniques being used for a long time, but no details given, so I still don't know why I never see anything like this actually used.
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Old 14th November 2004, 12:05 AM   #6
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Evil,

It's been thought of before (I can't remember where,
but it probably wasn't original then, either).

It works, but you would have to have a pretty lousy
gain stage for X1 to want to bother, and as you point
out yourself, it will be rife with stability issues.

On the other hand, it might sound great and you can
drive Halcro out of business. Now that would be really
evil
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Old 14th November 2004, 12:11 AM   #7
jcx is offline jcx  United States
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I believe this is just another one of the many possible composite amplifiers that in the end can be shown to have lower distortion due to added loop gain

Walt Jung has shown composite op amps with gain in the loop, Jerald Graeme discusses them in his books&articles, Soliman in WW, Randall Geiger in IEEE…
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Old 14th November 2004, 12:53 AM   #8
Mr Evil is offline Mr Evil  United Kingdom
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Quote:
Originally posted by Nelson Pass
Evil,

It's been thought of before (I can't remember where,
but it probably wasn't original then, either).

It works, but you would have to have a pretty lousy
gain stage for X1 to want to bother, and as you point
out yourself, it will be rife with stability issues.

On the other hand, it might sound great and you can
drive Halcro out of business. Now that would be really
evil
Stability needs extra attention, but isn't really a big problem. X1 should have a GBP lower than X2 by a factor of the gain of X2. So if X2 has a GBP of 10MHz and a gain of 10, then if X1 has a GBP of 1MHz everything is fine. That's how it is in the one I tested and it shows no overshoot on transients. The good point however is that even though X1 has reduced bandwidth, the overall GBP is still 10MHz. Higher bandwidth for X1 is possible if some overshoot is permissible.

Of course if the main amplifier is already good then it might not be worth the effort, but one attractive aspect is that it seems to reduce 3rd harmonic distortion a lot. 2nd and 3rd are reduced at lower frequencies, but as the phase shift of the main amp increases then the effect reduces to only 3rd harmonic reduction. Who wouldn't like to be able to put on the spec sheet that 3rd harmonic distortion is down 140dB at 20kHz, even if it doesn't make an audible difference!

I'm not in the commercial electronics business, so I won't be putting anyone out of business, even if it did prove to have practical advantages.


Quote:
Originally posted by jcx
I believe this is just another one of the many possible composite amplifiers that in the end can be shown to have lower distortion due to added loop gain

Walt Jung has shown composite op amps with gain in the loop, Jerald Graeme discusses them in his books&articles, Soliman in WW, Randall Geiger in IEEE…
Thanks for the info. Do you have any particular links, or specific books I could look at to learn more about it?
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Old 14th November 2004, 02:23 AM   #9
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What is the actual cct for real audio amplifier. If one block/main block is discrete amplifier, the feedback block should be made from the same configuration, or it can be made by an opamp?
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Old 14th November 2004, 02:24 AM   #10
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What is the actual cct for real audio amplifier. If one block/main block is discrete audio amplifier, the feedback block should be made from the same configuration, or it can be made by an opamp? If the 2 blocks consist of different configuration, will it work effectively?
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