'active feedback' for distortion cancellation

[scott wurcer]

I was looking through some old stuff and found this idea from Barrie Gilbert (1978). He always liked to promote what he called active feedback. The idea being to put an identical input stage in the feedback path and to the first order distortions will cancel. Really careful design and you can get .02% or so with a couple of volts dynamic range. One neat feature is you have diff or SE input of either phase. I don't know if this has been kicked around here before, I didn't see it explicitly in the cancelling odd harmonics thread.

[mikeks]

Hi Scott,

This looks like an elaboration of the opa627/637 design rationale....

...with the exception, of course, of the diff. pair to which the minor resistive feedback loop is returned.....

By invoking elementary signal tracing, it becomes evident that this last diff. pair works to reduce the error signal applied to the output diff. stage (represented by the op. amp.)...

I am assuming said error signal is a voltage...at least nominally...?

Looks interesting...

[mikeks]

Scott, You wouldn't happen to have a copy of Barrie's prose to this concept would you?

[scott wurcer]

The basic idea is certainly public domain by now, but Barrie's notes are company property and I can't post them in detail.

[mikeks]

Hi Scott,

You've got mail...

[jcx]

aren't the diff pair inputs labeled backwards?

i'm not sure i can see how to use the circuit yet but with repect to input diff pair distortion reduction i would think the designer of the AD797 should be convinced of the ability of high loop gain to "linearize" the input diff pair

3rd harmonic distortion is porportional to the square of the input diff signal - 10x more loop gain (ref to some ordinary design) will give 100x less 3rd harmonic

[mikeks]

Originally posted by jcx
aren't the diff pair inputs labeled backwards?

True...methinks...

[smoking-amp]

Seeing as how multiplication of non-linearities is associative (disregarding phase effects), any reason we couldn't unravel the amplifier at the OTHER end? ie, take feedback from before the output stage, run it thru a duplicate output stage on the way back to the input summation. This way the signal gets pre-distorted to correct for the actual output stage that is after the feedback pickoff. The output stage is usually considered more problematic than the input stage for distortion.
Off course a few complications come to mind, such as an accurate mimic for the speaker load. But then if we run the speaker thru both output stages in series, they will have the same load currents.

Don

[lumanauw]

Thanks Scott Wurcer, for bringing this AFA (Active Feedback Amplifier) idea of Barrie Gilbert. I've been searching this AFA since I read the "Are OpAmps Really Linear" article by Barrie Gilbert, but never find it.

How to implement this idea, if the input is not balanced, like in ordinary single RCA input power amp?

And will this idea rise the OL gain (I like low OL gain)?

[scott wurcer]

Yes, I might have switched sign (50/50 you know). Well here's more elaborate circuit. I was thinking maybe a gain of 5 line amp driving a 50k constant impedance volume control. The feature I was most interested in was the true high impedance differential input that rejected common mode and could be differential or single ended of either sign. Even this simple circuit sims out at .015% second and 3rd and above at <.002% at 10V p-p out.

Just some old food for thought. And yes the main idea is in the alternative input stage ideas seeking some of the properties mentioned in Barrie's articles. Output stage issues are still in a feedback loop.