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Thread: amplifier compensation and stability
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face BY : Bob Blick email (remove spam text)

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Hi Sean,
Sorry not to have replied sooner. Thanks very much for your input. I'd pretty much already done a lot of the things you suggested, so it was great to get the reinforcement. I'm trying to squeak a little too much from too few parts and mostly getting away with it. Unfortunately I don't have all the same test equipment that I used to, and what I have now is more RF than audio, and I don't have a low distortion oscillator anymore. But as I said, I'm not looking for super low distortion.

Running the amplifier open loop shows it to be pretty well behaved. But I am using mosfet output devices and the driver stage is running class A, so it's definitely slew rate limited due to the low available gate current. But it sounds good and I think I can get away with it up to about 100V between power rails. Unfortunately the power transformers I have will net me 110V so I'll probably add a class AB driver stage.

Mosfets are strange creatures in audio amps. I was hoping that by using them in their linear range I would not be up against the full input capacitance and gate charge, but unfortunately it has not worked out that way. I also need to run a lot of quiescent current or else they are a little squirrely(actually more opossum-y) around zero. I suppose a fancy delta-Vbe circuit might be able to moderate the need for high quiescent current. But this is a low parts-count design.

I'll post the schematic for this and also whatever it is I end up finishing with as soon as I get a chance.

Thanks again,
Bob ________________________________________
From: TakeThisOuTpiclist-bounces@spam@spamspamBeGonemit.edu <KILLspampiclist-bouncesTakeThisOuTspamRemoveMEmit.edu> on behalf of Sean Breheny
Sent: Thursday, August 24, 2017 8:28 PM
To: Microcontroller discussion list - Public.
Subject: Re: [EE] amplifier compensation and stability

Hi Bob,

My main suggestion is to characterize the behavior by providing various
input signals like impulses, steps, white noise, swept sine, etc. and
looking at the output on a scope. You might also vary the output load. This
gives you a good idea that you are not close to going unstable and you
aren't experiencing "peaking" around one frequency band or a null in one
band.

I've had problems in the past with circuits like this where the output
stage has a nonlinearity about zero (like crossover distortion) which can
make the gain be reduced substantially near zero. This can result in
instability by causing integrator wind-up while the output is in the
low-gain region, followed by massive overshoot when it exits the low-gain
region, followed by a compensatory wind-up in the other direction due to
the overshoot, etc. This problem can be dealt with by making sure that the
transistors in the output are always biased with some minimum current so
their input-output gain never goes below a certain value.

Note, too, that the op-amp can be treated as it's own finite gain stage,
where you close the loop locally around it and then close the loop again
around the whole system. This can make analysis of the entire loop easier
because once you guarantee that the op-amp gain stage is itself stable, you
can then treat it as a pure gain block or as a simple compensator (like an
integrator or integrator plus proportional gain or a lead compensator or
lead-lag compensator, etc.) You are sorta doing that already in that the
compensation cap is turning the op-amp into a fast integrator.

I have developed a "theorem" of simple control design (which I am sure has
been stated before but I've never seen it put this way) - given any stable
system P, one can always close the loop around P with an integrator H such
that the closed-loop system will have zero DC error and be stable, for some
value of integrator gain K in H, and the system will continue to be stable
and exhibit zero DC error for any integrator gain Kprime < K. To put it
more simply, you can always control a stable plant using a slow integrator
and obtain at least the improvement of zero DC error, but also usually some
amount of improvement in servo tracking and regulation against
disturbances.

Sean


On Thu, Aug 24, 2017 at 12:32 PM, Bob Blick wrote:

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