I've got so much to learn. Sean, what's your education and background? I'm a junior working on my EET and only have a hobbyist background. -- Sincerely, James Burkart *Filmmaker & Documentarian* *Burkart Studios* 925.667.7175 | Personal 415.738.2071 | Office *Web:* burkartstudios.com *Facebook:* facebook.com/burkartstudios On Thu, Aug 24, 2017 at 9:28 PM, Sean Breheny wrote: > 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. Th= is > 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 th= e > 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, y= ou > can then treat it as a pure gain block or as a simple compensator (like a= n > 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 ha= s > been stated before but I've never seen it put this way) - given any stabl= e > system P, one can always close the loop around P with an integrator H suc= h > that the closed-loop system will have zero DC error and be stable, for so= me > value of integrator gain K in H, and the system will continue to be stabl= e > 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 integrato= r > and obtain at least the improvement of zero DC error, but also usually so= me > amount of improvement in servo tracking and regulation against > disturbances. > > Sean > > > On Thu, Aug 24, 2017 at 12:32 PM, Bob Blick wrote: > > > Hi y'all, > > > > I'm trying to build some audio power amplifiers in order to use up some > > parts I have left over from a previous life. > > > > In the past I've always built the input stages from discrete parts but > > this one uses a conventional 4558-style opamp. Then I follow it with > > voltage and current boosting parts and finally enclose the whole mess i= n > > negative feedback. > > > > In this aforementioned past life I designed a lot of power amps so I kn= ow > > to design each stage to have as linear and balanced response as > reasonable, > > keep the gain low around the slowest components etc. > > > > Of course with the added gain and relatively low speed of the output > > stage, the internally compensated opamp is not able to keep things > stable, > > so I've added an external compensation capacitor. It does work fine, an= d > > I'm not looking for super high fidelity, but it's always nice to make > > simple changes that reap big benefits. > > > > So my question is, any ideas about compensation or changes I could make= ? > > > > I've attached a block diagram. Note that my output stage is the > right-hand > > amplifier block, it contains many discrete components, and most > > importantly, it is inverting. Therefore I am using the non-inverting > input > > of the opamp for the negative feedback. > > > > Thank you! > > > > Bob > > -- > > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > > View/change your membership options at > > http://mailman.mit.edu/mailman/listinfo/piclist > > > > > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .