2. amps-preamps
  3. 1092202479

Douglas Self on Negative feedback and distortion

I've been reading Douglas Self book on amplifier design and something he said that really makes me think twice.

As you have seen most amplifier makers claim that their amps either does not use global NFB at all or very little of it to improve dynamic (or transient response).

According to Self, the only parameter that matters is distortion and nothing else. I supposed he measures the extra harmonics that the amp produces given a sinusoidal input. In other words, distortion is measured in the frequency domain.

If I remember correctly in my Control Theory course way back in my college days, the frequency domain reponse cannot tell how the amp will response for a given step input. And the STEP RESPONSE is what can tell a lot about the behavior of an amp dynamic and transient response.

In his book, he is very adamant about his position that the only thing that matters is the amp frequency response.

I don't thing frequency response contains information about how any amp would respond to a step input but I could be wrong. Frequency response is only a steady state behavior of the amp. It cannot tell how much the amp would over-shoot, under-shoot, tendency to ringing, and so and so, given a step response. I don't think you can look at the frequency response and make any conclusion about the amp tendency to overshoot, undershoot, ringing and so on...

What do you think?

By the way, I think his book is excellent read into the theory an amplifier design if you can ignore some of his more dogmatic position.
andy2
marakanetz's avatar
marakanetz
5,473 posts
 
You're right,
The control theory states that negative feedback MAINLY exists to keep amplifier stable.
An amplifier at the same time can have multiple stages of amplifications which we also can consider as an amplifiers.
A global negative feedback i.e. over the whole amplifier not only should exist to decrease distortions over the larger freequency bandwidth but to prevent an amp from self-oscillations.
Ag insider logo xs@2x
hammy
176 posts
 
I don't trust ANYONE who is dogmatic about specs. We're not trying to reproduce sine waves at 1000Khz. It's ridiculous- all you have to do is compare a cheap yamaha with 100 watts and ultra low distortion to a 25 watt audiophile amp. How many compromises did yamaha engineer in to get those specs right? A little Naim Nait sounds better and plays louder- but the yamaha is better on paper.
rsross
6 posts
 
Re: Randy Sloans book (High Power Amplifier Design) I have read it a few times but have not seen the Self book (I have read a few of his articles). Sloans book is a good introduction to most all issues in classic power amplifier design but is reported to cover the same ground as Self. It appears to be more organized than it is and I am continually digging out my tattered copy of Horowitz and Hill to remind myself how many of the basic circuit elements Sloane throws about really work. His explanations of dominant pole capicators and amplifier stability are rather confusing to say the least. I hope Self does a better job.

BTW, Sloan is also dogmatic in his own way claiming that only Class B amps make any sense. Ironically he does include a small section on Class A biasing schemes and includes a supposed 'high-performance' Class A design. He is also a strong proponent of the wide-band frequency response/low distortion is everything school. I personally agree with these guys in that it is much much harder (nay impossible) to design a high performance amplifier without using feedback. In no other field of electronics would this even be discussed seriously.
eldartford
4,262 posts
 
Step response can tell you everything you need to know about an amplifier, but criteria are not well defined. From what points do you measure rise time? 10 percent to 90 percent of max amplitude? How many overshoots? How big does the overshoot have to be to be counted? Etc. Etc. You can look at a scope trace and say "that's good" or "thats bad", but that is not very definitive.

Frequency response is not the same thing as step function response, but it is correlated. And a lot easier to measure. If an amp is 0.5 dB down at 100 Kc it probably has good step response.

Negative feedback is most often thought of as a way to reduce distortion, and it does do that. However, it also extends bandwidth, most notably in the case of pentode tube amp output transformers, where the feedback is provided by dedicated windings of the transformer that are connected to auxillary grids of the output tubes.
andy2 OP
1,539 posts
 
Self actually says in his book that one should use as little of nfb as possible. He advices that each stage (which in itself has its own natural feedback) should be designed to give as small a distortion as possible. Then after each gain stage has been designed, as a final step, a nfb loop should be added to suit one's needs.

His argument is that global feedback is unfairly criticized by amplifier makers since each gain stage already has its own natural feedback built in. For example, an output stage complementary pair of PNP/NPN transister operating in class A/B has an inherent feedback with its emiter output resister ... So since you cannot avoid feedback in the first place, then why it is so bad?
My takes on this is that global feedback does have by far a larger impact on the behavior of the entire amp as compared to each gain stage feedback.

My one problem (among others) main problem is with his position on distortion measurement.
More to discover