Negative feedback, voltage and output impedance

Greg: I should have clarified that my "NO" feedback was in regards to "global" feedback. The use of small amounts of local feedback is not a big deal and results in less time lag. Sorry for my lack of clarification.

Tom: That was a good find. I'm sure that Theta won't mind the re-post here either : )

Marakanetz: I agree with your basic statements. See my response above to Greg.

El: My analogy applies more to "global feedback" than to "local feedback". Distortion and the correction of that distortion is like gossip. It can be quelled much faster and in a more effective manner "locally" than it can when trying to fix the problem "globally".

Think of "local feedback" as having a dedicated supervisor for each stage of amplification. If everything is good in section A, B & C but section D is a mess, only section D's supervisor has to get involved.

On the other hand, global feedback is kind of like having one supervisor by himself trying to keep track of what is going on in the entire "plant". The potential for more problems to slip through the cracks un-noticed is far greater. Once those problems are noticed, it is possible that the amount and type of correction applied may not be directly to the source of the problem, resulting in other problems. This is why global feedback is less desirable. Sean
>

Sean...Analogies can get you in trouble! One popular management approach today is the "self-directed workforce" where little or no supervision is exercised at the day to day level, and the only thing that is supervised is the overall product going out the door, ("global supervision").

Whatever floats your boat :)

The idea that:

>Amp's input and driving stages do definitely have a feedback so the statement "no feedback" isn't truthful in any case.

-is incorrect. It is quite possible to build an entire amplifier without feedback of any kind.

There are two types of feedback that are both considered 'negative' (in that the feedback can be used to lower distortion): Loop and degenerative.

Usually Loop feedback is the type that is considered malfesent, as it is the type that increases high odd-ordered distortion. This occurs because there is a very finite time that it takes for the input signal to propogate to the output of the amp, where the signal is then tapped off back to the input. What this means is that as frequency increases, the feedback signal becomes increasingly out of sync with the input signal. Hence the addition of odd-ordered harmonic content. Esentially at higher frequencies the loop feedback results in ringing unless steps are taken to control it (usually by tailoring the feedback circuitry).

Degenerative feedback occurs in real time and so does not contribute to odd ordered harmonics, however the output impedance of the circuit is increased.

Audio Note (UK) made amps that lacked either type of feedback, back in the early 90s, at least according to the designer whom I met at CES.