332 Chapter 10
10.6.1 Some Common Misconceptions About Negative Feedback
All of the comments quoted here have appeared many times in the hi-fi literature. All are
wrong.
NFB is a bad thing. Some audio commentators hold that, without qualifi cation, NFB is a
bad thing. This is of course completely untrue and based on no objective reality. NFB is
one of the fundamental concepts of electronics, and to avoid its use altogether is virtually
impossible; apart from anything else, a small amount of local NFB exists in every
common emitter transistor because of the internal emitter resistance. I detect here distrust
of good fortune; the uneasy feeling that if something apparently works brilliantly then
there must be something wrong with it.
A low NFB factor is desirable. Untrue; global NFB makes just about everything better,
and the sole effect of too much is HF oscillation, or poor transient behavior on the brink
of instability. These effects are painfully obvious on testing and not hard to avoid unless
there is something badly wrong with the basic design.
In any case, just what does low mean? One indicator of imperfect knowledge of NFB is
that the amount enjoyed by an amplifi er is almost always baldly specifi ed as so many dB
on the very few occasions it is specifi ed at all, despite the fact that most amplifi ers have
a feedback factor that varies considerably with frequency. A dB fi gure quoted alone is
meaningless, as it cannot be assumed that this is the fi gure at 1 kHz or any other standard
frequency.
My practice is to quote the NFB factor at 20 kHz, as this can normally be assumed to be
above the dominant pole frequency and so in the region where open-loop gain is set by
only two or three components. Normally the open-loop gain is falling at a constant 6-dB/
octave at this frequency on its way down to intersect the unity-loop-gain line and so its
magnitude allows some judgment as to Nyquist stability. Open-loop gain at LF depends
on many more variables, such as transistor beta, and consequently has wide tolerances
and is a much less useful quantity to know.
NFB is a powerful technique and therefore dangerous when misused. This bland truism
usually implies an audio Rakes’s progress that goes something like this: an amplifi er
has too much distortion and so the open-loop gain is increased to augment the NFB