378 Chapter 12
textbooks give a very full treatment of this stability criterion.) An intersection of 12 dB/
octave is defi nitely unstable. Working within this, there are two basic ways in which to
maximize the NFB factor.
- While a 12-dB/octave gain slope is unstable, intermediate slopes greater than
6 dB/octave can be made to work. The maximum usable slope is normally
considered to be 10 dB/octave, which gives a phase margin of 30°.This may
be acceptable in some cases, but I think it cuts it a little fi ne. The steeper
fall in gain means that more NFB is applied at lower frequencies and so less
distortion is produced. Electronic circuitry only provides slopes in multiples of
6 dB/octave, so 10 dB/octave requires multiple overlapping time constants to
approximate a straight line at an intermediate slope. This gets complicated, and
this method of maximizing NFB is not popular. - The gain slope varies with frequency so that maximum open-loop gain and
hence NFB factor is sustained as long as possible as frequency increases;
the gain then drops quickly, at 12 dB/octave or more, but fl attens out to
6 dB/octave before it reaches the critical unity loop-gain intersection. In this
case the stability margins should be relatively unchanged compared with the
conventional situation.
12.3 Maximizing Linearity Before Feedback ...............................................................
Make your amplifi er as linear as possible before applying NFB has long been a cliché.
It blithely ignores the diffi culty of running a typical solid-state amplifi er without any
feedback to determine its basic linearity.
Virtually no dependable advice on how to perform this desirable linearization has been
published. The two factors are the basic linearity of the forward path and the amount of
NFB applied to further straighten it out. The latter cannot be increased beyond certain
limits or else high-frequency stability is put in peril, whereas there seems no reason why
open-loop linearity could not be improved without limit, leading us to what in some
senses must be the ultimate goal—a distortionless amplifi er. This book therefore takes as
one of its main aims the understanding and improvement of open-loop linearity.