TABLE OF CONTENTS The compensation of an amplifier is the tailoring of its open-loop gain and phase characteristics so that is dependably stable when the global feedback loop is closed.
It must be said straight away that compensation is a thoroughly misleading word to describe the subject of this chapter. It implies that one problematic influence is being balanced out by another opposing force, when in fact it means the process of tailoring the open-loop gain and phase of an amplifier so that it is satisfactorily stable when the global feedback loop is closed. The derivation of the word is historical, going back to the days when all servomechanisms were mechanical, and usually included an impressive Watt governor pirouetting on top of the machinery.
An amplifier requires compensation because its basic open-loop gain is still high at frequencies where the internal phase-shifts are reaching 180 . This turns negative feedback into positive at high frequencies, and causes oscillation, which in audio amplifiers can be very destructive. The way to prevent this is to ensure that the loop gain falls to below unity before the phase-shift reaches 180 ; oscillation therefore cannot develop. Compensation is therefore vital simply because it makes the amplifier stable; there are other considerations, however, because the way in which the compensation is applied has a major effect on the closed-loop distortion behaviour.
The distortion performance of an amplifier is determined not only by open-loop linearity, but also the negative feedback factor applied when the loop is...
