Some considerations in the design of negative-feedback amplifiers

A brief survey is made of the application of a phase-amplitude theorem, due to Bode, to the design of feedback amplifiers with constant-phase margins, and a graphical method of estimating the phase change at any frequency from a knowledge of the amplitude characteristic is described. The use of constant-phase margin as a design criterion is criticized, and justification is given for a new criterion termed ¿stability margin.¿ Using this criterion, a technique is developed for employing several feedback paths, thus enabling an increased amount of feedback to be connected to circuits of increased complexity. A brief account of a miniature audio amplifier with two feedback paths is included, and a 100-watt amplifier covering 300 c/s¿108 kc/s with some 35 db of feedback is described which, although it is ¿conditionally¿ stable, is suitable for use in a multi-channel telephony system.