The Stabllity Problem in Feedback Amplifers

This paper is written for the engineer or student who has some general knowledge of feedback and who wishes to acquaint himself with the basic problem underlying the design of feedback amplifiers. The concept of complex frequency is introduced as a necessary background for the discussion of the stability problem. The criterion for stability is formulated in terms of fundmental restrictions on amplifier behavior at complex frequencies. From this a simplified criterion placing equivalent restrictions on the behavior at real frequencies leads directly to Nyquist´s criterion. The role played by the theory of functions of a complex variable is indicated, but a knowledge of the theory is not required. A survey of certain important relations between gain and phase-shift in minimum phase-shift networks provides the preparation needed for the concluding discussion of the principles by which stability is effected.