6 BELL SYSTEM TECHNICAL JOURNAL 



able amount of negative feedback is applied and properly controlled. Posi- 

 tive feedback is sometimes used to increase gain or selectivity, but stability 

 under such circumstances is poor. Any considerable amount of positive 

 feedback results in oscillation. 



The criterion by which stable feedback systems are distinguished from 

 unstable ones has been presented by Nyquist and verified by others.^' ^ 

 A closed feedback system having input and output terminals is illustrated 

 in Fig. 4. In Fig. 5 the loop is opened at some arbitrary point and a test 



INPUT 



OUTPUT 



FEEDBACK 

 CIRCUIT 



Fig. 4 — Typical feedback amplifier. 



TEST 

 GENERATOR 



TEST 

 DETECTOR 



FEEDBACK 

 CIRCUIT 



Fig. 5 — Test for stal^ility of feedljack amplifier. 



oscillator and detector are connected. Here as in Fig. 2 certain precautions 

 as to impedance are observed. The test generator must produce a pure 

 sinusoidal wave of such small magnitude that no part of the tested system 

 overloads and the vector ratio of the detector voltage to the generator voltage 

 is observed for a large number of frequencies. The polar plot of Fig. 3 

 applies directly to the feedback amplifier except that the radius vector 

 represents the transmission of a simple wave rather than of an envelope. 



^ H. Nyquist, "Regeneration Theory," Bell S\s. Tech. Jour., Vol. 11, pp. 126-147, 

 Jan., 1932. 



8 E. Peterson, J. G. Kreer, & L. A. Ware, "Regeneration Theory and Experiment," 

 Proc. LR.E., Vol. 22, pp. 1191-1210, Oct., 1934. 



