REGENERATION THEORY AND EXPERIMENT 



695 



0° 

 360° 



Fig. 10 — Transfer factor diagram for the amplifier of Fig. 9 with the feed-back 

 attenuator set at zero decibel. 



by the theoretical derivation. Where the transfer factor is zero at zero 

 frequency, only A{jiS) is required since the loop then closes for positive 

 values of w. In amplifiers transmitting d.c. , however, both positive and 

 negative values of oj are needed ^ to form a closed loop. In any case 

 ■4(— jco) is the mirror image of ^(jco) about the x-axis. 



Extensions of the Criterion 

 Nonlinear Amplifier 

 The stability criterion which was verified by the experiments re- 

 ported in the preceding section is framed for linear systems, those in 

 which the steady state response is linearly proportional to the applied 

 force. In vacuum tube circuits, linearity is best approximated at small 

 force amplitudes, and is departed from to an extent dependent upon 

 the impressed potentials, as well as upon tube and circuit characteris- 

 tics. The divergence from linearity becomes well marked when the 

 load capacities of the tubes are approached, or when grid current is 

 made to flow through large grid impedances. The question then arises 

 as to the form which the stability criterion takes when a tube circuit is 



* Log. cit. 



