16 



BELL SYSTEM TECHNICAL JOURNAL 



Because the limiter introduces no phase shift it follows that the envelope 

 loop transmission is merely an enlarged and reversed copy of that for the 

 filter. This can loop the (1,0) point only if there are at least three shunt 

 elements in the filter section. That is, instability can result only if the phase 

 shift of the filter system exceeds 180° for frequencies relatively near the 

 operating frequency. This circuit is therefore much less likely to produce 

 intermittent operation than any other circuit here considered. 



Fig. 20 — Schematic diagram of varistor stabilized oscillator. 



X. Negative Feedback in Oscillators 



Because positive feedback is the necessary condition for the operation 

 of an oscillator it is not obvious that the application of negative feedback is 

 ever desirable. Actually it is frequently possible to introduce negative 

 feedback into an oscillator with no loss of performance and under certain 

 circumstances advantages are gained. 



The circuit of Fig. 16 serves as a convenient example. Removal of the 

 cathode by-pass condenser is likely to reduce the amplifier gain by about 

 6 db and to increase the stabihty of the gain with respect to applied voltages 

 by a corresponding amount. Coincident with removal of the by-pass 

 condenser the operating level drops a small amount, the bridge loss decreases 

 6 db to reestablish equilibrium, and the stabilizing effect of the bridge is cut 

 in half. Accordingly the over-all stability of the output with respect to 

 applied voltages is unchanged. The advantages gained are that the loss 

 which must be held in the bridge is reduced so that stray reactances are less 

 likely to disturb the operation, and that the harmonic content of the output 

 is reduced. 



Stated in a different way, the output stabihty of an oscillator using a non- 

 feedback amplifier is limited in practice by the bridge balance which may 

 be maintained. After this value of gain has been reached additional stability 



