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BELL SYSTEM TECHNICAL JOURNAL 



voltage swings. If the time constant of the grid-leak-condenser combination 

 is long in comparison to the period of a modulation cycle the bias will not 

 be able to follow the applied voltage and the modulation of the output 

 will be larger than that of the input. Moreover it is in phase with that of 

 the input. When the modulating frequency is low the bias is able to follow 

 the level of modulation and the output modulation is very small. Thus the 

 transmission of a modulated signal is greatest at high modulating frequen- 

 cies, and the modulation output is in phase with the input. Because of the 



Fig. 14 — Envelope transmission of a modulated wave through a grid-leak-biased Class C 



amplifier. 



f = 



Fig. 15 — Nyquist diagram applying to Fig. 13. 



action of the grid-leak-condenser network a phase shift at intermediate 

 modulating frequencies occurs. This behavior is represented in polar form 

 in Fig. 14. 



The stabihty of the system is determined by combining in Fig. 15 the 

 separate diagrams of Figs. 14 and 10. As in the previous system a 

 thoroughly stable system results if the element values are such that the points 

 A and A' of Figs. 10 and 14 correspond to the same frequency. If on the 

 other hand the elements are such that B and B' correspond to the same fre- 

 quency the curve loops (1, 0) indicating instability. In general stability is 



