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BELL SYSTEM TECHNICAL JOURNAL 



C of Fig. 6 serves only as a high-frequency by -pass; the direct voltage across r 

 being the instantaneous difference between the peak voltage induced in L3 

 and that of the stabilizing battery B. For very high modulating frequencies 

 the modulation as well as the carrier is by-passed by C and no modulation 

 voltage appears across r. Thus the bias is constant and the output wave is 

 identical with the input wave. This corresponds to an envelope transmission 

 of (1, 0). For intermediate values of the modulating frequency the voltage 

 developed across r varies in magnitude and phase approximately as if a 

 constant current of the modulating frequency / were applied to r and C in 

 parallel. 



The output of the amplifier depends not only upon the bias developed 

 across r but also upon the input. For systems having a large amount of 

 control the action of the bias is predominant. Thus for a low modulating 

 frequency the variation of the bias overpowers the initial modulation, the 

 phase of the modulation is reversed, and the percentage magnified by the 



(1.6) 

 Fig. 11 — Envelope transmission of a modulated wave through controlled amplifier. 



action of the limiter. In Fig. 1 1 the envelope transmission is plotted in polar 

 form for conditions of relatively large and relatively small amounts of control. 



Loop Transmission 



The separate diagrams of Figs. 10 and 11 are combined in Fig. 12 to de- 

 termine the stability of the system. For any chosen frequency/ the vector 

 of Fig. 10 is multiplied by the vector of Fig. 11 corresponding to the same 

 frequency to locate one point of Fig. 12. The resultant vector has an angle 

 which is the sum of the two component angles and a magnitude which is the 

 product of the two component magnitudes. 



It is seen that the loop may be made to cross the axis considerably to the 

 left of the point (1,0) if the points --1 and A' of the previous figures cor- 

 respond to the same frequency. Similarly the loop may be made to come 

 very close to the point (1,0) by increasing the size of C or lowering the Q 

 of the tuned circuit so that the points B and B' correspond to the same 

 frequency. With the circuit elements drawn in Fig. 6 the stability margin 



