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BELL SYSTEM TECHNICAL JOURNAL 



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LOAD CURRENT-DB UP ON I MIL INTO 600 OHMS 



Fig. 12 — Comparison of feed-back attenuator settings required for the starting 

 of oscillations, and those deduced from the transfer diagram, plotted as functions of 

 the 60-kilocycle load. The two dashed curves correspond to the two points (roughly 

 4.5 and 8 kilocycles) at which the transfer factor diagram (Fig. 11) crosses the zero 

 phase axis. The gains of Figs. 11 and 12 cannot be compared directly because of a 

 change made in the amplifier circuit of Fig. 12 which increased the loop gain. 



Nyquist's criterion involving the transfer factor may be transformed so 

 as to formulate a more complete criterion involving such an impedance. 

 To do this we have to express the factor (1 —A), on which the 

 stability criterion was based, in terms of the circuit impedances. For 

 illustrative purposes we may quote the results obtained with the two 

 fundamental forms of feed-back circuits, the series and shunt types. ^^ 

 These results, while obtained for the input circuit of the amplifier, are 

 valid for any other point of the feed-back loop. Further, combinations 

 of the shunt and series type feed-back circuits may be used. 



Series Feed-Back 



The series circuit is shown in Fig. 13, so called because the feed- 

 back is applied in series with the amplied electromotive force and the 

 amplifier input. The passive impedances marked are those existing 

 when the feed-back loop is broken and terminated as indicated by the 



^2 Crisson, Bell Sys. Tech. Jour., vol. X, p. 485. 



