FEEDBACK AMPLIFIER DESIGN 



431 



This expression will be used as the fundamental formula for the loop 

 cut-off characteristic in the next section. 



Overall Feedback Loop Characteristics 



The survey just concluded shows what combinations of attenuation 

 and phase characteristics are physically possible. We have next to 

 determine which of the available combinations is to be regarded as 

 representing the transmission around the overall feedback loop. The 

 choice will naturally depend somewhat upon exactly what we assume 

 that the amplifier ought to do, but with any given set of assumptions 

 it is possible, at least in theory, to determine what combination is most 

 appropriate. 



Fig. 8 — Nyquist stability diagrams for various amplifiers. Curve I represents 

 "absolute" stability, Curve II instability, and Curve III "conditional" stability. 

 In accordance with the convention used in this paper the diagram is rotated through 

 180° from its normal position so that the critical point occurs at — 1,0 rather than 

 + 1,0. 



The situation is conveniently investigated by means of the Nyquist 

 stability diagram ^ illustrated by Fig. 8. The diagram gives the path 



^ Bell System Technical Journal, July, 1932. See also Peterson, Kreer, and Ware, 

 Bell System Technical Journal, October, 1934. The Nyquist diagrams in the present 

 paper are rotated through 180° from the positions in which they are usually drawn, 

 turning the diagrams in reality into plots of — m/3. In a normal amplifier there is one 

 net phase reversal due to the tubes in addition to any phase shifts chargeable directly 

 to the passive networks in the circuit. The rotation of the diagram allows this 

 phase reversal to be ignored, so that the phase shifts actually shown are the same as 

 those which are directly of design interest. 



