FEEDBACK AMPLIFIER DESIGN 



447 



feedback in the useful band satisfies a relation of the form 



'7r/2 





Adfp = constant, 



(12) 



where = sin~^///o. Thus the area under the varying characteristic, 

 when plotted against <^, should be the same as that under a correspond- 

 ing constant characteristic having the same phase and gain margins 

 and the same final asymptote. This is exemplified by Fig. 19, the 



4> IN DEGREES 



Fig. 19 — Diagram to illustrate the computation of available feedback when the 

 required feedback in the useful band is not the same at all frequencies. 



varying characteristic being chosen for illustrative purposes as a 

 straight line on an arithmetic frequency scale. 



The most important question has to do with the assumption that the 

 useful transmission band extends down to zero frequency. In most 

 amplifiers, of course, this is not true. It is consequently necessary to 

 provide a cut-off characteristic on the lower as well as the upper side 

 of the band. The requisite characteristics are easily obtained from 

 the ones which have been described by means of frequency transforma- 

 tions of a type familiar in filter theory. Thus if the cut-off characteris- 

 tics studied thus far are regarded as being of the "low-pass" type the 

 characteristics obtained from them by replacing f/fo by its reciprocal 

 may be regarded as being of the "high-pass" type. If the band width 

 of the amplifier is relatively broad it is usually simplest to treat the 

 upper and lower cut-offs as independent characteristics of low-pass and 

 high-pass types. In this event, the asymptote for the lower cut-off is 

 furnished by such elements as blocking condensers and choke coils in the 

 plate supply leads. The low-frequency asymptote is usually not so 



