FEEDBACK AMPLIFIER DESIGN 



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useful range is decreased as the assumed margins are increased, so that 

 it is generally desirable to choose as small margins as is safe. 



The essential feature in this situation is the requirement that the 

 diminution of the loop gain in the cutoff region should not be accom- 

 panied by a phase shift exceeding some prescribed amount. In view 

 of the close connection between phase shift and the slope of the attenu- 

 ation characteristic evidenced by (2) this evidently demands that the 

 amplifier should cut off, on the whole, at a well defined rate which is not 

 too fast. As a first approximation, in fact, we can choose the cutoff 



Fig. 9 — Diagram to illustrate definitions of phase and gain margins for the feedback 



loop. 



characteristic as an exactly constant slope from the edge of the useful 

 band outward. Such a characteristic has already been illustrated by 

 Fig. 5 and is shown, replotted,® by the broken lines in Fig. 10. If we 

 choose the parameter corresponding to k in Fig. 5 as 2 the cutoff rate 

 is 12 db per octave and the phase shift is substantially 180° at high 

 frequencies. This choice thus leads to zero phase margin. By choos- 

 ing a somewhat smaller k on the other hand, we can provide a definite 



• To prevent confusion it should be noticed that the general attenuation-phase 

 diagrams are plotted in terms of relative loss while loop cutoff characteristics, here 

 and at later points, are plotted in terms of relative gain. 



