304 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1956 



Since the phase crossover of A|S* is usually placed below this frequency, 

 the principal effect of the second cutoff is to introduce excess phase. This 

 excess phase can be minimized by operating the stage into the smallest 

 load resistance possible, thus maximizing Wc . j 



An undesirable property of the common emitter transmission charac- 

 teristic is that the corner frequency coi occurs at a relatively low fre- 

 quency. However, the corner frequency can be increased by using local 

 feedback as shown in Fig. 6(a). Shunt feedback is used in order to pro- 

 vide a low input impedance for the preceding stage to operate into. The 

 amplitude and phase of the current transmission is controlled prin- 

 cipally by the impedances Z\ and Z2 . If | A& \ is much greater than one, 

 and if /3 ;^ ^1/^2 , then from (7) the current transmission of the stage is 

 approximately equal to — Z2/Z1 . Because of the relatively small size of 

 A^ for a single stage, this approximation is only valid for a very limited 

 range of values of Zi and Z2 . If Zi and Zi are represented as resistances 

 R\ and Ri , then the current transmission of the circuit is given to a good 

 approximation by 



tto 



h. _ R2 1 — gp + 7 



^^ = /i= ~{R2 + n)r_^p_^ v' 



where 



7 = 



coi = 



COc = 



Co/ COaCOcCl — Oo + 7), 



R\ + Te _, Rl + Te 



R2 + ^6 



I 



(14) 



{R, -f re)Cc i 



By comparing (14) with (13), it is evident that the negative feedback 

 has reduced the low-frequency current gain from ao/(l — ao) (5 may 

 usually be neglected) to 



( 



R2 \ I «0 \ _ , ^2 



R2 + rj \1 - ao + 7/ ^1 + re 



(if 7 > 1 - ao) 



.-•! 



* The phase crossover of A/3 is equal to the frequency at which the phase shift 

 of A/3 from its low-frequency value is 180°. 



I 



