TRANSISTOR CIRCUITS FOR ANALOG AND DIGITAL SYSTEMS 305 



The half power frequency, however, has been increased from 



1— Oo , 1— Oo + 7 



t:^ 1 + 7 , 1 



as shown by the dashed curves in Fig. 5.* 



The bandwidth of the common emitter stage can be increased without 

 reducing the current gain at dc and low-frequencies by representing Zi 

 by a resistance Ri , and Z2 by a resistance R2 in series with a condenser 

 C2 . If I/R2C2 is much smaller than co/, then the current transmission of 

 the stage is given by (14) multiplied by the factor 



P 



1 + 



C04 

 P 



(15) 



where 



602 



Wi 



H^^i 



1 - cro + 



Ri + re 



C2(/?2 + r6)(l - ao + 7) 



The current transmission for this case is plotted in Fig. 6(b). The con- 

 denser d introduces a rising 6 db per octave asymptote with a corner 

 frequency at wi . At dc the current gain is equal to 



ao 



1 — ao + 5 



A second method of shaping the loop current transmission char- 

 acteristic of a feedback amplifier is by means of interstage networks. 

 These networks are usually used for reducing the loop current gain at 

 relatively low frequencies while introducing negligible phase lag near 

 the gainf and phase crossover frequencies. Interstage networks should 

 be designed to take advantage of the variable transistor input impedance. 

 The input impedance of a transistor in the common emitter connection 



* In Figs. 5 and 6(b), the factor R^/iRi + n) is assumed equal to unity. This is 

 ' a good approximation since in practice R2 is equal to several thousand ohms while 

 rt is equal to about 100 ohms. 



t The gain crossover frequency is equal to the frequency at which the magni- 

 tude of Al3 is unity. 



