298 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1956 



and 7? — ju. At frequencies less than Ua/^ir, (1) can be approximated by 



a = 



1 + ^ 



(2)' 



COa 



where ao is the low frequency value of 



a ^ 1 



l/TT 

 2U. 



and 



2.4Z). 



CCa = 



w 



(Dm is the diffusion constant for the minority charge carriers in the base 

 region). A readily measured parameter called alpha (a), the short 

 circuit current gain of a junction transistor in the common base connec- 



SCHEMATIC 



Zc = 



EQUIVALENT CIRCUIT 



=^ V\V 



aZcLe 



Tb 



(a) COMMON BASE 

 Lb 



: 



rb 



Zed -a) 



aZc'Lb 



'X, 



■re 



(b) COMMON EMITTER 



i^b rb 



aZc 



re 



aZcLb 



Zed -a) 



(C) COMMON COLLECTOR 



re 



1 + prcCc 



re = COLLECTOR RESISTANCE 

 Cc = COLLECTOR CAPACITANCE 



ZTT 



ALPHA-CUTOFF FREQUENCY 



Fig. 2 — Basic transistor connections. 



