DESIGN THEORY OF JUNCTION TRANSISTORS 1283 



Finally, the last two terms, the feedback and output admittances, are 

 smaller than the first two by approximately 



kT_ dw 

 qwo dVc 



which is usually 10~ or less, but the four form a rather symmetrical set. 

 The symmetry of the terms can be seen by removing the dissymme- 

 tries. The term /„« (1 + i(jOTne) in ?/«« results from diffusion of electrons 

 into the emitter from the base and is the emission loss term which 

 makes 7 less than unity. The factor (1 + anc/a-pc) which appears in yc» 

 and ycc is the collector multiplication factor a* which results from the 

 flow of electrons out of the collector body. If 7 and a* are assumed to be 

 unity, the admittances assume a more symmetrical form. In that case 

 Vce/yee = Vec/Vcc = " jS, the base transport factor, and the forward 

 and reverse current transmission ratios are identical. This can be seen 

 more clearly if the hyperbolic expressions are replaced by the first two 

 terms of their polynomial expansions, yielding 



(1 + iw/03a) 

 Vee = g, 



Vce = 



(1 + iui/303a) 



1 + ioj/SoJa 

 — ^qQcc 



1 + ^a)/3aja 



(1 + ico/Wa) 



V^2D,rJ 



is the low frequency value of the base transport factor. A lumped param- 

 eter equivalent circuit for these simplified admittances is shown in de- 

 tail in Fig. 4(a). ^ 



It is apparent here that a single parameter, Wa = 2DJwq , specifies 

 the frequency variation of all four admittances. This frequency depend- 

 ence, which has been commonly measured as the alpha cutoff frequency 



