TRANSISTORS 



It is perhaps unfortunate that although the earthed emitter circuit is 

 probably the most important, many manufacturers describe their transistor 

 parameters in terms of the (possibly more fundamental) earthed-base 

 circuit {Figure 45.25a). Fortunately the conversion factors to get from 

 Figure 45.24 to Figure 45.25a are quite simple : 



h = ^6 



re ^ (^K 

 r — R 



Alternatively, the common-base configuration may be used with a current 

 generator (Figure 45.25b) where ar^ = r^. 



bh 



r^ 



A/y/\y\/ \r^) — " — ° 





rOQi 



bL 



re 



out 



(a) 



(b) 



Figure 45.25 



Thus, transistors may be described in terms of r^, r^, r^ and r^ or r^, r^, 

 r^ and a. In one work of reference known to the writer, r^,, r^, r^ and /5 are 

 used, though this seems a little inelegant, since the first three belong to the 

 earthed base configuration, and the last to the earthed emitter. The exact 

 relationship between /? and a is /? = a/(l — a); a is slightly less than unity. 



Alternative schemes of description which follow regard the transistor as 

 a black box {Figure 45.26), and different sets of parameters are required 



Figure 45.26 



for each of the three transistor configurations. It is important not to confuse 

 the sets. One method used for labelling them is to leave earthed base 

 parameters unprimed, to give earthed emitter parameters one prime, and 

 earthed collector parameters two primes. 



The four-resistance system — This describes the transistor in terms of two 

 resistances and two mutual resistances or 'trans-resistances' : 



11 



ro, = 



21 



5/in J (5/out = 



692 



