TRANSISTORS IN SWITCHING CIRCUITS 



1223 



in the forward transfer characteristic. In the case of tlie transistor, the 

 behavior of the forward transfer properties is given by the forward 

 transfer impedance, Rn . 



Examining the Rn family in Fig. 13, it is seen that in the normal, 

 positi\'e emitter current region the slope, R^ , is high indicating the 

 possibility of high forward gain. When /, is negative, however, the 

 slope is zero or nearlj^ so, changing very abruptly at /« = 0. Further, 

 it is to be noted that as I^ is made negative, the collector voltage is 

 unaffected, remaining constant for further change in /« . Thus it may 

 be said that the collector voltage is saturated.* 



REGION I 



REGION n 



Fig. 12 — Broken-line idealization of negative resistance characteristic- 

 sion into regions. 



-divi- 



If, on the other hand, the emitter current is increased, at constant 

 collector current, it is found that at a critical emitter current the slope 

 again becomes zero or nearly so. There are also two further observa- 

 tions. First, the collector ^'oltage is reduced to a verj- small value and 

 second, that the critical emitter current is related to the collector 

 current. From the small-signal relation, 



Vc = R21L + RoJc 



(2) 



or 



Vc = R220CU + R22IC , 



(3) 



* It is tacitly assumed that in the relation y = f(x) that there are extremes at 

 which rj becomes essentially constant and independent of further change in the 

 independent variable x. The point farthest removed from the origin at which the 

 dependent variable becomes constant is termed saturation. The point closest to 

 the origin at which the dependent variable becomes constant is termed cutoff. 



