TRANSISTORS IN SWITCHING CIRCUITS 1237 



EVALUATION OF REGION II-REGION III TRANSITION 



Tlie hij^h-cuiTent (uniiiif>; point for the .short-circuit case is deter- 

 mined from a simultaneous sohition of the pertinent equations for 

 Regions II and UK ('((nations (27) and (30). Thus, 



VcTe 



V ?^ V 



I 1 1 1 III ' ' l\ 



(34) 



(re + /?6)/?6(l - a)J 



Where it ma>' l)e appi-oximated that /v » /f^ , as has ah'eady been 

 done in bringinsi; in a, (Hjuations (33) and (34) become, 



V '-^ V 



r, + Vc - r 



(36) 



Rb (1 - a) 



In this order of approximation, T^„ is nearly equal to T^ . Any \-aria- 

 tion in the lower trigger point is primarily with /e, , due chiefly to any 

 change in a. It is interesting to note that the trigger point will mo\^e 

 along the Region III curve given by (30). 



The ratio of F^t, to V,p is often of interest to estimate voltage swings 

 or perhaps as a design objective in some switching circuits. Thus from 

 (36) and (31), 



F.„ [R,(l - «) + r'/' + ;•:" - r'^'](R, + r,) 



V^v Rla - a) 



— a) then (37 



V,v Rb + Tc 



If r" + Tc — Vm « Rb{l — a) then (37) becomes: 



(37) 



(38) 



Vtp Rb 



If Rb is verj' large, the ratio approaches unity with the implication of 

 the existence of only two regions. This is eciuivalent to saying that the 

 negati\-e resistance becomes infinite over an infinitely short range. The 

 proper choice of Rb in terms of (38) may well be a design problem where 

 it is desired to ha\e a high i-atio of l\,, to T'^,. , as in lockout circuits. 



SYNOPSIS OF NEGATIVE RESISTANCE CHARACTERISTICS 



Synopsis for the three negative resistance characteristics are gi\-en 

 in Figs. 17, 18 and 19. The solution and analysis pi'ocedures are the 

 same as outlined for the emitter charactei-istic. It should be not(xl that 

 the base characteristic is short-circuit stable in distinction to the emitter 



