TRANSISTORS IN SWITCHING CIRCriT.-* 



1235 



&] 



I, 



(r. + n + Rb +R.) 



Synopsis 



in + Rb){n + Rb+ rj 

 '■b + Rb + i-c + Re 



(Vcc + /c.(r, - r'e")) 

 rb + Rb + re + Re 



(rb + Rb) 



xirnatc Short Circuit Case 



rb «: /?b ; r„ r'/' «: 74 ; Rb < r„, /■„.; /?< = /?. = 0; 



TcXrc - rn <^ Vc 



1 I 



a II 



a III 



F. = 7.r/ + 



VcRb 



Rb + re 



F, = LRoir - a) + 



VcRb 



Rb + re 



1 + 



i^bCl ^— 'a 



F„ i2b + re 



Vtp Rb 



nice characteristic and properties. 



for /, < and r„ > for I, > 0. This is not a precise assumption and 

 the turmng point will usually lie slightly in the positive emitter current 

 region. For very small triggers or more accurate calculations, considera- 

 tion must be given to closer approximations of r^ = /i(/«) and Rn = 

 hih). 



The consequences of equation (31) can be quite serious. Vc and ^6 

 are of course fixed, but Tc is variable from unit to luiit, with temperature 

 and perhaps with life. The variability of V,p can result in failure to 

 trigger, self-triggering or lock-up at high current. 



