TliANSlSTOUS I\ SW ITCHl.NCi CIKCUITS 1233 



Equation (27) is also the equation of a .strai{»ht line lia\-inf;- the xoltaftc 

 axis interc'ei)t of {\'cRb)/{Ih + /',) the same \'alue as in Kegion 1. The 

 slope, Rb(l — a), is negative provided a > 1. 



KVALUATIO.V IX REGION III 



In Region III it may be assumed that rb « lib , Vc « Rb and /'l" « Rb . 

 Other suitable approximations will depend largely upon the magnitude 

 of Re . From equation (23) 



F.I 



' "\ -i 



/// , ,, Rb{Rb + /'m ) 



''e -r tib — 



Rb + !•:" + RJ Rb + R 



V R,. 



L + ^' ' (28) 



If Re is large, that is, large compared to /% , but small compared to 

 Region II Tc , then (28) becomes, 



T' r>^ tibiae J 1^ V cc^b (i^(\\ 



* '"' - Ih+^e ^' + WVR. ^^^^ 



Under these conditions, the circuit is essentially independent of 

 de^'ice parameters. This is useful where a high independence of device 

 parameters is required, but does not focus the attention upon the 

 device parameters as does the Re ^ ^ case. This is the condition under 

 which the transistor might be operated when it is desired to obtain the 

 maximum ON current, or conversely the minimum internal switch re- 

 sistance. 



Where Re = 0, ecjuation (28) becomes, 



T^in = [/•:" + r'e" - r::']L + F. (30) 



Since /% and (r<; — r^ ) are quite small the short-circuit currents 

 may be very high. Where the transistor is considered as a switch between 

 emitter and cbllectbr circuits, the "switch" voltage drop, as V^c , is 

 given by the first term of equation (30). 



KVALIATIOX OF REGION J-REGION II TRANSITION 



Earlier, trigger sensitivities were mentioned as being the small voltage 

 and current differences between the turning points of the negative 

 resistance characteristic and the stable operating points. The determina- 

 tion of the turning points and their stability is of great importance 

 since it is usually desired to obtain maximum stable sensitivity. The 

 voltage and current at the two turning points* have been given the 

 subscript p and v for the low and high current conditions respectively 

 as shown in the synopses of Fig. 17, 18 and 19. V^p and I^p in theshort- 



* Sometimes termed "peak" and "valley". 



