TRANSISTORS IN SWITCHING CIRCUITS 



1247 



Variation iu characteristics will arise from self-enfj;en(lere(l heat, that 

 is, dissipation. Transistors may be thermally unstable under constant 

 Noltage conditions. Since the switching properties are e.xhibited under 

 short-circuit or constant voltage terminations, thermal propei'ties are 

 of concern. The limitations involved are similar to those of any positive 

 feedback circuit. If the thermal loss through radiation and conduction 

 exceeds the heat input, the system Avill be stable. The practical sig- 

 nihcance is to place limitations on dissipation and to employ designs 

 which result in rapid heat loss. Other design criteria such as miniaturiza- 

 tion may limit the latter. 



If perfect switching characteristics were obtainable, dissipation would 

 be of little consequence in switching. This is akin to saying that neither 

 a short-circuit nor an open-circuit dissipates any energy. Further, the 

 perfect device has zero transition time and therefore involves no loss. 

 The transistor has finite resistance both open and closed and a finite 

 although rapid transition time. There is some advantage however. A 

 constant dissipation cur\'e sho\Mi as a dotted line has been included in 

 Fig. 21. Small-signal operation at mid-range currents and voltages 

 results in fairly low limitations on both cvuTent and voltage. The inter- 

 section with the i?22 voltage saturation line (/« = 0) is at fairly high 

 voltage. Similarly, the intersection with the collector voltage cut off line 

 is at high current. For constant dissipation, approximately, 



Voltage saturation: 



Voltage cutoff: 



V 



Ills,* 



Depending upon the circuit the assumed dissipation limit may or may 

 not be exceeded during the transitions. Should the limit be exceeded, 



«, 



60 



i ^° 



? 40 



u 



Z 30 



< 



K 

 (/) 



7> 20 



to 



20 30 40 50 60 70 80 



20 30 40 50 60 70 80 

 TEMPERATURE IN DEGREES CENTIGRADE 



Fig. 24 — Temperature behavior of characteristics of developmental switching- 

 type transistor (M1689). 



* This includes both emitter and collector dissipation. See equation (30). 



