1320 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1953 



applied to the collector and the emitter current are shown. The condition 

 which limit the ac voltage on the collector to this value will be discussed 

 later. 



In region 1 the net voltage on the emitter to base circuit is negative 

 and it has no effect on the current which flows in the collector. The cur- 

 rent which flows in the collector will therefore depend purely on the re- 

 verse resistance of the collector acting as a diode and the voltage applied 

 between collector and base. This is a positive resistance but it is not 

 always linear. Since the only voltage active in this region is that obtained 

 from the tank circuit, this resistance acts as a load on the tank circuit. 



In region 2 the voltage applied to the emitter circuit is positive and 

 the current that flow^s as a result of this voltage exercises control over 

 the collector current. The external resistance that is used in the emitter 

 circuit is sufficiently high so that the small changes that occur in the 

 emitter input resistance as the emitter current is varied are insignificant. 

 The current that flows in the emitter circuit can therefore be considered 

 vary linearly with the ac voltage. Since for each incremental change in 

 collector voltage a proportional change in emitter current will be ob- 

 tained, a plot of this relationship for a given ratio of emitter to collector 

 ac voltage will result in a straight line. The slope will be determined by 

 the amount of feedback (emitter voltage). Since a decrease in the abso- 

 lute voltage on the collector results in an increase in collector current, 

 the line has a negative slope. It therefore represents a negative resistance. 



In region 3 the emitter current exercises very little control over the 

 collector current. 



A negative resistance represented by —R2 is therefore obtained from 

 to TT of the ac wave. If a second transistor were added of identical charac- 



lil 16 



II 

 Si 



> 

 I 



30 



15 10 5 



COLLECTOR CURRENT, I^ , IN MILLIAMPERES 



Fig. 3. — Idealized transistor characteristics with operating regions for oscilla- 

 tor indicated. 



