398 



THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



perature coefficient of resistance. At the resonant frequency of the tuned 

 circuit 



Rt 

 R. 



1 



+ 1 



where Rx is the equivalent resistance of the tuned circuit at resonance. 

 The circuit values are chosen so that Rt is smaller than Rx and therefore the 

 feedback is positive. As the ampUtude of oscillation builds up, the increas- 

 ing signal level across Rt increases its temperature thereby increasing its 



Fig. 7 — A bridge-stabilized oscillator and the transistor dual. 



resistance and bringing the bridge nearer to balance, so that the amount of 

 positive feedback is reduced. This results in a stable amplitude of oscillation 

 sufficient to make Rt slightly smaller than Rx . 



Figure 7(b) shows the transformed equations and the dual transistor oscil- 

 lator. In this case, Rt' is a thermistor with a negative temperature coefficient 

 of resistance. At the resonant frequency of the series tuned circuit 



1 _ ^ 



ie Rx' 



1 4- 



R.' 



where Rx' is the effective series resistance of the tuned circuit at resonance. 

 The circuit values are so chosen that Rt' is greater than Rx' and therefore 



