TRANSISTOR CIRCUITS FOR ANALOG AND DIGITAL SYSTEMS 



323 



the load R^ as it appears at the primary of the transformer T2 . Ri is a 

 function of the dc voltage across the diode Z)i . At a prescribed value of 

 Ri , the comparator circuit becomes unstable and switches. The relation- 

 ship between this critical value of Ri and the transistor and circuit 

 parameters is obtained by evaluating the characteristic equation for the 

 circuit and by determining the relationship which the coefficients of the 

 equation must satisfy in order to have a root of the equation lie in the 

 right hand half of the complex frequency plane. To a good approxima- 

 tion, the critical value of Ri is given by the expression 



R, -\-R„ + n = 



Mao 



RiCc -\- 





(26) 



N'^Rr 



where M is the mutual inductance of transformer Ti and R2 — ly h^l 

 Since the transistor parameters which appear in expression (26) have only 

 a small variation with temperature, the critical value of Ri is independent 

 of temperature (to a first approximation). 



It will now be shown that the comparator can be designed for an ac- 

 curacy of ±5 millivolts throughout a temperature range of 20°C to 40°C. 

 In order to establish this accuracy it will be assumed that the critical 

 value of 7^1 is equal to 30,000 ohms. This assumption is based on the 



30/iF 



TO LC FILTER 



IN MAGNETTOR 



NPUT CIRCUIT 



4/iF 



+33V 



I+33V 



Fig. 17(b), 900-cycle carrier oscillator. 



