OPERATION OF THERMIONIC VACUUM TUBE CIRCUITS 457 

 When c is a resistance, R, the expression for //, reduces to 



''- rp+R {rp+Ry'^ 



^ ~ (^Vhfe^ "' (52) 



9ai 



dEp 



r p{r p+ R) - ixr pr p 



If jji is constant, this becomes 



whicli shows that the circuit then acts as though a voltage, (fxe-\-E) 

 had been impressed in series with the plate circuit. 



OscilJalioii 



The subject of vacuum tube oscillators has been so extensively 

 treated elsewhere that but little new material has thus far been 

 obtained from the general equations now offered. The method of 

 handling the problem is, however, illuminating as it gives an example 

 of what is meant by the statement that no sharply drawn line should 

 be placed between oscillation, detection, amplification, or other uses 

 of the thermionic vacuum tube. 



In treating the oscillator problem we consider the amplification 

 term of the general equations; namely 



tp= "^ 



{rp-\-Zn) 



L ^ "^ ' V rp-\-z„J 



The oscillating conditions require that current shall flow witiiout a 

 driving voltage. Hence, as e is zero, ip can be finite only if one of 

 the factors in the denominator is zero. Thus either 



rp+zn = (54) 



or 



r,+gnil-^-~^=0 (55) 



V rp-\-z„ 



