THERMIONIC VACUUM TUBFS 



83 



met by trial and adjustment taking into account Such general theo- 

 retical considerations as follows. 



Vacuum tube oscillators make a convenient way of obtaining 

 large high frequency currents at small voltages and large a.c. voltages 

 at small currents for testing purposes. 36 A special oscillator circuit 

 for giving a very pure sine wave output of constant frequency is dis- 

 cussed in Sec. 53. 



45. Equivalent Resistance of the Oscillator Circuit. Oscillator cir- 

 cuits are of many types, but the fundamental action of all of them 

 can be reduced to common terms, and to simplify the discussion the 

 type of circuit illustrated in Fig. 54 will be discussed. 



It will be noted that a d.c. voltage is applied between filament and 

 plate by means of a battery in series with a choke coil. This choke 



Eb 



Fig. 54 



may be considered as having zero d.c. resistance and virtually an 

 infinite resistance to the a.c. generated by the oscillator. An oscillat- 

 ing circuit consisting of inductance, resistance and capacity is also 

 connected between filament and plate, and by means of another in- 

 ductance joining filament and grid a portion of the output energy is 

 fed back to the input by virtue of the inductive coupling M. 



In circuits as usually constructed the ohmic resistances r± and r 2 

 of the oscillating circuit are very small compared to the impedances 

 of Li and C. It follows that the frequency of oscillation differs but 

 slightly from the natural frequency l/2Ty/L 2 C, and the oscillating 

 circuit may therefore, be looked upon as introducing nothing more 

 than a pure resistance (so far as the fundamental component of I p is 

 concerned) into the output circuit of the tube. Except for consider- 

 ations of feed-back, we may therefore imagine- the oscillating circuit 

 Lit C, replaced by an equivalent resistance whose value is given by 



the equation K = - -y . 1 his equivalent resistance is import- 

 ant in dealing with oscillating circuits. 

 "See W. C. White, Gen. Elect. Rev., Vol. 20, p. 635, 1917. 



