THERMIONIC VACUUM TUBES 



85 



phase. This latter condition is obviously required for maximum 

 power output by an a.c. generator. In order that I P be a maximum 

 when E p is a minimum, E g must be a maximum at this time. 



47. Dynamic Charac 'eristic. A tube when operating into an output 

 resistance follows a dynamic characteristic (also called derived char- 

 acteristic) 37 whose slope is somewhat less than that of the static (see 

 Sec. 12). The dynamic characteristic is flatter than the static char- 

 acteristic since the oscillating circuit Li, C acts as an equivalent re- 

 sistance. In fact, the dynamic characteristic of the oscillating tube 

 may differ in one important respect from that of Sec. 12, for in an 

 oscillator the grid potential has a relatively very high positive value 

 for a portion of the cycle. Consequently the dynamic plate char- 

 acteristic very frequently turns downward at its upper end (point B, 

 Fig. 56). As will be pointed out later, this feature of the dynamic 



Gnd Potential 



Fig. 56 



characteristic is likely to be one of the factors tending to limit the 

 amplitude of oscillation. 



48. Amplitude of Oscillation. As yet no very comprehensive 

 formula has been derived, either theoretically or empirically, to express 

 the amplitude of oscillation in terms of the tube and circuit constants 

 and the applied Ep. However, certain general statements can be 

 made which will serve as useful guides. 



We shall not consider further the circuit shown in Fig. 54; it shows 

 a negative grid battery in the grid filament branch, which, if sufficiently 

 large to control the oscillator when in operation, usually makes the 

 starting difficult and uncertain, and is therefore undesirable. To 

 eliminate the grid battery and yet supply sufficient negative potential, 

 once oscillations have started, circuits are usually supplied with a 

 grid blocking condenser and high resistance leak (see Figs. 64 and 

 65). Since for a portion of each oscillation the grid is positive, it 



37 See L. A. Hazeltine, Procd. Inst, of Radio Engrs., April, 1918. 



