THERMIONIC VACUUM TUBES 



45 



To a first approximation, 26 the alternating component / of the 

 space current, when a voltage e = e cos pt is applied to the grid is given 

 by 



/= 



r+r p 



e cos pt ■ 



r p r p e i 

 2\(r + r p ) 



r(l+COs2/>/). 



(4) 



In this equation r p is the internal resistance of the tube and r p is its 

 derivative with respect to the effective voltage ( r--E«J. It will be 



noted that the second term on the right side of Equation 4, which 

 gives the first harmonic, diminishes rapidly with r as was to be ex- 

 pected from the preceding paragraph. The more or less straightened 



-X> -30 -V> -20 



Eg Vo'ts 



Fig. 14 — Average I p , ^Characteristics for Five Tubes. Average /x =5.92; average 

 internal impedance =6,000 ohms for £& = 130 volts and E g =-9 volts. Plate bat- 

 tery connected to +end of filament and grid battery to —end. Western Electric 



type 101-B tubes. 



characteristic resulting from the effect of r is known as the dynamic 

 characteristic; see Fig. 14, which curves fit a tube whose r =6,000 

 ohms. As will be pointed out in the section on amplifier circuits, 

 the dynamic characteristic is a useful guide in selecting tubes as 

 amplifiers. 



Equation 4 also expresses the important fact that the application 

 of the voltage e to the grid is equivalent, so far as current in the plate 

 circuit is concerned, to the application of the voltage p. e in the plate 

 circuit. 



13. Internal Capacities and Effect on Input Impedances. In certain 

 uses to which the vacuum tube may be put, a knowledge of the in- 

 fluence which the internal electrostatic capacities have on the input 

 impedance is important. The equivalent circuit of the tube 27 is 



J6 J. R. Carson, Proc. Inst. Radio Engrs., Vol. 7, page, 187, 1919. In case the 

 output circuit of a tube contains reactance as well as resistance, Eq. 4 becomes 

 much more complicated as Carson shows. 



17 H. W. Nichols, Phys. Rev., Vol. 13, p. 405, 1919; J. M. Miller, Bureau of Stand- 

 ards, Bulletin No. 351. 



