SPACE CHARGE BETWEEN PARALLEL PLANE ELECTRODES 71 



is desired; however, type B solutions with some reflected electrons 

 would be objectionable. The permissible operating range must 

 therefore lie in the C region on Fig. 7. If, for example, we operate 

 the tube with Vz = Vi as the quiescent point and with a resistance 

 load, the operating curve is a straight line through the point 7 = 0, 

 ^ = 1. The upper excursion of this line is set by the upper limit, 

 curve a, of the C region. For characteristics which cross curve d and 

 enter the D region a loss of suppression would result. Maximum 

 efficiency occurs with the largest possible excursion of V2, that is for 

 a line tangent to the d curve at the quiescent point. The minimum 

 instantaneous value of F2 for the conditions specified is seen to be 

 25 per cent of its quiescent value as contrasted with a usual value of 

 about 10 per cent for a pentode when similarly operated. For the 

 instantaneous value of V2 to fall to 10 per cent of its initial value 

 (without loss of suppression) the quiescent point must be at 7 = 0, 

 ^ = 1.3 with the operating line tangent to curve d at about <p = 0.5. 

 Operation in the D region above ^ = 1 does not result in a secondary 

 current from plate to grid. 



//. Triode Operation in the Positive Grid Region 

 Figure 1 may be taken as representing an idealized triode. The 

 second plane represents the plate so that V2 = Vp and the first plane 

 represents the grid plane with effective potential Fi. We let 



Fi = F, + Fp/m, 



where n will be a variable if space charge is formed in the plate-grid 

 region and may differ considerably from its "cut-off" value. 



If the fraction of the electrons stopped by the grid is negligible and 

 if operation does not enter the B region, with disturbing effects 

 produced by reflected electrons, then Fig. 1 becomes an accurate 

 representation and in view of the choice of units a is a geometrical 

 constant of the tube. Hence the operating characteristic is a constant 

 7 line (7 = 0"^) on Fig. 7.^^ 



For normal operation the characteristic must not extend into the B 

 region. If the grid is to be driven far positive it is desirable to stay 

 in the C region for all values of ^ < 1 to prevent secondary electrons 

 from leaving the plate. The minimum value of ^ as a function of 7 

 may be read from the intersections of the constant 7 lines with limiting 

 curve a if 7 > 2 and limiting curve d ii y < 2. Now since 



Vp Vp 





Fi Vp 



' Va-\- — 



1^ Proof of the relationship 7 = o-^ is contained in the appendix. 



