44 BELL SYSTEM TECHNICAL JOURNAL 



Fig. 8, may be greater than the error produced by neglecting the 

 varying cut-off. In this event it would hardly be worthwhile to use 

 the method of Fig. 11, except in extreme cases where the bias differs 

 greatly from the cut-off potential. 



The dynamic output current characteristics calculated in Fig. 10 

 are shown plotted to scale in Fig. 12. It should be noted that for 

 the case of bias above cut-off {Ec = — 250) the curvature of the dy- 

 namic is reversed at the lower end. This will tend to allow a better 

 approximation to a straight line for the overall curve than can be 

 obtained for curves of bias below cut-off. 



In general the highest output impedance admissible will give the 

 straightest dynamic curve for a given tube. Comparing two tubes of 

 equal power rating, the one with the highest mutual conductance will 

 give the straightest dynamic characteristic with a given output im- 

 pedance. 



It will be noticed that the dynamic curves tend to show a saturation 

 effect at their upper ends. This is predicted, however, from the static 

 characteristics. If the grid excitation voltage is sufficient to allow the 

 grid potential to approach the plate potential very closely, the elec- 

 tron current taken by the grid will increase rapidly. Since this cur- 

 rent taken by the grid would otherwise have been taken by the plate, 

 the result is a reduction in the value of ip which causes the plate cur- 

 rent characteristic to depart from the three-halves power law. A 

 slight saturation effect at the upper end of the dynamic curve may help 

 the curve to approximate a straight line more closely. The output 

 impedance may be chosen so as to realize this advantage provided 

 the grid becomes positive during a sufficient portion of the cycle. 



The Screen Grid Tube 

 The foregoing theory, with a few alterations, will apply equally well 

 to the screen grid tube. In any screen grid tube where the screening 

 is sufficient to reduce the grid-plate capacity to the point where opera- 

 tion at high frequencies without neutralization is feasible, the screen 

 voltage will determine the plate current at a given grid voltage almost 

 entirely, the plate voltage having very little effect. In this case the 

 cut-off point will be given approximately by 



E^ 

 e,, cut-oft" = -, (14) 



where ju here is the ju of a three-element tube with the plate in place 

 of the screen. A more exact formula would be 



e„ cut-off = ,. . x' , (15) 



m(1 + p) 



