THEORY OF MULTI-ELECTRODE VACUUM TUBES 



57 



Since the transconductance, Sm, has a value not greatly different from 

 the normal value for a triode, and since Rp is very large, m, which is 

 proportional to Rp, also must be very large. Referring to equation 14 



•2. 

 Z 6 



< 3 



10 



08 2 



07 5 



. -to 

 ^D 2 



Z _l 



iij •^ 



20 



40 



60 



80 



160 



180 200 220 240 



100 120 140 

 PLATE VOLTS, Ep 



Fig. 3 — Characteristics of a screen-grid tetrode similar to those shown in Fig. 2 

 except that E, = 90 volts. 



and its analogy with the generator equation, it may be seen that, 

 despite the fact that the screen-grid tube considered as a generator has 

 a very high internal resistance, it gives normal output because it is 

 provided also with a very large electromotive force, y.eg. 



From this analysis of the characteristics of the screen-grid tube, 



