THE OPERATION OF VACUUM TUBES 31 



as also will io- At the instant when 



ip = lb, io = 0, 

 as illustrated by point 2, Fig. 2. Then as R,, is further increased, ip 



Fig. 2 — Plate current and plate voltage relations for sinusoidal plate current in 



circuit of Fig. 1. 



becomes less and /o starts to increase in the negative direction until 

 ip is zero at cut-off; 



Rp = ^ , io = — lb from (3). 



Then from (2) and the above, 



ep = Eb -{- IbRo, (5) 



which is the peak value of Cp. Thus the power in Rq is io"Ro averaged 

 over a c>cle and the power dissipated in the tube is ip-Rp averaged 

 over a cycle. 



In actual operation, the plate current is not sinusoidal but goes from 

 zero to peak value and back to zero in about half the cycle and remains 



