596 



BELL SYSTEM TECHNICAL JOURNAL 



equilibrium may be obtained. For this purpose (1) is written as 

 /i = 





1 



(2) 



The corresponding network representation is as shown in Fig. 3, where the 

 energizing source in the plate circuit consists of a constant current generator 



M 

 of strength — ~ Fi impressed across the output terminals 2. 



© 



li G 



® 



■op 



Co- 



■oC 



Fig. 3 — Equivalent circuit of a negative grid triode at low frequencies. 



ru)/igV2 



Fig. 4 — Equivalent circuit of a positive grid triode at low frequencies. 



Let US now consider a further step and assume that the grid is positive so 

 that grid current is also flowing. In regard to the grid circuit there is a 

 theorem called the ''Equivalent-grid-circuit Theorem" which is exactly 

 similar to the corresponding plate circuit theorem. The theorem says that 

 the a-c grid current can be calculated by assuming that an e.m.f. iig W 

 acts in series with a resistance rg where //^ is the reflex factor and Vg the 

 internal grid resistance. In symbols and by using the potation of Fig. 1 

 this is expressed by: 



Fi = ;u, F2 + h Tg (3) 



By combining the two theorems the equivalent circuit of Fig. 4 results. 

 Again by writing (3) as 



/i = - Ki - ^ K2 



fo r„ 



(4) 



a corresponding network based upon current equilibrium may be found. 



