EQUIVALENT CIRCUITS OF NETWORKS 



595 



all displacement or capacity currents can be disregarded, and let it first be 

 supposed that the grid is negatively polarized with respect to the cathode so 

 that grid current is absent. This represents the most primitive form of 

 operation, governed by a law which has been termed "The equivalent-plate- 

 circuit Theorem."^ With reference to the assumed current direction this 

 theorem may be expressed by saying that the application of the voltage Fi to 

 the grid is equivalent, so far as phenomena in the plate circuit are concerned, 

 to the application of the voltage — /^Fi in series with a resistance r^ , where 

 H is the amplification factor and r-p the internal plate resistance. The 



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(D 



Fig. 1 — Current-voltage relations for a general four-pole. 



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co- 



Fig. 2 — Equivalent circuit of a negative grid triode at low frequencies. 



equivalent circuit for this mode of operation is thus as shown in Fig. 2, where 

 the input terminals 1 are represented by the grid G and the cathode C and 

 the output terminals by the anode P and the cathode C. In terms of analy- 

 sis the circuit is described by the two equations 



/i = 



(1) 



Equations (1) and their associated circuit Fig. 2 are expressions of the 

 fact that in any closed loop the voltages must be in equiUbrium. 



By a slight rearrangement of (1) a network representation based on current 



1 Chaffee, "Theory of Thermionic Vacuum Tubes," page 192. 



