The Alternating-Current Equivalent Circuit

Author: E.E. Kimberly

For the circuit of Fig. 27-13, the equation for plate current change is

Also,

Therefore,

By simplifying the equation, we obtain

Since gmrp = μ,

Fig. 27-14. Equivalent Circuit Corresponding to the Circuit of Fig. 27-13

It follows, therefore, that a generator having a voltage μ Δe_e and connected in series with the anode would produce the same change Δt_p in the plate current as Δeg in the grid circuit actually does. It is therefore feasible to replace the actual circuit with an equivalent circuit such as that shown in Fig. 27-14. The battery voltage Ebb is not shown because Δi_p is only the usable incremental part of the total plate current. If the simple equivalent circuit is to be used, it is necessary that the triode be operated on the linear portion of its characteristic.

If eg is a sinusoid, then Equation (27-12) may be written as a vector equation as follows:

(27-13)

where

The alternating component of plate voltage is

The gain of the amplifier of Fig. 27-13 is then expressed as follows: