Amplification Factor, Plate Resistance, and Transconductance

Author: E.E. Kimberly

The plate current of a triode in its circuit may be made to change by changing either the plate voltage or the grid potential. The amplification factor relates these two functions. If, from a given operating point on the mutual characteristic, a change of grid voltage Δe_c results in a change of plate current Δi_b and a subsequent change of plate voltage -Δe_b will restore the plate current to its original value, then the amplification factor is

For example, if a change of 40 volts on the plate will restore a plate-current change caused by a change of + 2 volts on the grid, the amplification factor is 20.

The plate resistance rp is found by dividing a small change in plate voltage by the accompanying change in plate current (in amperes).

The grid-plate transconductance or mutual conductance gm is found for any fixed plate voltage by dividing a small change in plate current Δi_b by the change in grid voltage Δe_c that produced it. Therefore,

and

where g_m is in mhos and r_p is in ohms.