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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 Δec results in a change of plate current Δib and a subsequent change of plate voltage -Δeb 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 Δib by the change in grid voltage Δec that produced it. Therefore,



where gm is in mhos and rp is in ohms.

Last Update: 2010-11-27