The Mutual Conductance

Author: J.B. Hoag

Very often we are interested in the change of the plate current for different changes in the grid voltage. This is expressed by means of a quantity called mutual conductance, g_m, or, as it is sometimes called, the transconductance s_m. It is defined as the change of plate current for a change of 1 volt on the grid. Thus

Since the plate resistance changes with the voltages on the tube, so also does the mutual conductance. For different triodes on the market, the mutual conductance ranges from 200 to 5,000 micro-mhos (units of conductance). A value of 5,000 micro-mhos means that the plate current will change by 5 milliamperes when the voltage change of the grid is 1 volt.