Transconductance

Transconductance (for transfer conductance), also infrequently called mutual conductance, is the electrical characteristic relating the current through the output of a device to the voltage across the input of a device. Conductance is the reciprocal of resistance.

Transadmittance (or transfer admittance) is the AC equivalent of transconductance.

It is very often denoted as a conductance, g_m, with a subscript, m, for mutual. Transconductance is defined as follows:

For small signal alternating current, the definition is simpler:

The SI unit, the siemens, with the symbol, S; 1 siemens = 1 ampere per volt replaced the old unit of conductance, having the same definition, the mho (ohm spelled backwards), symbol, ℧.

Transresistance (for transfer resistance), also infrequently referred to as mutual resistance, is the dual of transconductance. It refers to the ratio between a change of the voltage at two output points and a related change of current through two input points, and is notated as r_m:

The SI unit for transresistance is simply the ohm, as in resistance.

Transimpedance (or, transfer impedance) is the AC equivalent of transresistance, and is the dual of transadmittance.

For vacuum tubes, transconductance is defined as the change in the plate(anode)/cathode current divided by the corresponding change in the grid/cathode voltage, with a constant plate(anode)/cathode voltage. Typical values of g_m for a small-signal vacuum tube are 1 to 10 millisiemens. It is one of the three characteristic constants of a vacuum tube, the other two being its gain μ (mu) and plate resistance r_p or r_a. The Van der Bijl equation defines their relation as follows:

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