Transconductance

Author: N.H. Crowhurst

Measuring transconductance

How is it that a small change in voltage at the grid can be converted into a larger one at the plate? Two things control how many electrons get away from the cathode of the tube: the plate voltage and the grid voltage. The number of electrons flowing inside the tube fixes the current in the outside circuit. Using a fixed plate voltage, the grid voltage can be changed and plate current measured each time it is changed.

Each time the grid voltage is changed, the plate current changes, until the potential on the grid is so negative that no electrons leave the plate, at which point the current becomes zero. The amount of change in plate current for each volt change in grid voltage is called the mutual conductance or transconductance of the tube. It is, of course, different for each change, but the figure usually quoted for a tube is the change that occurs when the voltage and current are nearest to the practical operating values used in a circuit.

Plate current versus grid voltage curve used to obtain transconductance

Resistance is the opposition that a circuit presents to current flow - the larger the resistance, the smaller the current at the same voltage. Conductance is the opposite of resistance - a measure of the ease with which current can flow through a circuit. A higher conductance means that more current can flow at the same voltage, or that the same current will flow at a smaller voltage. Transconductance is a measure of the ease with which current in one circuit (the plate circuit) can be affected by the voltage in another (the grid circuit).

The relationship for resistance is the well-known Ohm's law, which can be expressed in symbols as R = E/L The unit for conductance is the reciprocal of that for resistance: instead of being volts per ampere, it is amperes per volt. This unit is the mho (ohm spelled backwards). The conductance form of Ohm's law is G = I/E.

In tubes, a change of a volt at the grid only produces a change of milli-amperes at the plate. (A milliampere is one-thousandth of an ampere.) For this reason, transconductance (symbolized as gm) is generally quoted in milliamperes per volt. (We use g rather than G, because conductance applies to direct readings of current and voltage, whereas the tube works on changes in voltage and current.) Milliamperes per volt could also be called mil-limhos. But this terminology is never used; instead, the unit of transconductance is the micromho, which is a microampere per volt. (A microampere is a millionth of an ampere.) Thus, 2 milliamperes per volt is called 2000 micromhos.

The meaning of transconductance