FET Parameters


As you know the primary gain parameter of a standard bipolar transistor is beta. Beta describes the ratio of the current flow through the base relative to the current flow through the collector. In case of FETs, the primary gain parameter is called trans-conductance (Gm). This trans-conductance is the ratio that defines the effect that a gate-to source voltage (VGS) variation will have on the drain current (ID).

Transconductance is usually defined in terms of micromhos (mho is the basic unit used for expressing conductance).Normal transconductance values for common FETs range from 2000 to 15,000 micromhos. The equation for determining transconductance is,


Referring to figure below, assume that this example has the same value  trans-conductance as determined in the previous illustration. Just a 1-volt variation in the gate-to-source voltage (that is our input) will be sufficient to cause a 10-milliamp change in the drain current. As per Ohm’s law, a 10-milliamp change in current through the 1-Kohm drain resistor (RD) will make a 10-volt change throughout the drain resistor (10 milliamps x 1000 ohms = 10 volts). This 10-volt change will be visible at the output. Thus, because a 10-volt change at the output  happens due to only 1-volt change at the gate, this circuit has a voltage gain (Ae) of 10.


In many different ways, the FET circuits are compared with the standard bipolar transistor circuits. The circuit shown in figure below, is analogous to the common-emitter configuration, and it is suitably called a common-source configuration. The output is inverted from the input, and has the capacity of voltage gain. If we take the output from source, instead of the drain, then it will be a common-drain configuration.

The output shall not be inverted, and the voltage gain would be a 1 approximately. Yes of course, in bipolar design the common-drain FET amplifier is analogous to the common-collector amplifier.

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