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,

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FET Biasing

Referring to the diagram below, note in the diagram that the gate is effectively placed close to the same potential as circuit common through resistor RG.

When there is no input applied, the gate voltage value (relative to circuit common) is zero. However, this does not lead to mean that the gate-to-source voltage is equal to zero. Consider that the source resistor (RS) is 100 ohms, and the drain current, which is the equal as the source current, is 15 milliamps. This 15-milliamp current will flow through RS would cause it to drop 1.5 volts, placing the source lead of the FET at a positive 1.5-volt potential ‘above circuit common.’ If the value of source is 1.5 volts more positive than the value of gate, it could also be mentioned that the gate is 1.5 volts more negative that of the source. (Is an 8-ounce glass, with 4 ounces of water in it, whether it is half-full or half-empty?)Ultimately, the level of gate-to-source voltage in this scenario is

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