Transconductance is an expression of the performance of a bipolar transistor or field-effect transistor (FET). In general, the larger the transconductance figure for a device, the greater the gain(amplification) it is capable of delivering, when all other factors are held constant.
Formally, for a bipolar device, transconductance is defined as the ratio of the change in collector current to the change in base voltage
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over a defined, arbitrarily small interval on the collector-current-versus-base-voltage curve. For an FET, transconductance is the ratio of the change in drain current to the change in gate voltage over a defined, arbitrarily small interval on the drain-current-versus-gate-voltage curve.
The symbol for transconductance is gm. The unit is thesiemens, the same unit that is used for direct-current (DC) conductance.
If dI represents a change in collector or drain current caused by a small change in base or gate voltage dE, then the transconductance is approximately:
gm = dI / dE
As the size of the interval approaches zero -- that is, the change in base or gate voltage becomes smaller and smaller -- the value of dI / dE approaches the slope of a line tangent to the curve at a specific point. The slope of this line represents the theoretical transconductance of a bipolar transistor for a given base voltage and collector current, or the theoretical transconductance of an FET for a given gate voltage and drain current.