Q.Find the value of $g_m$, given $V_{GS} = -2$ V, $V_P = -6$ V and $g_{mo} = 8$ mS.

Transconductance $g_m$, also called the forward transconductance, measures how effectively the gate voltage controls the drain current of a JFET. It is defined as the ratio of a small change in drain current to the small change in gate-source voltage that produced it, at constant drain-source voltage:

Its unit is the siemens (S); because the values are small it is usually quoted in millisiemens (mS). A large $g_m$ means a small change in gate voltage produces a large change in drain current — that is, a more effective control and higher gain.

An expression for $g_m$ follows by differentiating Shockley's equation with respect to $V_{GS}$:

$g_m = g_{mo}\left(1 - \frac{V_{GS}}{V_P}\right), \qquad g_{mo} = \frac{-2\,I_{DSS}}{V_P}$ …