Transconductance

The dynamic current amplification is a measure of how well a bipolar transistor amplifies a signal. The JFET analog of this is known as dynamic mutual conductance or transconductance.

Suppose that E_G is a certain value, having a corresponding ID resulting. If the gate voltage changes by the small amount dEG then drain current will change by a certain increment dID. The transconductance is ratio dI_D/dE_G. Geometrically, this translates to slope of a line tangent to curve as shown in the figure.

The value of dI_D/dE_G is not the same everywhere along the curve. When JFET is biased beyond the pinchoff, in region marked Y in the figure, the slope of the curve is zero. There is no drain current, even if gate voltage change. Only when the channel is conducting there will be a change in I_D when there is change in E_G. The region where transconductance, dI_D/dE_G, is the largest is the region marked X, where slope of the curve is steepest. This is where most gain can be obtained from JFET.