Q. What is transconductance? Explain its significance from the transfer characteristics.

Transconductance is the transistor gain of the JFET; it indicates the amount of control the gate voltage has on the drain current. The transconductance is defined as

    gm = d(ID)/d(VGS)

Using the expression for the ideal drain currents derived, we can write the expressions for the transconductance.

The drain current for n-channel depletion mode device in the non-saturation region region was given by equation for the pinch off current.

The derivation of the equation for the transconductance leads to the relation shown below

    gms = (-2IDSS)(1-VGS/VP)/VP

The transconductance shows the significance of the transfer characteristics of a JFET.

The transfer characteristic of an FET relate the transfer of the input directly with the output. That is the drain current is directly plotted with the gate source voltage. The graph on the left slowly diminishes in its amplitude as it approaches the x axis. In theory the graph touches the x axis and channel pinches off. But in practical sense the graph does not tough the x axis but goes parallel to it with out touching it. For this reason the gm of the JFET is always a constant value. The transconductance of the JFET relates the ease with which the drain current flows at a particular value of the gate source voltage. The measure of transconductance is done in siemens.