JFET Low Frequency ac Equivalent Circuit:

Fig. 5.7 shows the small signal low frequency ac equivalent circuit for n-channel JFET. The relation of $I_D$ by $V_GS$, is included as a current source $g_m$ $V_GS$, connected from drain to source.

The input impedance is represented by the open circuit at its input terminals, since gate current $ I_G$ is zero. The output impedance is represented by $ r_d$ from drain to source.

The JFET parameters are the major components of low frequency small signal model for JFET. We know that, drain to source current of JFET is controlled by gate to source voltage.

The change in the drain current due to change in gate to source voltage can be determined using the transconductance factor $g_m$. It is given as,

$\Delta I_d=g_m\Delta V_GS$

In BJT the relation between an output and input quantity is given by amplification factor β, whereas in JFET this relation is given by transconductance factor $g_m$.

The another important parameter of JFET is drain resistance $r_d$ . It is given by

$r_d=\frac{\Delta V_DS}{\Delta I_D}|V_GS=constant$

It determines the output impedance $Z_O$ of the JFET amplifier.