Common Gate JFET Amplifier and its AC Equivalent Circuit:

The circuit of a simple Common Gate JFET Amplifier is shown in Fig. 13.53 and its ac equivalent circuit is shown in Fig. 13.54. In this Amplifier Circuit, input signal is applied to the source terminal S and output is taken from the drain terminal D, as illustrated in Fig. 13.53.

The gate is grounded and external load resistor R_L is connected to the drain terminal D through coupling capacitor C₂. In ac equivalent circuit, current source is connected between the drain and source terminals, as usual. However, since source and drain are the input and output terminals respectively, g_mV_gs appears between the input and output.

Input Impedance: Ignoring current through R_S and R_gs, the input current

So input impedance of FET,

The input impedance of the circuit is Z_s || R_S. So,

Actually speaking r_d and R_D are also involved in Z_in but their effect is negligible.

Output Impedance: The output of a Common Gate JFET Amplifier Circuit is taken from the drain terminal, just as in the case of a common source circuit. So the output impedance at the drain terminal is

Again this equation gives the impedance looking into the device drain terminal. The circuit output impedance

Voltage gain,

Usually r_d ≫ R_D || R_L

So

The above equation for voltage gain is the same as that in case of a common source circuit, except that there is no minus sign. It means that output voltage and input voltage are in phase.