Common Source Amplifier With Fixed Bias

Common Source Amplifier With Fixed Bias

Figure shows Common Source Amplifier With Fixed Bias. The coupling capacitor C1 and C2 which are used to isolate the d.c biasing from the applied ac signal act as short circuits for ac analysis.

The following figure shows the low frequency equivalent model for Common Source Amplifier With Fixed Bias. It is drawn by replacing

·        All capacitors and d.c supply voltages with short circuit

·        JFET with its low frequency a.c Equivalent circuit

Input Impedance Zi

Z_i = R_G

Output Impedance Zo

It is the impedance measured looking from the output side with input voltage Vi equal to Zero. As Vi=0,Vgs =0 and hence g_mVgs =0 . And it allows current source to be replaced by an open circuit.

So,

If the resistance rd is sufficiently large compared to R_D, then