Common Source Amplifier With
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.
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
JFET with its low frequency a.c Equivalent circuit
Input Impedance Zi
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 gmVgs =0 . And it allows current
source to be replaced by an open circuit.
resistance rd is sufficiently large compared to RD, then