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.

**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_{m}Vgs =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

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Electronic Circuits : JFET and MOSFET Amplifiers : Common Source Amplifier With Fixed Bias |

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