JFET common source amplifier realized with N-channel JFET is given in figure bellow.
Simulate JFET common source amplifier.
JFET common source amplifier is unipolar FET single stage amplifier equivalent to bipolar BJT common emitter amplifier. It means that for JFET single stage amplifiers this common source topology is with the highest gain. In order to have as highest gain as possible, JFET has to operate in saturation that is equivalent to active regime of bipolar BJT. In that sense, PN junction gate-to-source has to be inverse biased and for n-channel JFET gate has to be on potential lower then the source. For that reason, parallel connection of RS resistor and CE capacitor is to keep source voltage level higher then ground. Now, positive voltage on the gate can still be lower then positive source voltage, so JFET common source amplifier can operate with only one battery. Equivalent scheme of JFET common source amplifier for small signals is given in figure bellow.
Transconductance for given equivalent scheme is:
In here, IDSS is Zero-Gate Voltage Drain Current, IDQ is quiescent drain current and VP is pinch voltage.
Voltage gain of JFET common source amplifier is
In here, parameter RDS is Drain-to-Source resistance and it is Early voltage over VP i.e.
Since RDS affects overall voltage gain, and Early voltage is less for JFET transistors comparing to bipolar transistors BJT, JFET common source amplifier usually has smaller gain and it has bigger non-linearity then common emitter amplifier, but input impedance is higher.
Simulate jfet common source amplifier.