The most frequently encountered configuration for a JFET amplifier is the common source circuit. The source is common to the input and output as shown in the diagram.
The common source configuration for a FET is similar to the common emitter bipolar transistor configuration. Current Id flows through Rs to develop a voltage that reverse-biases the PN gate junction. Rg is provided for a dc return path.
The common source amplifier can provide both a voltage and current gain as the input resistance of FET is extremely large, but the voltage gain is generally inferior to that available from a bipolar device. The source by-pass capacitor is connected to provide a low impedance path to ground for high frequency components. As a result of presence of by-pass capacitor AC signals will not cause a fluctuation in the bias voltage.