An amplifier circuit with an FET. Component designators are discussed in the text.
Above figure shows an N-channel JFET hooked up as a common source amplifier. The input signal passes through C2 to the gate. Resistor R2 provides the bias. Resistor R1 and capacitor C1 give the source a dc voltage relative to ground, while grounding it for signals. The output signal goes through C3. Resistor R3 keeps the output signal from being short-circuited through the power supply.
A JFET has a high input impedance, and therefore the value of C2 should usually be small. If the device is a MOSFET, the input impedance is higher still, and C2 will be smaller yet, sometimes 1 pF or less. The resistor values depend on the application. In some instances, R1 and C1 are not used, and the source is grounded directly. If R1 is used, its optimum value will depend on the input impedance and the bias needed for the FET. For a weak-signal amplifier, typical values are 680 Ω for R1, 10 kΩ for R2, and 100 Ω for R3.