Basic FET amplifier  circuit

In the figure given below, an N-channel JFET can be hooked up as a common-source amplifier. The input signal passes through C2 to gate. Resistor R2 gives the bias. Resistor R1 and capacitor C1 give source a direct current voltage relative to ground, while grounding it for alternating current signals. The alternating current output signal goes through capacitor C3. Resistor R3 keeps the alternating current output signal from being short-circuited through the power supply.

                                                              Figure-- An amplifier using an FET.

Concerning values of capacitors, the same considerations apply for this amplifier, as apply in bipolar circuit. A JFET amplifier always has a high input impedance, and thus the value of C2 will usually be small. If device is a MOSFET, the input impedance is even higher, and C2 will be smaller yet, sometimes as small as 1 pF or less.
 
The resistor values are dependent on application. In some examples,R1 and C1 are not used, and source is directly grounded. If R1 is used, its value will depend on input impedance and bias required for the FET. Nominal values can be R1= 680 ?, R2=10 K?, and R3 = 100 ? for a weak-signal, wideband amplifier.

If circuit is used as a power amplifier, the values of resistors will vary. It might be required to bias the gate negatively with respect to source, by using a 2nd power supply with the voltage negative relative to ground.