Connections and Applications of a JFET

There are 3 basic connections of a JFET which are:

1.   Common Source Connection
2.   Common Drain Connection
3.   Common Gate Connection

Common Source Connection

 
This is the most widely used connection because the connection provides high input impedance, voltage gain and a moderate output impedance. However the circuit a 180 out of phase output signal with regards to the input signal.

Common Drain Connection

Common Gate Connection

Applications of a JFET

1.   Buffer Amplifier:  Because a JFET has a high input impedance and a low output impedance. It can act as an excellent buffer amplifier. The high input impedance of a JFET means light loading of the preceding stage. This permits almost the entire output from the first stage to appear at the buffer input. On the other hand, the low output impedance of the JFET can drive heavy loads. This ensures that all the output from the buffer reaches the input of the second stage.

2.   Phase Shift Oscillator
3.   RF Amplifier:  JFET is used in communication electronics instead of BJT for this reasons:
    A.  It has a low noise level
    B.  It is a voltage controlled device, therefore the current from the weak signal is not responsive.