Posted by Tittu Thomas , 7/29/2012
JUNCTION FIELD EFFECT TRANSISTOR (JFET) is a unipolar voltage controlled device. Because Drain current is controlled by the voltage applied to gate-source terminal.
Likewise in Transistors, JFET amplifiers can also be setup in the three configurations namely, Common Drain, Common Source and Common Gate.
Common source configuration is almost similar to Common emitter configuration of BJT's (Common Emitter RC Coupled Amplifier). JFET can also be biased as Voltage Divider or Self Bias.
COMMON SOURCE AMPLIFIER USING VOLTAGE DIVIDER BIASVoltage divider bias is used to maintain stable bias voltage at the FET terminals, independent of drain current. Voltage divider bias can be obtained by a small modification in the self biasing circuit. Two high valued Resistors R1 and R2 establishes Voltage divider bias. Inorder to maintain a negative voltage across Gate and source terminals, R1 and R2 are selected such a way that potential at the source is more than that at Gate. Self bias maintains drain current and mutual conductance gm relatively constant. The gate to source junction is reverse biased, which offers High Impedence.