FET amplifier

Chapter 1: Low Power Amplifiers

Sub point: 1.5   FET Amplifier: Common Source amplifier, working principle and applications 

Explanation and operation of FET common source amplifier with applications. 

Figure: Common Source FET Amplifier 

(Note: Drain (D) and Source (S) terminals are interchangeable and if you are asked to draw Common Drain amplifier then.....) 

             So far we have seen transistor amplifiers in sub points 1.1 to 1.4, in this sub topic (1.5) we are going to see FET common source amplifier. 

             Above figure shows Common Source N-channel FET amplifier in which source terminal is common between the input and output hence it is called as common source amplifier (Same as Common-base (CB), Common emitter (CE) and Common collector (CC)configurations of transistor). Resistors R1 and R2 are used as voltage divider biasing circuit. Cin & Cout capacitors are input and coupling capacitors respectively that are used used to couple input AC signal & output. Cs is the bypass capacitor which keeps the source of FET effectively. 

Working: 

DURING POSITIVE HALF CYCLE: 

• As the gate to source voltage increases, the drain current also increases. 

• As a result of this, the voltage drop across resistor RD also increases. 

• This causes the drain voltage to decrease. As VDS = VDD – IDRD. 

• It means that the positive half cycle of the input produces negative half cycle of the output voltage. 

• In other words output voltage is 180 out of phase with the input voltage. 

DURING NEGATIVE HALF CYCLE: 

• As the gate to source voltage decreases, the drain current also decreases. 

• As a result of this, the voltage drop across resistor RD also decreases. 

• This causes the drain voltage to increase. As VDS = VDD – IDRD. 

• It means that the negative half cycle of the input produces positive half cycle of the output voltage. 

• In other words output voltage is 180 out of phase with the input voltage. 

Applications of Common Source FET amplifier: 

• As a pre-amplifier in audio circuits. 

• As a voltage amplifier. 

• In the public address system. 

• In radio & TV amplifier circuit.