FET amplifier

What are CS, CD, and CG amplifiers?

Common Source amplifier is on in which the Source terminal is common to both input and output circuit.

Common Drain amplifier is on in which the Drain terminal is common to both input and output circuit.

Common Gate amplifier is on in which the Gate terminal is common to both input and output circuit.

Give expressions for Ri, Ro, Voltage gain of CS, CD, CG?

The following equations are provided for MOSFET’s with voltage divider bias arrangement having Rg1 and Rg2 as biasing resistors at the gate terminal and constant current source at the source terminal.

Amplifier/Parameter Input resistance output resistance Voltage gain
Common Source amplifier Rg ro//Rd -gm*(ro//Rd//Rl)
Common Drain amplifier(Neglecting ro) Rg Rd -gm*(Rd//Rl)
Common Gate amplifier(Neglecting ro) 1/gm Rd gm*(Rd//Rl)

Where Rg = Rg1//Rg2, gm is Transconductance, Rd is the resistance at the drain terminal.

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