User:PrathameshD.elec/sandbox

INTRODUCTION

The main advantage of FET is used for amplification is that it has very high input impedance. Also it shows low output impedance. These two are main conditions for amplifier.

In detail

The transconductance is given by,

gm=Id/Vgs

On rearranging we get,

Id=gm*Vgs ...(1)

Thus, from this equation we can tell that output current Id is product of input voltage Vgs and the transconductance gm.

Equivalent circuit

A FET equivalent circuit is given by equation number (1). The internal resistance r'gs , between gate and source and a current source given by equation (1) appears between drain and source. r'ds is internal resistance between drain and source. As r'gs is very high, it is taken to be infinite and r'ds is neglected.

Voltage Gain

For ideal FET equivalent circuit, voltage gain is given by,

Av=Vds/Vgs

From the equivalent circuit,

Vds=Id*Rd

and from the definition of transconductance,

Vgs=Id/gm

Thus we get,

Av=gm*Rd ...(2)

Types of FET amplifiers

  There are three types of FET amplifiers depending upon the common terminal used as input and output similar to BJT amplifier.

1. Common Gate Amplifier

  In this the gate terminal is common to both input and output.
  Common gate

1. Common Source Amplifier

  In this the source terminal is common to both input and output.
  Common source

1. Common Drain Amplifier

  In this the drain terminal is common to both input and output. It is also known as 'Source Follower'.
  Common drain