User:TheUnnamedNewbie/sandbox

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TODO:

• Start by including a short discription of the basic small-signal model
  • Make/find a vector graphic of the small signal model

• Mention bulk effect

An FET amplifier is an amplifier which uses one or more field-effect transistors (FETs). The main advantage of an FET used for amplification is that it has very high input impedance and low output impedance. These are two desirable features for an amplifier.

Equivalent circuit

The internal resistance r'gs, between gate and source 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. ^[1]

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) ^[1]

Types of FET amplifiers

There are three types of FET amplifiers depending upon the common terminal used as input and output. This is similar to a bipolar junction transistor (BJT) amplifier.

Common gate amplifier

In a common gate amplifier, the gate terminal is common to both input and output.

Common source amplifier

In a common source amplifier, the source terminal is common to both input and output.

TODO:

• Go into the difference between cascode and common source (High Z or Low Z)

Common drain amplifier

In a common drain amplifier, the drain terminal is common to both input and output. It is also known as a "source follower".^[2]

References