FET amplifier

A FET amplifier is an amplifier that uses one or more field-effect transistors (FETs). The most common type of FET amplifier is the MOSFET amplifier, which uses metal–oxide–semiconductor FETs (MOSFETs). The main advantage of a FET used for amplification is that it has very high input impedance and low output impedance.

Generalised FET as an amplifier

In detail

The transconductance is given by

${\displaystyle g_{m}={I_{\mathrm {D} } \over V_{\mathrm {GS} }}}$

On rearranging, we get

${\displaystyle I_{\mathrm {D} }=g_{m}V_{\mathrm {GS} }}$

Equivalent circuit

The internal resistance R_gs, between gate and source appears between drain and source. R_ds is the internal resistance between the 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,

$${\displaystyle A_{v}={\frac {V_{DS}}{V_{GS}}}}$$

From the equivalent circuit,

$${\displaystyle V_{DS}=I_{D}\cdot R_{D}}$$

and from the definition of transconductance,

$${\displaystyle V_{GS}={\frac {I_{D}}{g_{m}}}}$$

we get^[1]

$${\displaystyle A_{V}=g_{m}\cdot R_{D}}$$

Types of FET amplifiers

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

Common gate amplifier

The gate is common to both input and output.

Common source amplifier

The source is common to both input and output.

Common drain amplifier

The drain is common to both input and output. It is also known as a "source follower".^[2]

History

Further information: Field-effect transistor and Amplifier

The basic principle of the field-effect transistor (FET) amplifier was first proposed by Austro-Hungarian physicist Julius Edgar Lilienfeld in 1925.^[3] However, his early FET concept was not a practical design.^[4] The FET concept was later also theorized by Oskar Heil in the 1930s and William Shockley in the 1940s,^[5] but there was no working practical FET built at the time.^[4]

MOSFET amplifier

Main article: MOSFET

1957, Diagram of one of the SiO2 transistor devices made by Frosch and Derrick^[6]

In 1955, Carl Frosch and Lincoln Derrick accidentally grew a layer of silicon dioxide over the silicon wafer, for which they observed surface passivation effects.^[7] By 1957 Frosch and Derrick, using masking and predeposition, were able to manufacture silicon dioxide transistors and showed that silicon dioxide insulated, protected silicon wafers and prevented dopants from diffusing into the wafer.^[7][8] J.R. Ligenza and W.G. Spitzer studied the mechanism of thermally grown oxides and fabricated a high quality Si/SiO₂ stack in 1960.^[9][10][11]

Following this research, Mohamed Atalla and Dawon Kahng proposed a silicon MOS transistor in 1959^[12] and successfully demonstrated a working MOS device with their Bell Labs team in 1960.^[13][14] Their team included E. E. LaBate and E. I. Povilonis who fabricated the device; M. O. Thurston, L. A. D’Asaro, and J. R. Ligenza who developed the diffusion processes, and H. K. Gummel and R. Lindner who characterized the device.^[15][16]

See also

• Audio power amplifier
• LDMOS
• Power electronics
• Power semiconductor device
• Power MOSFET
• RF power amplifier