A FET amplifier is an

amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost the v ...

that uses one or more

field-effect transistor The field-effect transistor (FET) is a type of transistor that uses an electric field to control the flow of current in a semiconductor. FETs (JFETs or MOSFETs) are devices with three terminals: ''source'', ''gate'', and ''drain''. FETs contro ...

s (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 The input impedance of an electrical network is the measure of the opposition to current ( impedance), both static ( resistance) and dynamic ( reactance), into the load network that is ''external'' to the electrical source. The input admittance (the ...

and low

output impedance The output impedance of an electrical network is the measure of the opposition to current flow (impedance), both static ( resistance) and dynamic ( reactance), into the load network being connected that is ''internal'' to the electrical source. The ...

In detail

The

transconductance Transconductance (for transfer conductance), also infrequently called mutual conductance, is the electrical characteristic relating the current through the output of a device to the voltage across the input of a device. Conductance is the reciproca ...

is given by :

g_m =

On rearranging, we get :

I_\mathrm = g_m V_\mathrm

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.

Voltage gain

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

A_v = \frac

From the equivalent circuit,

V_ = I_D \cdot R_D

and from the definition of transconductance,

V_ = \frac

we get

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 A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. In contrast, a unipolar transistor, such as a field-effect transistor, uses only one kind of charge carrier. A bipolar ...

(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".

History

The basic principle of the

(FET)

was first proposed by Austro-Hungarian physicist

Julius Edgar Lilienfeld Julius Edgar Lilienfeld (April 18, 1882 – August 28, 1963) was an Austro-Hungarian, and later American (where he moved in 1921) physicist and electrical engineer, who was credited with the first patent on the field-effect (FET) (1925). Be ...

in 1925.Lilienfeld, Julius Edgar (1926-10-08) "Method and apparatus for controlling electric currents" However, his early FET concept was not a practical design. The FET concept was later also theorized by

Oskar Heil Oskar Heil (20 March 1908, in Langwieden – 15 May 1994, San Mateo, California) was a German electrical engineer and inventor. He studied physics, chemistry, mathematics, and music at the Georg-August University of Göttingen and was awarded h ...

in the 1930s and

William Shockley William Bradford Shockley Jr. (February 13, 1910 – August 12, 1989) was an American physicist and inventor. He was the manager of a research group at Bell Labs that included John Bardeen and Walter Brattain. The three scientists were jointly ...

in the 1940s, but there was no working practical FET built at the time.

MOSFET amplifier

A breakthrough came with the work of Egyptian engineer

Mohamed M. Atalla Mohamed M. Atalla ( ar, محمد عطاالله; August 4, 1924 – December 30, 2009) was an Egyptian-American engineer, physicist, cryptographer, inventor and entrepreneur. He was a semiconductor pioneer who made important contributions to ...

in the late 1950s. He developed the method of

surface passivation A surface, as the term is most generally used, is the outermost or uppermost layer of a physical object or space. It is the portion or region of the object that can first be perceived by an observer using the senses of sight and touch, and is t ...

, which later became critical to the

semiconductor industry The semiconductor industry is the aggregate of companies engaged in the design and fabrication of semiconductors and semiconductor devices, such as transistors and integrated circuits. It formed around 1960, once the fabrication of semiconduct ...

as it made possible the mass-production of

silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...

semiconductor A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...

technology, such as

integrated circuit An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny ...

(IC) chips. For the surface passivation process, he developed the method of

thermal oxidation In microfabrication, thermal oxidation is a way to produce a thin layer of oxide (usually silicon dioxide) on the surface of a wafer. The technique forces an oxidizing agent to diffuse into the wafer at high temperature and react with it. The rat ...

, which was a breakthrough in silicon semiconductor technology. The surface passivation method was presented by Atalla in 1957. Building on the surface passivation method, Atalla developed the

metal–oxide–semiconductor The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...

(MOS) process, with the use of thermally oxidized silicon. He proposed that the MOS process could be used to build the first working silicon FET, which he began working on building with the help of Korean recruit

Dawon Kahng Dawon Kahng ( ko, 강대원; May 4, 1931 – May 13, 1992) was a Korean-American electrical engineer and inventor, known for his work in solid-state electronics. He is best known for inventing the MOSFET (metal–oxide–semiconductor field-effe ...

. The MOS field-effect transistor (MOSFET) amplifier was invented by Mohamed Atalla and Dawon Kahng in 1959. They fabricated the device in November 1959, and presented it as the "siliconsilicon dioxide field induced surface device" in early 1960, at the Solid-State Device Conference held at

Carnegie Mellon University Carnegie Mellon University (CMU) is a private research university in Pittsburgh, Pennsylvania. One of its predecessors was established in 1900 by Andrew Carnegie as the Carnegie Technical Schools; it became the Carnegie Institute of Technology ...

. The device is covered by two

patents A patent is a type of intellectual property that gives its owner the legal right to exclude others from making, using, or selling an invention for a limited period of time in exchange for publishing an enabling disclosure of the invention."A p ...

, each filed separately by Atalla and Kahng in March 1960. (1960)

References

{{Reflist Transistor amplifiers MOSFETs Arab inventions Austrian inventions Egyptian inventions Hungarian inventions South Korean inventions