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AnFET amplifier is anamplifier that uses one or morefield-effect transistors (FETs). The most common type of FET amplifier is theMOSFET amplifier, which usesmetal–oxide–semiconductor FETs (MOSFETs). The main advantage of a FET used for amplification is that it has very highinput impedance and lowoutput impedance.
Thetransconductance is given by
On rearranging, we get
The internal resistance Rgs, between gate and source appears between drain and source. Rds is the internal resistance between the drain and source.As Rgs is very high, it is taken to be infinite, and Rds is neglected.[1]
For ideal FET equivalent circuit, voltage gain is given by,
From the equivalent circuit,
and from the definition of transconductance,
we get[1]
There are three types of FET amplifiers, depending on which terminal is the common input and output. (This is similar to abipolar junction transistor (BJT) amplifier.)
The gate is common to both input and output.
The source is common to both input and output.
The drain is common to both input and output. It is also known as a "source follower".[2]
The basic principle of thefield-effect transistor (FET)amplifier was first proposed by Austro-Hungarian physicistJulius Edgar Lilienfeld in 1925.[3] However, his early FET concept was not a practical design.[4] The FET concept was later also theorized byOskar Heil in the 1930s andWilliam Shockley in the 1940s,[5] but there was no working practical FET built at the time.[4]
A breakthrough came with the work of Egyptian engineerMohamed M. Atalla in the late 1950s.[6] He developed the method ofsurface passivation, which later became critical to thesemiconductor industry as it made possible the mass-production ofsiliconsemiconductor technology, such asintegrated circuit (IC) chips.[7][4][8] For the surface passivation process, he developed the method ofthermal oxidation, which was a breakthrough in silicon semiconductor technology.[9] The surface passivation method was presented by Atalla in 1957.[10] Building on the surface passivation method, Atalla developed themetal–oxide–semiconductor (MOS) process,[7] with the use of thermally oxidized silicon.[11][12] 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 recruitDawon Kahng.[7]
TheMOS field-effect transistor (MOSFET) amplifier was invented by Mohamed Atalla and Dawon Kahng in 1959.[5] Theyfabricated the device in November 1959,[13] and presented it as the "silicon–silicon dioxide field induced surface device" in early 1960,[14] at the Solid-State Device Conference held atCarnegie Mellon University.[15] The device is covered by two now long-expiredpatents, each filed separately by Atalla and Kahng in March 1960.[16][17]