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.
In detail.
The transconductance is given by
formula_1
On rearranging, we get
formula_2
Equivalent circuit.
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.
Voltage gain.
For ideal FET equivalent circuit, voltage gain is given by,
formula_3
From the equivalent circuit,
formula_4
and from the definition of transconductance,
formula_5
we get
formula_6
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".
History.
The basic principle of the field-effect transistor (FET) amplifier was first proposed by Austro-Hungarian physicist Julius Edgar Lilienfeld in 1925. However, his early FET concept was not a practical design. The FET concept was later also theorized by Oskar Heil in the 1930s and William Shockley in the 1940s, but there was no working practical FET built at the time.
MOSFET amplifier.
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. 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. J.R. Ligenza and W.G. Spitzer studied the mechanism of thermally grown oxides and fabricated a high quality Si/SiO2 stack in 1960.
Following this research, Mohamed Atalla and Dawon Kahng proposed a silicon MOS transistor in 1959 and successfully demonstrated a working MOS device with their Bell Labs team in 1960. 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.
References.
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