Editing Amplifier (section)

== History ==
=== Vacuum tubes ===
The first practical prominent device that could amplify was the [[triode]] [[vacuum tube]], invented in 1906 by [[Lee De Forest]], which led to the first amplifiers around 1912.  Vacuum tubes were used in almost all amplifiers until the 1960s–1970s when [[transistors]] replaced them.  Today, most amplifiers use transistors, but vacuum tubes continue to be used in some applications.{{Cn|date=December 2024}}
[[File:First Audion amplifier 1914.jpg|thumb|upright=0.7|De Forest's prototype audio amplifier of 1914. The [[Audion]] (triode) vacuum tube had a voltage gain of about 5, providing a total gain of approximately 125 for this three-stage amplifier.]]

The development of audio communication technology in form of the [[telephone]], first patented in 1876, created the need to increase the amplitude of electrical signals to extend the transmission of signals over increasingly long distances. In [[telegraphy]], this problem had been solved with intermediate devices at stations that replenished the dissipated energy by operating a signal recorder and transmitter back-to-back, forming a [[relay]], so that a local energy source at each intermediate station powered the next leg of transmission.
For duplex transmission, i.e. sending and receiving in both directions, bi-directional relay repeaters were developed starting with the work of [[C. F. Varley]] for telegraphic transmission. Duplex transmission was essential for telephony and the problem was not satisfactorily solved until 1904, when H. E. Shreeve of the [[American Telephone and Telegraph Company]] improved existing attempts at constructing a [[Repeater#Telephone repeater|telephone repeater]] consisting of back-to-back [[carbon microphone|carbon-granule transmitter]] and electrodynamic receiver pairs.<ref name="gherardi">Gherardi B., Jewett F.B., ''Telephone Repeaters'', Transactions of the AIEE 38(11), 1 Oct 1919, p.1298</ref> The Shreeve repeater was first tested on a line between Boston and Amesbury, MA, and more refined devices remained in service for some time.  After the turn of the century it was found that negative resistance [[mercury lamp]]s could amplify, and were also tried in repeaters, with little success.<ref name="Sungook">{{cite book
  | last = Sungook
  | first =  Hong
  | title = Wireless: From Marconi's Black-Box to the Audion
  | publisher = MIT Press
  | year = 2001
  | pages = 165
  | url = https://books.google.com/books?id=UjXGQSPXvIcC&pg=PA165
  | isbn = 978-0262082983
 }}</ref>

The development of [[vacuum tube|thermionic valves]] which began around 1902, provided an entirely electronic method of amplifying signals. The first practical version of such devices was the [[Audion]] [[triode]], invented in 1906 by [[Lee De Forest]],<ref name="De Forest">{{cite journal
  | last = De Forest
  | first = Lee
  | title = The Audion; A New Receiver for Wireless Telegraphy
  | journal = Trans. AIEE
  | volume = 25
  | pages = 735–763
  | date = January 1906
  | url = https://zenodo.org/record/1432498
  | doi = 10.1109/t-aiee.1906.4764762
  | access-date = March 30, 2021
 }} The link is to a reprint of the paper in the ''Scientific American Supplement'', Nos. 1665 and 1666, November 30, 1907 and December 7, 1907, p.348–350 and 354–356.</ref><ref name="Godfrey">{{cite encyclopedia
  | last = Godfrey
  | first = Donald G. 
  | title = Audion
  | encyclopedia = Historical Dictionary of American Radio 
  | pages = 28
  | publisher = Greenwood Publishing Group
  | year = 1998
  | url = https://books.google.com/books?id=4l_2kkv5aeMC&q=%22Audion&pg=PA28
  | access-date = January 7, 2013| isbn = 9780313296369 
  }}</ref><ref name="Newnes">{{cite encyclopedia
  | last = Amos
  | first = S. W.
  | title = Triode
  | encyclopedia = Newnes Dictionary of Electronics, 4th Ed. 
  | pages = 331
  | publisher = Newnes 
  | year = 2002
  | url = https://books.google.com/books?id=lROa-MpIrucC&q=triode&pg=PA331
  | access-date = January 7, 2013
 | isbn = 9780080524054
  }}</ref> which led to the first amplifiers around 1912.<ref name="Nebeker" />  Since the only previous device which was widely used to strengthen a signal was the [[relay]] used in [[electric telegraph|telegraph]] systems, the amplifying vacuum tube was first called an ''electron relay''.<ref name="McNicol">{{cite book
  | last1  = McNicol
  | first1 = Donald
  | title  = Radio's Conquest of Space
  | publisher = Murray Hill Books 
  | date   = 1946
  | pages  = 165, 180
  | isbn = 9780405060526
 | url    = https://archive.org/stream/radiosconquestof00mcnirich#page/180/mode/2up
 }}</ref><ref name="McNicol1917">{{cite journal
  | last1  = McNicol
  | first1 = Donald 
  | title = The Audion Tribe
  | journal = Telegraph and Telephone Age
  | volume = 21
  | pages = 493
  | date = November 1, 1917
  | url = https://books.google.com/books?id=JZc2AQAAMAAJ&q=%22electron+relay%22&pg=PA493
  | access-date = May 12, 2017
 }}</ref><ref name="Americana">{{cite book
  | title  = Encyclopedia Americana, Vol. 26
  | publisher = The Encyclopedia Americana Co.
  | date   = 1920
  | pages  = 349
  | url    = https://books.google.com/books?id=CF4fijqC9GgC&q=%22electron+relay%22+audion&pg=PA349
 }}</ref><ref name="Hong2">{{cite book| url = https://books.google.com/books?id=UjXGQSPXvIcC&dq=relay&pg=PA177| title = Hong 2001, ''Wireless: From Marconi's Black-Box to the Audion'', p. 177| isbn = 9780262082983| last1 = Hong| first1 = Sungook| year = 2001| publisher = MIT Press}}</ref>  The terms ''amplifier'' and ''amplification'', derived from the Latin ''amplificare'', (''to enlarge or expand''),<ref name="Harper">{{cite web|url=http://www.etymonline.com/index.php?term=amplify&allowed_in_frame=0|title=Amplify|last=Harper|first=Douglas|date=2001|publisher=Etymonline.com|work=Online Etymology Dictionary|access-date=July 10, 2015}}</ref> were first used for this new capability around 1915 when triodes became widespread.<ref name="Harper" />

The amplifying vacuum tube revolutionized electrical technology.<ref name="Nebeker">{{cite book
  | last1  = Nebeker
  | first1 = Frederik 
  | title  = Dawn of the Electronic Age: Electrical Technologies in the Shaping of the Modern World, 1914 to 1945
  | publisher = John Wiley and Sons
  | date   = 2009
  | pages  = 9–10, 15
  | url    = https://books.google.com/books?id=xwmH6-q5O5AC&pg=PA15
  | isbn   = 978-0470409749
 }}</ref>  It made possible long-distance telephone lines, [[public address system]]s, [[radio broadcasting]], [[talking motion picture]]s, practical [[audio recording]], [[radar]], [[television]], and the first [[computer]]s.  For 50 years virtually all consumer electronic devices used vacuum tubes.    Early tube amplifiers often had [[positive feedback]] ([[regenerative circuit|regeneration]]), which could increase gain but also make the amplifier unstable and prone to oscillation.  Much of the mathematical theory of amplifiers was developed at [[Bell Telephone Laboratories]] during the 1920s to 1940s.  Distortion levels in early amplifiers were high, usually around 5%, until 1934, when [[Harold Stephen Black|Harold Black]] developed [[Negative feedback amplifier|negative feedback]]; this allowed the distortion levels to be greatly reduced, at the cost of lower gain.  Other advances in the theory of amplification were made by [[Harry Nyquist]] and [[Hendrik Wade Bode]].<ref name="Bode">{{cite journal
  | last1  = Bode
  | first1 = H. W. 
  | title = Relations Between Attenuation and Phase in Feedback Amplifier Design
  | journal = Bell Labs Technical Journal
  | volume = 19
  | issue = 3
  | pages = 421–454
  | date = July 1940
  | doi = 10.1002/j.1538-7305.1940.tb00839.x
  }}</ref>

The vacuum tube was virtually the only amplifying device, other than specialized power devices such as the [[magnetic amplifier]] and [[amplidyne]], for 40 years.  Power control circuitry used magnetic amplifiers until the latter half of the twentieth century when power semiconductor devices became more economical, with higher operating speeds. The old Shreeve electroacoustic carbon repeaters were used in adjustable amplifiers in telephone subscriber sets for the hearing impaired until the transistor provided smaller and higher quality amplifiers in the 1950s.<ref>AT&T, Bell System Practices Section C65.114, ''Telephone Sets for Subscribers with Impaired Hearing — 334 Type''</ref>

===Transistors===
{{Further|History of the transistor|MOSFET|Audio power amplifier|RF power amplifier}}

The first working [[transistor]] was a [[point-contact transistor]] invented by [[John Bardeen]] and [[Walter Brattain]] in 1947 at [[Bell Labs]], where [[William Shockley]] later invented the [[bipolar junction transistor]] (BJT) in 1948. They were followed by the invention of the [[metal–oxide–semiconductor field-effect transistor]] (MOSFET) by [[Mohamed M. Atalla]] and [[Dawon Kahng]] at Bell Labs in 1959. Due to [[MOSFET scaling]], the ability to scale down to increasingly small sizes, the MOSFET has since become the most widely used amplifier.<ref>{{Cite web|url=https://www.computerhistory.org/siliconengine/timeline/|title = Timeline &#124; the Silicon Engine &#124; Computer History Museum}}</ref>

The replacement of bulky electron tubes with transistors during the 1960s and 1970s created a revolution in electronics, making possible a large class of portable electronic devices, such as the [[transistor radio]] developed in 1954.  Today, use of vacuum tubes is limited to some high power applications, such as [[radio transmitter]]s, as well as some [[musical instrument amplifier#Tube amplifiers|musical instrument]] and high-end [[audiophile]] amplifiers.{{Cn|date=December 2024}}

Beginning in the 1970s, more and more transistors were connected on a single chip thereby creating higher scales of integration (such as small-scale, medium-scale and [[large-scale integration]]) in [[integrated circuit]]s. Many amplifiers commercially available today are based on integrated circuits.{{Cn|date=December 2024}}

For special purposes, other active elements have been used. For example, in the early days of the [[satellite communication]], [[parametric amplifier]]s were used. The core circuit was a diode whose capacitance was changed by an RF signal created locally. Under certain conditions, this RF signal provided energy that was modulated by the extremely weak satellite signal received at the earth station.{{Cn|date=December 2024}}

Advances in [[digital electronics]] since the late 20th century provided new alternatives to the conventional linear-gain amplifiers by using digital switching to vary the pulse-shape of fixed amplitude signals, resulting in devices such as the [[Class-D amplifier]].{{Cn|date=December 2024}}