Editing Amplifier (section)

=== Active devices ===
All amplifiers include some form of active device: this is the device that does the actual amplification. The active device can be a [[vacuum tube]], discrete solid state component, such as a single [[transistor]], or part of an [[integrated circuit]], as in an [[operational amplifier|op-amp]].{{Cn|date=December 2024}}

[[Transistor]] amplifiers (or solid state amplifiers) are the most common type of amplifier in use today. A transistor is used as the active element. The gain of the amplifier is determined by the properties of the transistor itself as well as the circuit it is contained within.{{Cn|date=December 2024}}

Common active devices in transistor amplifiers include [[bipolar junction transistor]]s (BJTs) and [[metal oxide semiconductor field-effect transistor]]s (MOSFETs).{{Cn|date=December 2024}}

Applications are numerous. Some common examples are audio amplifiers in a home stereo or [[public address|public address system]], RF high power generation for semiconductor equipment, to RF and microwave applications such as radio transmitters.{{Cn|date=December 2024}}

Transistor-based amplification can be realized using various configurations: for example a bipolar junction transistor can realize [[common base]], [[common collector]] or [[common emitter]] amplification; a MOSFET can realize [[common gate]], [[common source]] or [[common drain]] amplification. Each configuration has different characteristics.{{Cn|date=December 2024}}

[[Valve amplifier|Vacuum-tube amplifiers]] (also known as tube amplifiers or valve amplifiers) use a [[vacuum tube]] as the active device. While semiconductor amplifiers have largely displaced valve amplifiers for low-power applications, valve amplifiers can be much more cost effective in high power applications such as radar, countermeasures equipment, and communications equipment. Many [[#Microwave amplifiers|microwave amplifiers]] are specially designed valve amplifiers, such as the [[klystron]], [[gyrotron]], [[traveling wave tube]], and [[crossed-field amplifier]], and these microwave valves provide much greater single-device power output at microwave frequencies than solid-state devices.<ref>{{cite journal | author=Robert S. Symons | title=Tubes: Still vital after all these years | journal=IEEE Spectrum | year=1998 | volume=35 | issue=4 | pages= 52–63 | doi=10.1109/6.666962 }}</ref> Vacuum tubes remain in use in some high end audio equipment, as well as in [[instrument amplifier|musical instrument amplifiers]], due to a preference for "[[tube sound]]".{{Cn|date=December 2024}}

[[Magnetic amplifier]]s are devices somewhat similar to a [[transformer]] where one winding is used to control the saturation of a magnetic core and hence alter the impedance of the other winding.<ref>{{Cite web|url=http://www.ti.com/lit/ml/slup129/slup129.pdf|title=Magnetic Amplifier Control for Simple, Low-Cost, Secondary Regulation|last=Mammano|first=Bob|date=2001|publisher=Texas Instruments}}</ref> They have largely fallen out of use due to development in semiconductor amplifiers but are still useful in [[HVDC]] control, and in nuclear power control circuitry due to not being affected by radioactivity.{{Cn|date=December 2024}}

[[Negative resistance]]s can be used as amplifiers, such as the [[tunnel diode]] amplifier.<ref>{{Cite web|url=http://users.tpg.com.au/users/ldbutler/NegativeResistance.htm|title=Negative Resistance Revived|website=users.tpg.com.au|access-date=2016-06-20}}</ref><ref>{{Cite journal|last=Munsterman|first=G.T.|date=June 1965|title=Tunnel-Diode Microwave Amplifiers|url=http://techdigest.jhuapl.edu/views/pdfs/V04_N5_1965/V4_N5_1965_Munsterman.pdf|journal=APL Technical Digest|volume=4|pages=2–10|access-date=2016-06-20|archive-date=2016-10-09|archive-url=https://web.archive.org/web/20161009041943/http://techdigest.jhuapl.edu/views/pdfs/V04_N5_1965/V4_N5_1965_Munsterman.pdf|url-status=dead}}</ref>