Editing Amplifier (section)

== Categories ==
=== 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>

===Power amplifiers===
{{See also|Audio power amplifier| RF power amplifier}}
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[[File:Sagem VS4 - Skyworks SKY77328-13-9827.jpg|thumb|Power amplifier by [[Skyworks Solutions]] in a [[Smartphone]].]]

A power amplifier is an amplifier designed primarily to increase the power available to a [[Electrical load|load]]. In practice, amplifier power gain depends on the source and load [[Electrical impedance|impedances]], as well as the inherent voltage and current gain. A [[radio frequency]] (RF) amplifier design typically optimizes impedances for power transfer, while audio and instrumentation amplifier designs normally optimize input and output impedance for least loading and highest signal integrity. An amplifier that is said to have a gain of 20&nbsp;dB might have a voltage gain of 20&nbsp;dB and an available power gain of much more than 20&nbsp;dB (power ratio of 100)—yet actually deliver a much lower power gain if, for example, the input is from a 600&nbsp;Ω microphone and the output connects to a 47&nbsp;[[Ohm|kΩ]] input socket for a power amplifier. In general, the power amplifier is the last 'amplifier' or actual circuit in a signal chain (the output stage) and is the amplifier stage that requires attention to power efficiency.

Audio power amplifiers are typically used to drive [[loudspeaker]]s. They will often have [[Stereophonic sound|two output channels]] and deliver equal power to each. An RF power amplifier is found in radio [[transmitter]] final stages. A [[Servo drive|servo motor controller]] amplifies a control voltage to adjust the speed of a motor, or the position of a motorized system.

Power amplifier circuits (output stages) are classified as A, B, AB and C for [[analog circuit|analog]] designs—and class D and E for switching designs. The [[power amplifier classes]] are based on the proportion of each input cycle (conduction angle) during which an amplifying device passes current.<ref>{{Cite web|url=http://electronicdesign.com/analog/understanding-amplifier-operating-classes|title=Understanding Amplifier Operating "Classes"|website=electronicdesign.com|access-date=2016-06-20|date=2012-03-21}}</ref> The image of the conduction angle derives from amplifying a sinusoidal signal. If the device is always on, the conducting angle is 360°. If it is on for only half of each cycle, the angle is 180°. The angle of flow is closely related to the amplifier [[Electrical efficiency|power efficiency]].{{Cn|date=December 2024}}

=== Operational amplifiers (op-amps) ===
[[File:LM741CN.jpg|thumb|An LM741 general purpose [[Operational amplifier|op-amp]]]]
{{Main|Operational amplifier|Instrumentation amplifier}}{{Unreferenced section|date=December 2024}}
An operational amplifier is an amplifier circuit which typically has very high open loop gain and differential inputs. Op amps have become very widely used as standardized "gain blocks" in circuits due to their versatility; their gain, bandwidth and other characteristics can be controlled by [[feedback]] through an external circuit.  Though the term today commonly applies to integrated circuits, the original operational amplifier design used valves, and later designs used discrete transistor circuits.

A [[fully differential amplifier]] is similar to the operational amplifier, but also has differential outputs. These are usually constructed using [[Bipolar junction transistor|BJTs]] or [[Field-effect transistor|FETs]].

=== Distributed amplifiers ===
{{Main|Distributed amplifier}}{{Unreferenced section|date=December 2024}}
These use balanced [[transmission line]]s to separate individual single stage amplifiers, the outputs of which are summed by the same transmission line. The transmission line is a balanced type with the input at one end and on one side only of the balanced transmission line and the output at the opposite end is also the opposite side of the balanced transmission line. The gain of each stage adds linearly to the output rather than multiplies one on the other as in a cascade configuration. This allows a higher bandwidth to be achieved than could otherwise be realised  even with the same gain stage elements.

=== Switched mode amplifiers ===
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These nonlinear amplifiers have much higher efficiencies than linear amps, and are used where the power saving justifies the extra complexity. [[Class-D amplifier]]s are the main example of this type of amplification.

=== Negative resistance amplifier ===
{{Unreferenced section|date=December 2024}}
A negative resistance amplifier is a type of regenerative amplifier that can use the feedback between the transistor's source and gate to transform a capacitive impedance on the transistor's source to a negative resistance on its gate. Compared to other types of amplifiers, a negative resistance amplifier will require only a tiny amount of power to achieve very high gain, maintaining a good noise figure at the same time.