Editing Amplifier (section)

=== Function ===
Other amplifiers may be classified by their function or output characteristics. These functional descriptions usually apply to complete amplifier systems or sub-systems and rarely to individual stages.{{Cn|date=December 2024}}
*A '''[[servo drive|servo amplifier]]''' indicates an integrated [[feedback loop]] to actively control the output at some desired level. A DC [[servomechanism|servo]] indicates use at frequencies down to DC levels, where the rapid fluctuations of an audio or RF signal do not occur. These are often used in mechanical actuators, or devices such as [[DC motor]]s that must maintain a constant speed or [[torque]]. An '''AC servo''' amp. can do this for some AC motors.<ref>{{Cite web |title=Servo Motor vs Servo Amplifier: Understanding the Differences |url=https://sasaki.com.my/blog/servo-motor-vs-servo-amplifier-differences/ |access-date=2025-05-11 |language=en-US}}</ref><ref>{{Cite web |date=2024-09-04 |title=What Is Feedback on a Servo Loop Circuit? {{!}} Precision Control |url=https://www.indmallautomation.com/faq/what-is-feedback-on-a-servo-loop-circuit/ |access-date=2025-05-11 |language=en-US}}</ref>
*A '''linear''' amplifier responds to different frequency components independently, and does not generate [[harmonic distortion]] or [[intermodulation]] distortion. No amplifier can provide ''perfect'' linearity (even the most linear amplifier has some nonlinearities, since the amplifying devices—[[transistor]]s or [[vacuum tubes]]—follow nonlinear [[power laws]] such as square-laws and rely on circuitry techniques to reduce those effects).<ref>{{Cite web |date=2020-03-06 |title=Linearity in Audio, Part One |url=https://www.psaudio.com/blogs/copper/linearity-in-audio-part-one |access-date=2025-05-11 |website=PS Audio |language=en}}</ref><ref>{{Cite web |title=What is Intermodulation Distortion? - everything RF |url=https://www.everythingrf.com/community/what-is-intermodulation-distortion |access-date=2025-05-11 |website=www.everythingrf.com}}</ref>
*A '''nonlinear''' amplifier generates significant distortion and so changes the harmonic content; there are situations where this is useful. Amplifier [[non-linear circuits#Non-linear networks|circuits]] intentionally providing a non-linear [[transfer function]] include:{{Cn|date=December 2024}}
** a device like a [[silicon controlled rectifier]] or a [[transistor#Transistor as a switch|transistor used as a switch]] may be employed to turn either fully ''on'' or ''off'' a load such as a lamp based on a threshold in a continuously variable input.
** a non-linear amplifier in an [[analog computer]] or [[true RMS converter]] for example can provide  a special transfer function, such as logarithmic or square-law.
** a [[Power amplifier classes#Class C|Class C]] [[radio frequency|RF]] amplifier may be chosen because it can be very efficient—but is non-linear. Following such an amplifier with a so-called ''[[LC circuit|tank]]'' [[tuned circuit]] can reduce unwanted harmonics (distortion) sufficiently to make it useful in [[transmitters]], or some desired [[harmonic]] may be selected by setting the [[Electronic filter|resonant frequency]] of the tuned circuit to a higher [[frequency]] rather than [[fundamental frequency]] in [[frequency multiplier]] circuits.
** [[Automatic gain control]] circuits require an amplifier's gain be controlled by the time-averaged amplitude so that the output amplitude varies little when weak stations are being received. The non-linearities are assumed arranged so the relatively small signal amplitude suffers from little distortion (cross-channel interference or [[intermodulation]]) yet is still [[modulated]] by the relatively large gain-control [[direct current|DC]] voltage.
** [[Amplitude modulation|AM]] detector circuits that use amplification such as [[anode-bend detector]]s, [[precision rectifier]]s and [[infinite impedance detector]]s (so excluding ''unamplified'' detectors such as [[cat's-whisker detector]]s), as well as [[Peak detector#Peak detector|peak detector circuits]], rely on changes in amplification based on the [[signal (electrical engineering)|signal]]'s instantaneous amplitude to derive a [[direct current]] from an [[alternating current]] input.
** [[Operational amplifier applications#Comparators and detectors|Operational amplifier comparator and detector circuits]].
*A '''wideband''' amplifier has a precise amplification factor over a wide frequency range, and is often used to boost signals for relay in communications systems. A '''narrowband''' amp amplifies a specific narrow range of frequencies, to the exclusion of other frequencies.{{Cn|date=December 2024}}
*An '''RF''' amplifier amplifies signals in the [[radio frequency]] range of the [[electromagnetic spectrum]], and is often used to increase the sensitivity of a [[receiver (radio)|receiver]] or the output power of a [[transmitter]].<ref>{{cite journal|last=Roy|first=Apratim|author2=Rashid, S. M. S.|title=A power efficient bandwidth regulation technique for a low-noise high-gain RF wideband amplifier|journal=Central European Journal of Engineering|date=5 June 2012|volume=2|issue=3|pages=383–391|doi=10.2478/s13531-012-0009-1|bibcode = 2012CEJE....2..383R |s2cid=109947130|doi-access=free}}</ref>
*An '''audio amplifier''' amplifies [[sound reproduction|audio]] frequencies. This category subdivides into small signal amplification, and power amps that are optimised to driving [[loudspeaker|speaker]]s, sometimes with multiple amps grouped together as separate or bridgeable channels to accommodate different audio reproduction requirements. Frequently used terms within audio amplifiers include:{{Cn|date=December 2024}}
** [[Preamplifier]] (preamp.), which may include a [[phono stage]] with [[RIAA equalization]], or [[tape head]] preamps with [[Comité consultatif international pour la radio|CCIR]] equalisation filters. They may include [[filter (signal processing)|filter]]s or [[tone control]] circuitry.
** [[Power amplifier]] (normally drives [[loudspeaker]]s), [[headphone amplifier]]s, and [[public address#Amplifiers|public address amplifiers]].
** [[Stereo]] amplifiers imply two channels of output (left and right), though the term simply means "solid" sound (referring to three-dimensional)—so [[quadraphonic]] stereo was used for amplifiers with four channels. 5.1 and 7.1 systems refer to [[Home theatre system]]s with 5 or 7 normal spatial channels, plus a [[subwoofer]] channel.
*[[Buffer amplifiers]], which may include [[emitter follower]]s, provide a high [[Electrical impedance|impedance]] input for a device (perhaps another amplifier, or perhaps an energy-hungry load such as lights) that would otherwise draw too much current from the source. [[Line driver]]s are a type of buffer that feeds long or interference-prone interconnect cables, possibly with [[differential signaling|differential]] outputs through [[twisted pair]] cables.{{Cn|date=December 2024}}