Editing Amplifier (section)

== Classification of amplifier stages and systems ==
=== Common terminal ===
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One set of classifications for amplifiers is based on which device terminal is common to both the input and the output circuit. In the case of [[bipolar junction transistor]]s, the three classes are common emitter, common base, and common collector. For [[field-effect transistor]]s, the corresponding configurations are common source, common gate, and common drain; for [[vacuum tube]]s, common cathode, common grid, and common plate.

The common emitter (or common source, common cathode, etc.) is most often configured to provide amplification of a voltage applied between base and emitter, and the output signal taken between collector and emitter is inverted, relative to the input. The common collector arrangement applies the input voltage between base and collector, and to take the output voltage between emitter and collector. This causes negative feedback, and the output voltage tends to follow the input voltage. This arrangement is also used as the input presents a high impedance and does not load the signal source, though the voltage amplification is less than one. The common-collector circuit is, therefore, better known as an emitter follower, source follower, or cathode follower.

=== Unilateral or bilateral ===
An amplifier whose output exhibits no feedback to its input side is described as 'unilateral'. The input impedance of a unilateral amplifier is independent of load, and output impedance is independent of signal source impedance.<ref>{{Cite web|url=http://www.microwaves101.com/encyclopedias/active-directivity-of-amplifiers|title=Microwaves101 {{!}} Active Directivity of Amplifiers|last=Administrator|website=www.microwaves101.com|access-date=2016-06-20}}</ref>

An amplifier that uses feedback to connect part of the output back to the input is a ''bilateral'' amplifier. Bilateral amplifier input impedance depends on the load, and output impedance on the signal source impedance.
All amplifiers are bilateral to some degree; however they may often be modeled as unilateral under operating conditions where feedback is small enough to neglect for most purposes, simplifying analysis (see the [[common base]] article for an example).{{Cn|date=December 2024}}

=== Inverting or non-inverting ===
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Another way to classify amplifiers is by the phase relationship of the input signal to the output signal.  An 'inverting' amplifier produces an output 180 degrees out of phase with the input signal (that is, a polarity inversion or mirror image of the input as seen on an [[oscilloscope]]). A 'non-inverting' amplifier maintains the phase of the input signal waveforms. An [[emitter follower]] is a type of non-inverting amplifier, indicating that the signal at the emitter of a transistor is following (that is, matching with unity gain but perhaps an offset) the input signal. Voltage follower is also non-inverting type of amplifier having unity gain.

This description can apply to a single stage of an amplifier, or to a complete amplifier system.

=== 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}}

=== Interstage coupling method ===
{{See also|Multistage amplifier}}{{Unreferenced section|date=December 2024}}
Amplifiers are sometimes classified by the coupling method of the signal at the input, output, or between stages. Different types of these include:
;Resistive-capacitive (RC) coupled amplifier, using a network of resistors and capacitors: By design these amplifiers cannot amplify DC signals as the capacitors block the DC component of the input signal. RC-coupled amplifiers were used very often in circuits with vacuum tubes or discrete transistors. In the days of the integrated circuit a few more transistors on a chip are much cheaper and smaller than a capacitor.
;Inductive-capacitive (LC) coupled amplifier, using a network of inductors and capacitors: This kind of amplifier is most often used in selective radio-frequency circuits.
;[[Transformer]] coupled amplifier, using a transformer to match impedances or to decouple parts of the circuits :Quite often LC-coupled and transformer-coupled amplifiers cannot be distinguished as a transformer is some kind of inductor.
;[[Direct coupled amplifier]], using no impedance and bias matching components: This class of amplifier was very uncommon in the vacuum tube days when the anode (output) voltage was at greater than several hundred volts and the grid (input) voltage at a few volts minus. So they were used only if the gain was specified down to DC (e.g., in an oscilloscope). In the context of modern electronics developers are encouraged to use directly coupled amplifiers whenever possible. In FET and CMOS technologies direct coupling is dominant since gates of MOSFETs theoretically pass no current through themselves. Therefore, DC component of the input signals is automatically filtered.

=== Frequency range ===
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Depending on the frequency range and other properties amplifiers are designed according to different principles.

Frequency ranges down to DC are used only when this property is needed. Amplifiers for direct current signals are vulnerable to minor variations in the properties of components with time.  Special methods, such as [[chopper (electronics)|chopper]] stabilized amplifiers are used to prevent objectionable drift in the amplifier's properties for DC. "DC-blocking" [[capacitor]]s can be added to remove DC and sub-sonic frequencies from audio amplifiers.

Depending on the frequency range specified different design principles must be used. Up to the MHz range only "discrete" properties need be considered; e.g., a terminal has an input impedance.

As soon as any connection within the circuit gets longer than perhaps 1% of the wavelength of the highest specified frequency (e.g., at 100&nbsp;MHz the wavelength is 3&nbsp;m, so the critical connection length is approx. 3&nbsp;cm) design properties radically change. For example, a specified length and width of a [[Printed circuit board|PCB]] trace can be used as a selective or impedance-matching entity.
Above a few hundred MHz, it gets difficult to use discrete elements, especially inductors. In most cases, PCB traces of very closely defined shapes are used instead ([[stripline]] techniques).

The frequency range handled by an amplifier might be specified in terms of [[Bandwidth (signal processing)|bandwidth]] (normally implying a response that is 3&nbsp;[[Decibel|dB]] down when the frequency reaches the specified bandwidth), or by specifying a '''frequency response''' that is within a certain number of [[decibel]]s between a lower and an upper frequency (e.g. "20&nbsp;Hz to 20&nbsp;kHz plus or minus 1&nbsp;dB").