Editing Amplifier (section)

==Theory of Operation==
[[File:Dependent Sources.PNG|thumbnail|300px|The four types of dependent source—control variable on left, output variable on right]]
In principle, an amplifier is an electrical [[two-port network]] that produces a signal at the output port that is a replica of the signal applied to the input port, but increased in magnitude.{{Cn|date=December 2024}}

The input port can be idealized as either being a voltage input, which takes no current, with the output proportional to the voltage across the port; or a current input, with no voltage across it, in which the output is proportional to the current through the port.  The output port can be idealized as being either a [[dependent voltage source]], with zero source resistance and its output voltage dependent on the input; or a [[dependent current source]], with infinite source resistance and the output current dependent on the input. Combinations of these choices lead to four types of ideal amplifiers.<ref name="Patronis"/> In idealized form they are represented by each of the four types of [[dependent source]] used in linear analysis, as shown in the figure, namely:{{Cn|date=December 2024}}

{| class="wikitable" style="text-align:center"
|-
! Input
! Output
! Dependent source
! Amplifier type
! Gain units
|-
|-
| I
| I
| Current controlled current source, CCCS
| Current amplifier
| [[Dimensionless quantity|Unitless]]
|-
|-
| I
| V
| Current controlled voltage source, CCVS
| [[Transresistance]] amplifier
| [[Ohm]]
|-
|-
| V
| I
| Voltage controlled current source, VCCS
| [[Transconductance]] amplifier
| [[Siemens (unit)|Siemens]]
|-
|-
| V
| V
| Voltage controlled voltage source, VCVS
| Voltage amplifier
| [[Dimensionless quantity|Unitless]]
|}

Each type of amplifier in its ideal form has an ideal input and output resistance that is the same as that of the corresponding dependent source:<ref>This table is a [http://www.swemorph.com/ma.html "Zwicky box"]; in particular, it encompasses all possibilities. See [[Fritz Zwicky]].</ref>

{| class="wikitable" style="text-align:center;"
|-
! Amplifier type
! Dependent source
! Input impedance
! Output impedance
|-
| Current
| CCCS
| 0
| ∞
|-
| Transresistance
| CCVS
| 0
| 0
|-
| Transconductance
| VCCS
| ∞
| ∞
|-
| Voltage
| VCVS
| ∞
| 0
|}

In real amplifiers the ideal impedances are not possible to achieve, but these ideal elements can be used to construct [[equivalent circuit]]s of real amplifiers by adding impedances (resistance, capacitance and inductance) to the input and output.   For any particular circuit, a small-signal analysis is often used to find the actual impedance. A small-signal AC test current ''I<sub>x</sub>'' is applied to the input or output node, all external sources are set to AC zero, and the corresponding alternating voltage ''V<sub>x</sub>'' across the test current source determines the impedance seen at that node as ''R = V<sub>x</sub> / I<sub>x</sub>''.<ref>{{Cite web|url=http://www.eeherald.com/section/design-guide/index|title=Small signal analysis of Complex amplifier circuits|website=www.eeherald.com|access-date=2016-06-20|url-status=dead|archive-url=https://web.archive.org/web/20161009041842/http://www.eeherald.com/section/design-guide/signal_analysis_amplifiers.html|archive-date=2016-10-09}}</ref>

Amplifiers designed to attach to a [[transmission line]] at input and output, especially [[RF power amplifier|RF amplifiers]], do not fit into this classification approach.  Rather than dealing with voltage or current individually, they ideally couple with an input or output impedance matched to the transmission line impedance, that is, match ''ratios'' of voltage to current.  Many real RF amplifiers come close to this ideal.  Although, for a given appropriate source and load impedance, RF amplifiers can be characterized as amplifying voltage or current, they fundamentally are amplifying power.<ref>{{cite book | title = Vsats: Very Small Aperture Terminals | author = John Everett | publisher = IET | year = 1992 | isbn = 978-0-86341-200-4 | url = https://books.google.com/books?id=MWuXmf4V4NwC&q=amplifier+impedance+transmission-line++dB&pg=PA84 }}</ref>