Editing Transconductance (section)

== Amplifiers ==

=== Transconductance amplifiers ===
A '''transconductance amplifier''' (''g''{{sub|m}} amplifier) puts out a current proportional to its input voltage. In '''[[Network analysis (electrical circuits)|network analysis]]''', the transconductance amplifier is defined as a ''{{dfn|voltage controlled current source}}'' ({{abbr|VCCS}}). These amplifiers are commonly seen installed in a [[cascode]] configuration, which improves the frequency response.

An ideal transconductance amplifier in a voltage follower configuration behaves at the output like a resistor of value {{math|1/''g''{{sub|m}}}}, between a buffered copy of the input voltage and the output. If the follower is loaded by a single capacitor {{math|''C''}}, the voltage follower transfer function has a single pole with time constant {{math|''C''/''g''{{sub|m}}}},<ref>{{cite web | url=https://hasler.ece.gatech.edu/Courses/ECE6414/Unit3/gmCFilter01.pdf | title=Basics of Transconductance - Capacitance Filters | website=hasler.ece.gatech.edu | first=Paul | last=Hasler}}</ref> or equivalently it behaves as a 1st-order low-pass filter with a [[half-power bandwidth|{{val|-3|u=dB}} bandwidth]] of {{math|''g''{{sub|m}}/2''πC''}}.

==== Operational transconductance amplifiers ====
{{Main article|Operational transconductance amplifier}}
An [[operational transconductance amplifier]] (OTA) is an integrated circuit which can function as a transconductance amplifier. These normally have an input to allow the transconductance to be controlled.<ref name="MAX3724">{{cite web |title=3.2&nbsp;Gbps SFP Transimpedance Amplifiers with RSSI |url=https://datasheets.maximintegrated.com/en/ds/MAX3724-MAX3725.pdf |website=datasheets.maximintegrated.com |publisher=Maxim |access-date=15 November 2018}}</ref>

=== Transresistance amplifiers ===
{{Main article|transimpedance amplifier}}
A '''transresistance amplifier''' outputs a voltage proportional to its input current. The transresistance amplifier is often referred to as a '''transimpedance amplifier''', especially by semiconductor manufacturers.

The term for a transresistance amplifier in network analysis is ''current controlled voltage source'' (''CCVS'').

A basic inverting transresistance amplifier can be built from an [[operational amplifier]] and a single resistor. Simply connect the resistor between the output and the inverting input of the operational amplifier and connect the non-inverting input to ground. The output voltage will then be proportional to the input current at the inverting input, decreasing with increasing input current and vice versa. 

Specialist chip transresistance (transimpedance) amplifiers are widely used for amplifying the signal current from photo diodes at the receiving end of ultra high speed fibre optic links.