Editing Variable-gain amplifier (section)

{{Short description|Electronic amplifier that varies its gain depending on a control voltage}}

A '''variable-gain''' ('''VGA''') or '''voltage-controlled amplifier''' ('''VCA''') is an [[electronic amplifier]] that varies its [[Gain (electronics)|gain]] depending on a control voltage (often abbreviated CV).

VCAs have many applications, including [[audio level compression]], [[synthesizers]] and [[amplitude modulation]].

A crude example is a [[Op-amp#Inverting amplifier|typical inverting]] [[op-amp]] configuration with a [[light-dependent resistor]] (LDR) in the feedback loop.  The gain of the amplifier then depends on the light falling on the LDR, which can be provided by an [[LED]] (an [[optocoupler]]).  The gain of the amplifier is then controllable by the current through the LED.  This is similar to the circuits used in optical [[audio compressor]]s.

A voltage-controlled amplifier can be realised by first creating a [[voltage-controlled resistor]] (VCR), which is used to set the amplifier gain. The VCR is one of the numerous interesting circuit elements that can be produced by using a [[JFET]] (junction field-effect transistor) with simple biasing. VCRs manufactured in this way can be obtained as discrete devices, e.g. VCR2N.

Another type of circuit uses [[operational transconductance amplifier]]s.

In audio applications [[logarithm]]ic gain control is used to emulate how the ear hears [[Fletcher–Munson curves|loudness]]. [[David E. Blackmer]]'s [[Dbx, Inc.|dbx]] 202 VCA, based on the [[Blackmer gain cell]], was among the first successful implementations of a logarithmic VCA.<ref>{{cite web |url=http://www.thatcorp.com/History_of_VCAs.shtml |archive-url=https://web.archive.org/web/20200703115259/http://www.thatcorp.com/History_of_VCAs.shtml |archive-date=2020-07-03 |title=A Brief History of VCA Development}}</ref>

[[Analog multiplier]]s are a type of VCA designed to have accurate linear characteristics, the two inputs are identical and often work in all four voltage quadrants, unlike most other VCAs.