Variable-gain amplifier

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A voltage-controlled amplifier is an electronic amplifier that varies its gain depending on a control voltage (often abbreviated CV).

They have many applications, including audio level compression, synthesizers, and amplitude modulation.

A crude example is a 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 optocouple). The gain of the amplifier is then controllable by the current through the LED. This is similar to the circuits used in optical audio compressors.

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 amplifiers.

[edit] In sound desks

Some sound desks come equipped with VCAs in series with the fader on each channel. The voltage that controls the gain of these amplifiers comes from a master fader elsewhere on the desk. There are several VCA masters, and each channel can be switched to be controlled by any of the masters. By assigning multiple channels to the same VCA group, you can control them all simultaneously without changing the position of the master fader.