Voltage Controlled Amplifier

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.
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