Home > Circuits > Voltage Controlled Amplifier
Modified:21:50, 22 October 2013

This circuit is based upon another I came across when looking for a Voltage-Controlled-Amplifier - an amplifier whose gain is controlled by a second input voltage. Very useful for modulators and sound effects. With a bit of experimenting it can also work as a multiplier where the output is the sum of the Signal and Gain inputs.

Voltage controlled amplifier - as a modulator

The circuit is fairly simple based around an op-amp utilising FET's to control the gain. Without going into detail, basically the gain of the amplifier is controlled by the the level of voltage at the GAIN input. This will cause the SIGNAL to vary in amplitude depending on the GAIN. VR1 sets the depth of modulation - the amount of effect the GAIN voltage has.

MODULATION

The circuit can act like a modulator with the SIGNAL input as the carrier and the GAIN input as the modulation signal. The trace below shows the effect of using an SIGNAL of 400Hz modulated with a 25Hz sine wave.

Simulated function:
Red trace = modulation input,
Blue trace = output signal
SIGNAL input: 400Hz sine @ 500mV
GAIN input: 25Hz sine wave @ 500mV

With the GAIN input at 0v, the gain of the amplifier is = 1.As the voltage at the GAIN rises the output signal falls, as the GAIN goes negative the output signal rises. The results for this circuit show that +/- 0.5v at the GAIN results in 60% cut/boost in the output signal. R5 controls the overall gain of the amplifier and can be increased to produce greater effect.

Voltage controlled amplifier - as a multiplier

By changing Q2 from a P-channel FET to an N-channel FET the circuit functions like that of a multiplier.

Basically the SIGNAL input is multiplied by the voltage present at the GAIN input. The result is the sum of the two inputs.

This is sometimes known as a Ring Modulator and can produce some excellent sound effects.The trace below shows the effect of this 'summing'.

Simulated function:
Red trace = modulation input,
Blue trace = output signal
SIGNAL input: 400Hz sine @ 500mV
GAIN input: 25Hz sine wave @ 500mV

The difference is clear to see. There are now bursts of sound. Notice how the SIGNAL is amplified on the positive and negative voltages at the GAIN input.

SOUND EFFECTS

This can produce some interesting sound effects if you connect a mic and pre-amp into the signal input, a low frequency sine wave in the GAIN and the OUT to an audio amplifier.

DOWNLOADS

- Circuit simulation of switchable multiplier/modulator

Designed and Written by Phil Townshend 2010
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