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The VCA or Voltage Controlled Amplifier



The VCA  – a Voltage Controlled Amplifier which doesn’t amplify.

The VCA, in this case used in an audio instrument such as an audio synthesiser is usually the last block in the chain.  VCAs have a control voltage, on peak control voltage the entire signal is allowed through, on zero volts or lower the signal is blocked.  VCAs can also be termed voltage controlled attentuators.

The VCA is usually of 2 quadrant or 4 quadrant type, the output is blocked when the input control voltage is at zero. Although a VCA can be made to amplify an input signal, here the VCA is used and predominantly so as an attentuator. 2 quadrant designs have no output when the CV is at or just below 0V, 4 quadrant designs invert the output signal, and with gain set to an absolute value of CV below 0V the gain then rises. Four quadrant designs are used in amplitude and ring modulation effects. VCAs are therefore ideal for envelope shaping. The  MOTM-190 can be switched to either 2 or 4 quadrant mode.

  • VCAs are often used in circuits for compressing, companding (compressing/expanding to improve S/N) and limiting.
  • DCAs or digital controlled amplifiers are another variant, describing the controller type which varies the gain.

Although years back VCAs were usually of the discrete type; getting to grips now with most VCAs one would be looking at a high quality, multi-bit controlled unit in an IC package.

VCA using an LM101
VCA using an LM101

The VCA above is gain controlled by FET 2N5457 which acts as a variable voltage resistor. The circuit is not unique, many opamps may and will have better performance but this is a typical design where the linear voltage change across the FET ultimately results in well controlled gain of the LM101 (or other).

Another circuit showing similar principles below.

vca - linear voltage control
Linear Voltage Control VCA

Although many circuit designers are using operational amplifiers in their circuits there are some that feel with a discrete component approach the output quality can be greatly improved. See below, under further reading.

As discussed before, the VCA is used in limiting, companding and compressing resulting in the use of additional controls which vary attack, delay and sustain, release times. (In synth language ADSR) or AGC (automatic Gain Control).

VCA - 2 and 4 quadrant multipliers
2 and 4 Quadrant Multipliers (Courtesy Texas Instruments LM13700)

Above we have the 2 quadrant multiplier or amplitude modulator and the ring modulator both using dual operational transconductance amplifiers, the LM13700 with linearizing diodes and buffers.

For the AM or 2-quadrant multiplier refer to chapters 20 and 21 of the application notes which explains the VCA.

Note the additional path in the RM where a resistor (RM) is installed between out and invert input. RC for Vin2 is a potentiometer.

In our previous article on RM we cover the 4 diode bridge circuit used in SSB (RF).

In our next article we look at how VCF or voltage controlled filters work.

Further Reading: 

VCA Techniques by the honourable guru Rod Elliott. Good solid write-up, as always.

Analog Devices – AN213 / AD539

The Ring-Bridge Modulator

 

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