Voltage Controlled Amplifier

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A Voltage Controlled Amplifier ( VCA , English for voltage controlled amplifier ) is an electronic amplifier whose gain factor can be controlled by a control voltage . A voltage-controlled amplifier can be distinguished from an analog multiplier in that there is a control signal and an amplified signal. Only the amplified signal has to be reproduced linearly and usable up to the limit frequency, the control signal, however, may be distorted non-linearly and does not have to reach the same limit frequency as the amplified signal. If both signal inputs are identical in terms of linearity and cut-off frequency, an analog multiplier is available.

Despite the name "amplifier", some voltage-controlled amplifiers work with gain factors smaller than 1 and are therefore actually voltage-controlled voltage dividers. If a higher gain is required, a normal amplifier can be connected downstream.

An algorithm in a digital audio system can have the same function . The gain of a controllable gain can be set not only with a voltage but also with an electric current or a digital control value.


Voltage controlled amplifiers in the strict sense

  • Transductor - a control signal brings an iron core into saturation to a greater or lesser extent, thereby modulating an inductance.
  • Multi-grid tube - electron tubes with several grids (e.g. pentode , tetrode ) allow the penetration and thus the gain of a circuit to be changed by changing the screen grid voltage .
  • Diodes - The differential resistance of a diode is controlled by a high DC current, and the diode is then inserted into a voltage divider or negative feedback of an amplifier. Usually, two diodes are connected in series through which the control current flows, and the signal is coupled into the connection node via a series resistor. The voltage fluctuations at the node follow the input voltage and are reduced in amplitude by the direct current. If the control current is much larger than the signal current, the input voltage is transmitted in a sufficiently linear manner.
  • Photo resistor - A photo resistor behaves perfectly linearly in relation to the U / I characteristic curve and can be adjusted by exposing it to a lamp (light bulb, LED ...). If it is used in the negative feedback of an amplifier or in a voltage divider, it can be used to control the gain factor.
  • Field effect transistor - The source-drain path behaves similarly to an ohmic resistor and can be used to modulate an amplifier. For good linearity, it is important that the fluctuations in the source-drain voltage have as little influence as possible on the gate-source voltage. This can be achieved with a relatively low signal voltage or with a low pass (capacitor between gate and source).

Analog multiplier


  • In electronic music , VCAs are widely used in compressors , limiters , gates and numerous other effects devices and electronic musical instruments . Mostly it is about electronic components, as they are from companies such. B. dbx, THAT or Analog Devices . The term amplifier is a bit misleading because the input signal is reduced or looped through unchanged at most, which corresponds to a "gain" by a factor of 1. This is why VCAs are sometimes referred to as Voltage Controlled Attenuators.
  • In entertainment electronics and radiotelephony , VCAs are used to automatically level out signals or to suppress background noise . In part, these applications are similar to those in music electronics; although in the end, especially in the professional area, much higher quality leveling methods and components are used; see also squelch , or squelch gate.

See also