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What Is a Voltage-Controlled Amplifier?

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  • Written By: B. Leslie Baird
  • Edited By: Michelle Arevalo
  • Last Modified Date: 05 September 2016
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    2003-2016
    Conjecture Corporation
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In a voltage-controlled amplifier (VCA), the gain is controlled by the voltage level of the signal. The gain is a ratio measurement of the output amplitude divided by the input amplitude. This indicates how much a signal, or sound, has been strengthened. These amplifier systems can be used for music and in public address (PA) or conference systems.

Sound systems are often used in mixing consoles for the recording and playback of music. Consoles use multiple channels to control different features of instruments and vocal sections. Each channel can have its own voltage-controlled amplifier circuit, allowing sections to be mixed and faded separately. The circuit establishes the volume level of a note and quiets the output when the note ends. A master control can allow sections of the music to become louder or softer without interfering with the total mix.

Variable-gain is another term that may be used to refer to a voltage-controlled amplifier. The circuits normally have a gain ratio of one, but the gain can be varied. A gain of one means that the circuit attenuates instead of simply amplifying. Attenuation filters the sounds and can remove unwanted noise while preserving the required tones.

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Electronic components found in these systems include operational amplifiers (op amps), junction field-effect transistors (JFETs), and voltage-controlled resistors (VCRs). Voltage-controlled resistors, or varistors, decrease in resistance as voltage increases. A very simple version of a linear voltage-controlled amplifier can be created with two JFETs and one op amp. In this circuit, the JFETs will work as resisters for linear control.

A voltage-controlled amplifier can be classified as two-quadrant or four-quadrant. In a two-quadrant version, if the controlling voltage drops to zero or less, the amplifier will have no output. Using a four-quadrant amplifier, when the voltage reaches zero, the output gain rises, but this output is inverted in its phase. Four-quadrant systems are used for amplitude modulation. Amplitude modulation (AM) varies the strength of a signal, while frequency modulation (FM) varies its frequency.

Amplifiers are available in a wide assortment of dynamic and gain ranges. They offer a great deal of range control with low and low distortion. A voltage-controlled amplifier tends to have a longer life span due to less electronic-part problems than other types.

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