In an amplifier, the term "gain" means the amount of amplification it can produce. Turning up the volume of an audio amplifier increases the sound level, but the total gain available depends on the design of the amplifier. A lower power VGA may produces a maximum of 10 watts, whereas a larger one could produce 1,200 watts or more. Amplifier systems for concerts or auditoriums may use a VGA to control the input of a high-power fixed gain amplifier system that distributes the sound to larger speakers that produce several thousand watts.
Any amplifier that has a control to change the output level -- such as the volume control on a radio or TV -- is a variable gain amplifier. Many engineers and technicians confuse the term with automatic gain control, which keeps the output level of an amplifier constant, regardless of the input.
In most home entertainment systems, audio amplifiers use an input of zero dBm or 1 milliwatt. Referred to as "line level," audio signals from devices such as CD players and cable and satellite TV receivers enter the audio input jacks of the amplifier at this standard level. Inputs from phonographs or digital inputs from iPods may have pre-amps to set the signals to line level before going to the VGA. When the volume control is all the way down, the VGA has no gain at all and the output is the same as or less than the input.
Gain Control and Output
Turning up the volume or gain control of a VGA increases the output to the maximum level of its design. The control may have calibration points from "1" to "10" or a digital display giving a number relative to the output power. Because the control isn't linear, an audio VGA produces roughly 50 percent of its available power at 70 percent of the volume control range. It reaches 100 percent power at roughly 80 percent of the control range; above that, circuits automatically limit the power to keep it at the maximum rated level, but begin to distort the quality.
A VGA may have an automatic gain control to keep the output level stable for varying input levels. A car radio, for example, may receive weaker signals with increased distance from the station, but AGC circuits keep the speaker volume the same. Industrial systems use VGAs for applications outside the audio frequency range, and some have automatic remote control features. For example, an industrial two-way microwave radio system may have a VGA on the transmitter. The receiver at the distant end returns information telling it to increase power during extreme weather, and then return to normal power when the storm passes.