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An amplifier is a device that uses a small amount of energy to control a larger amount. The relationship of the input to the output--usually expressed as a function of the input frequency--is called the transfer function of the amplifier, and the magnitude of the transfer function is termed the gain. Amplifiers are typically utilized over a specific range of frequencies and are generally of maximal utility if the gain is constant in that range.

Electronic Amplifiers

The most common type of amplifier is the electronic amplifier, commonly used in radio and television transmitters and receivers, high-fidelity ("hi-fi") stereo equipment, microcomputers and other electronic digital equipment, and guitar and other instrument amplifiers. Its critical component is an active device, such as a vacuum tube or transistor.

Transistor Amplifers

Most common actice devices in transistor amplifiers are bipolar junction transistors (BJTs); metal oxide semiconductor field-effect transistors (MOSFETs) are also used. The essential role of this active element is to magnify an input signal to yield a significantly larger output signal. The amount of magnification (the "forward gain") is determined by the external circuit design as well as the active device.

Vacuum Tube (Valve)Amplifiers

In the earlier years of audio, vacuum tubes filled the active device role.

Amplifiers using OPAMPS

Awaiting contributions.

Fluidic amplifiers

Another type of amplifier is the fluidic amplifier, based on the fluidic triode.

Other Amplifier Types

There are also mechanical amplifiers, such as the automotive servo used in braking.See also: low noise amplifier.

External links

• The Audio Circuit - Information on and user reviews of loudspeakers, headphones, amplifiers, and playback equipment

• Amplifiers