Science Fair Project Encyclopedia
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.
See also: low noise amplifier.
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.
The critical component of an electronic amplifier is an active device, such as a vacuum tube or transistor, typically a bipolar junction transistor (BJT) but occasionally a metal oxide semiconductor field-effect transistor (MOSFET). The essential role of the BJT is to dramatically magnify an alternating current (AC) base current to yield significantly larger AC collector current and emitter current . The amount of magnification (the "AC forward gain") is determined by interconnecting the BJT within a bias circuit that establishes a fixed ("quiescent") operating point, that is, a specific set of direct current (DC) values for the base, collector, and emitter currents. While the AC input signal to the amplifier (say, the tiny sound evidenced when a mechanical needle rides in the groove of a vinyl LP record) is typically the transistor base current, the AC output signal (say, used to drive a pair of eight-ohm stereo loudspeakers) of the amplifier could be a collector or emitter voltage (with respect to ground) or even some other value that varies with the collector current in a rigidly predeterminate manner.
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