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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.
Another type of amplifier is the fluidic amplifier , based on the fluidic triode. There are also mechanical amplifiers, such as the automotive servo used in braking.
The critical component of an electronic amplifier is an active device, such as a vacuum tube or transistor, typically a bipolar junction transistorA Bipolar Junction Transistor (BJT), is a type of transistor, an amplifying or switching device constructed of doped semiconductor. A bipolar transistor is a sandwich of differently doped sections, either NPN or PNP. The center section is called the base ( BJT) but occasionally a metal oxide semiconductor field-effect transistor ( MOSFETThe MOSFET or Metal-Oxide-Semiconductor Field-Effect Transistor is by far the most common Field effect transistor in both digital and analog circuits. The MOSFET is composed of a channel of n-type or p-type semiconductor material (see article on semicondu). The essential role of the BJT is to dramatically magnify an alternating currentAn alternating current AC is an electrical current, where the magnitude of the current varies in a cyclical form, as opposed to direct current, where the polarity of the current stays constant. The usual waveform of an AC circuit is generally that of a pe ( ACAC may stand for: in electricity, the abbreviation of alternating current in history, Latin ante Christum that is to say before Christ; see Anno Domini in medicine, a chemotherapy regime comprising adriamycin and cyclophosphamide see AC chemotherapy Atlan) 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 biasBias has several different meanings, most relating to an offset or prejudice of some sort. Viewpoint A bias is a prejudice in a general or specific sense, usually in the sense for having a predilection to one particular point of view or ideology. One is s circuit that establishes a fixed ("quiescent") operating point, that is, a specific set of direct currentDirect current DC or continuous current ) is the continuous flow of electricity through a conductor such as a wire from high to low potential. In direct current, the electric charges flow always in the same direction, which distinguishes it from alternati ( 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.
Common emitter, common base, common collector types.
See also valve sound ( tube sound) car tube amp.
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