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An electronic component known as a semiconductor. A semiconductor can be an excellent conductor of electricity under some conditions while it can resist conducting electricity under other conditions. There are basically two types, the bipolar transistor (also called the junction transistor), and the field effect transistor (FET). Transistors are typically designed with at least three terminals, one of which (the base) serves as a sort of control gate (for lack of a better term). A supply voltage is connected across the other two terminals and the (signal) voltage present at this third terminal serves to “turn on” the transistor to varying degrees allowing current flow. In a common amplifier circuit, for example, a large supply voltage is placed in series with the transistor and a load (such as a speaker), and then a small varying voltage (the source signal) is applied to the base. This causes the transistor to allow a varying amount of current to flow from the supply source (usually a power supply) through its load as the source signal’s voltage rises and falls. This is a very, very rudimentary description of an amplifier circuit, which is the basic circuit used across a good portion of all analog audio (including equalizers, mixers, crossovers, amps, etc.). It is also worth noting that in many ways the transistor mimics its predecessor, the vacuum tube. There are some distinct and important differences to us audio people, but the basic function is the same.

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