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The transistor is a semiconductor device that can amplify signals and switch power. Most used types are bipolar (BJT, for Bipolar Junction Transistor), UJT (Unijunction transistor) and MOSFET (FET, for Field Effect Transistor)

Transistor is a family of a semiconductor-device containing 3 electrodes. It is basically a semiconductor-version of triode. The word transistor means "transfer-resistor", since it can amplify and transfer a signal from low-resistance circuit to high-resistance circuit.

We can think a transistor as a variable-resistor whose resistance could be varied according an electrical signal.

BJTs are current controlled current sources, whereas FETs are voltage controlled current sources. Both can be used as amplifier, or as switch.

There are basically 3 possible circuit-pattern around a BJT transistor: 1. common emitter circuit, 2. common base circuit, 3. common collector circuit

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