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

The transistor is a family of semiconductor devices, usually with three terminals. It is basically a semiconductor version of a triode. The word transistor comes from "transfer-resistor", since it can amplify and transfer a signal from a low-resistance circuit to a high-resistance circuit.

We can think of a transistor as a variable resistor whose resistance can be varied by an electrical signal.

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

There are three basic circuit patterns around a BJT transistor:

  1. common emitter circuit
  2. common base circuit
  3. common collector circuit

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