A transistor is a semiconductor device used to amplify and switch electronic signals and electrical power. It is composed of semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current through another pair of terminals. Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify a signal.
The BC557 (RM0385) is a PNP transistor and a heavy-duty drop-in replacement for a 2N2222 or 2N3906. The three terminals of a bipolar transistor are called the emitter, base, and collector. A small current into the base controls a large current flow from the collector to the emitter. The current at the base is typically one-hundredth of the collector-emitter current. Moreover, the large current flow is almost independent of the voltage across the transistor from collector to emitter.
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