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Transistor Symbols and Connections

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Whether a transistor uses NPN or PNP construction, its three electrodes are labelled collector, base, and emitter. Note that each junction within the transistor, whether it be collector-base or base-emitter, constitutes a P-N junction - just like a diode.

In normal operations, the base-emitter junction is forward biased whilst the collector-base junction is reverse biased. The base region is, however, made very narrow so that carriers are swept across it from emitter to collector and only a relatively small current flows in the base. As an example, the current flowing in the emitter circuit of a small-signal transistor is typically 100 times greater than that flowing in the base. Note that the direction of conventional current flow is from emitter to collector in the case of a PNP transistor, and collector to emitter in the case of an NPN device.

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