How a transistor works

A transistor may be used to switch or to amplify. The image to the right represents a typical transistor in a circuit. Its three components are the base, emitter and collector which correspond to regions of the mixed semiconductors from which the transistor is made. Current may flow from the emitter to the collector depending on the voltage applied to the base, but only if this voltage exceeds a certain value: this is depicted in the graph below at A and is referred to as V_be.

Transistor as a switch

It can be seen from the graph that once the base voltage reaches a certain level, shown at B, no more current will flow and the output will be held at a fixed voltage. The transistor is then said to be saturated. Hence, values of input voltage can be chosen such that the output is either completely off, or completely on. The transistor is acting as a switch, and this type of operation is common in digital circuits where only "on" and "off" values are relevant.

Transistor as an amplifier
A varying base voltage, V_in, as long as it exceeds V_be, controls current through the transistor and thus influences the output voltage V_out. The slope of the graph is such that small swings in V_in will produce large changes in V_out.

This occurs because the base voltage controls how much of the power supply voltage V_cc causes current through the transistor itself, and how much of it causes current through a load driven by V_out. It is important that the operating parameters of the transistor are chosen and the circuit designed such that as far as possible the transistor operates within a linear portion of the graph, such as that shown between A and B, otherwise the output signal will suffer distortion.

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