Transistor as a Switch

When a bipolar junction transistor (BJT) is used in any circuit, its function is determined by the devices characteristic curves.

For a BJT there are input, output and transfer characteristic curves, the most useful being the output characteristic curve. The output curves dictate the range of collector-emitter voltage Vce for variations in collector current, Ic. For use as an amplifier, biasing is arranged so that the linear part of the output curves (the almost horizontal sections) are used. If the circuit uses the transistor as a switch, then biasing is arranged to operate on regions of the output curves known as saturation and cut-off.

Because a transistor's collector current is proportionally limited by its base current, it can be used as a sort of current-controlled switch. A relatively small flow of electrons sent through the base of the transistor has the ability to exert control over a much larger flow of electrons through the collector.

	While transistors have many uses, one of the less known uses by amateurs is the ability for bipolar transistors to turn things on and off. While there are limitations as to what we can switch on and off, transistor switches offer lower cost and substantial reliability over conventional mechanical relays. In this article, we will review the basic principles for transistor switches using common bipolar transistors.