::: Online Electronics Course

A transistor may be thought of as an electronic tap able to control a large flow of electrons with only small variations of the 'handle'.

The 'handle' in the case of a transistor is called the "base". The in and out 'pipes' are called the "emitter" and the "collector".

Voltage changes at the base of the transistor result in changes to the flow of electricity through the transistor.

A transistor may be thought of as an electronic 'tap'.

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Symbol for a Transistor

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A small-signal Transistor

A high-current Transistor

When we think of a water tap we know that large amounts of water can be controlled by very small movements of the spigot (the tap handle). It is useful to think of electricity flowing around a circuit in the same way we think of water flowing in pipes. With an electronic 'tap' the flow of electricity is controlled by varying the voltage between the emitter and base of the transistor. The main 'flow' is a path between the emitter and the collector.

Transistors are used in two basic ways:
1. As an electronic 'switch'. ie they are either fully ON, or fully OFF. (eg to switch a relay)
2. To provide amplification of a small changing voltage ( a 'signal') present at the Base. (eg in an audio amplifier)

As outlined in the downloadable booklet in the section on the 'Bi-polar' transistor, in order to turn on the emitter-collector pathway, a forward bias of at least 0.6 volts is needed between the emitter and base. In a PNP type, this means the base must be at least 0.6 volts 'more negative' than the emitter and in a PNP type, the base must be at least 0.6 volts 'more positive'.

The actual base current is chosen in order to provide a 'linear' response to the applied signal. (i.e. the amplified signal is proportional to the changing base current).

Biasing Transistors:
To keep the transistor operating within this useful range, resistors are used to establish a predetermined potential difference between emitter and base and base and collector. These resistors are called 'bias' resistors. Sometimes a resistor is also used in the base circuit to limit the flow of current into the base. There are a variety of ways to provide the correct 'bias' to a transistor, one of the most common is presented below:

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Resistors R1 and R2 form a 'voltage divider' which establishes correct bias and ensures a 'linear' response.

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A Transistor using the small current available from a computer Printer Port to control a Relay

General:
Transistors come in all shapes and sizes. Generally they have three 'legs', but not all three-legged components are transistors. The size of a transistor is usually determined by the amount of current they are required to handle. Large-current transistors are physically large and often have enhanced cooling features such a metal case. For our purposes we are only interested in small current transistors such as the BC547/338 types used in the interfaces described on this site to switch LEDs and relays.