The Transistor

A transistor is a current amplifying device used in the field of electronic engineering. It is composed of three junctions, where a small current injected to the base junction, controls a larger current flowing through the emitter-collector junctions.

The invention of the transistor replaced valve technology, and it became the main amplifying component used in electronic circuits. Its invention revolutionised the field of electronics, which led to the subsequent invention of integrated circuits (IC).


NPN Transistor Symbol

A bipolar junction transistor (BJT) is a semiconductor device consisting of three layers of semiconductor materials. Each layer connects to a terminal marked base, emitter, and collector. A small current injected at the base terminal (b) results in a large flow of current through the collector (c).

The NPN transistor symbol provides clue to how the component works. The arrow shows the conventional flow of current. The small current through the base controls a larger current through the collector junction.

In the diagram above, you can see that the base current flows to ground through the emitter junction. The amount of current flow in this loop controls the current flowing through the collector-emitter junctions.


Although many people think of transistors as amplifiers, they are also capable of behaving as switches, and therefore used extensively in digital electronics.


If the transistor was the father or modern electronics, then the relay was the mother-in-law, because before the invention of the transistor, engineers used relays to switch power to loads. Telephone networks consisted of large buildings with equipment full of relays chattering day and night.

The problem with relays is that they are noisy, bulky, power hungry and very unreliable because their contacts burn out periodically. With the use of transistors, the buildings became smaller, and quieter, and the systems were more reliable.

Compared to a valve or relay, a transistor is much smaller in size and weight. A transistor utilises semiconductor technology, which is more reliable than electromechanical technology.

A transistor is a simpler device to construct, and the manufacturing process is automated which means that these devices may be manufactured in mass and cost very little. In comparison, a relay consists of mechanical parts such as the armature and coil, which requires work force to manufacture and assemble hence it is more expensive.

Transistor switching is faster than a relay, which is why transistor oscillator circuits produce high-frequency signals.

With physical switches and relays, contact bounce is a huge issue, and requires further components to solve.


A transistor has many disadvantages though. It can only switch DC loads and not AC. They are capable of handling low current and low voltage loads.

When to use a transistor

Use a transistor when you need to switch low voltage, low current DC loads to drive a small relay coil, DC motor, LED, and piezoelectric speaker.

Transistor as a Switch

Transistor as a Switch Animation

Here is an animation showing a BC548 NPN bipolar transistor operating as a switch. In this simple circuit example, when the base switch is closed, a small amount of current flows through the 1 kΩ resistor to the base junction of the transistor. This current switches the transistor fully ON, and therefore it is saturated.

When using the transistor as a switch it is desirable to have it fully saturated because that way it will exhibit a minimum voltage drop across the collector-emitter junctions.

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Transistor as a Switch PDF