The Transistor as a Switch

Transistors can be thought of as electronic switches. A transistor is used to turn various devices such as motors, lights and speakers on and off. Just like the light switch in a room, a transistor can turn a light on and off. This is useful so that a small voltage source can be used to turn on and off a large voltage source. Lets look at a simple example using a light bulb.

Here we have an FET transistor hooked up to two different batteries and a light bulb. Lets look at the left half of the circuit first.

• The tiny batteries negative terminal is connected to the Source of the transistor.
• The tiny batteries positive terminal is connected to the Gate of the transistor.

In this configuration, the transistor is said to be On. You can see that a tiny current is flowing through the transistor from Gate to Source. Now lets look at the right half of the schematic.

• The large batteries negative terminal is connected to the Source of the transistor.
• The large batteries positive terminal is connected to one leg of the light bulb.
• The other leg of the light bulb is connected to the transistors Drain.

Since this transistor is On (from above) then a large current is flowing through the light bulb, and through the transistor from Drain to Source. If you disconnect the tiny battery, then the transistor will be off, and the light bulb will be off as well.

Here you can see that since we have disconnected the tiny battery, the FET is no longer on, so the light bulb is no longer on. Notice that the transistor is acting as a switch turning the light bulb on and off in response to the voltage being applied at the gate. This is where the gate gets its name. It is used to gate, or control the flow of current through the FET.

This circuit as it stands is not particularly useful. However, when we replace the tiny battery with other sources of voltage then the transistor switch becomes much more interesting.