What a transistor is

A diode has two legs and one trick. A transistor has three, and the third one changes what a circuit can be: it is the first part on this site whose job is to let one current decide what another current does.

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Small in, large out

That is the whole idea, and it is worth saying in the smallest possible way:

• A trickle goes into the base.
• A much larger current is allowed through the collector.
• Both leave together at the emitter.

The trickle is doing no work. It is not pushing the big current along — the supply does that. It is holding a gate open, the way your finger on a tap is not what makes the water come out.

The ratio between the two is about 100 for the small transistors in a beginner's kit. Put 100 microamps into the base and around 10 milliamps is allowed through the collector. That number has a name, gain, and the next page is about where it comes from and how far you can trust it.

It costs 0.7 V to start

Nothing at all happens until the base is about 0.7 V above the emitter. Below that the transistor is off — not dim, not slightly on, off.

If that number looks familiar it is because it is the same 0.7 V from the diode chapter, and it is the same physics. There is a junction in there, and you have to pay to cross it.

The three legs

The symbol tells you which leg does what. It does not tell you which leg is which on the actual part, and the two transistors in most starter kits are mirror images of each other. Read the number printed on the flat face, then read that number's datasheet. This is not fussiness: a transistor fitted backwards usually still does something, which is worse than doing nothing.

Two jobs, and you only need one of them

A transistor can sit anywhere between fully off and fully on, and that is what makes it an amplifier: a small wobble on the base becomes a large wobble at the collector. Radios, microphones and guitar pedals are built out of that.

You will not be doing that. In every project on this site the transistor is either fully off or shoved fully on, because that is what a switch is and because the in-between state is where the part gets hot. Which is the last page of this chapter: a transistor as a switch.