Transistor Crash Course

Today we will run through a brief overview of the two main variations of transistors commonly used in circuits, NPN and PNP transistors.

It is critical to understand the basic functioning of these transistors to understand any circuit of moderate complexity.

The first thing to note is that the basic function of transistors is to control the flow of current through a circuit.

Both the NPN and PNP transistors have 3 leads or legs, the Base, Emitter and Collector. The main difference between NPN and PNP transistors is how they control the flow of current.

If we imagine the current flowing through a circuit being water flowing through a pipe, a transistor would act like a faucet or tap. It can control the water going through it by opening or closing its valve.

And this is where the NPN and PNP transistors differ. A NPN transistor is closed by default, not allowing current to flow from its Emitter leg to its Collector leg, and will only allow current through when it is “opened” by applying a small amount of current to its Base leg, whereas the PNP transistor is open by default, allowing current to flow from its Emitter leg to its Collector leg, and applying current to its Base leg “closes” it thus stopping the flow of current between the Emitter and Collector.

Below are the schematic symbols for the 2 transistors:


Where the numbers represent:

1 – Base
2 – Emitter
3 – Collector

On an unrelated topic, I use open source software called Fritzing for all my electronic schematics and drawings. It is a great piece of software and can be downloaded from

Transistor Crash Course

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