how transistors work

 This easy-to-read guide will teach you how transistors work so that you can use them in your next circuit.


 

How Transistors Work (BJT)
Let’s start with the classic NPN transistor. It’s a Bipolar Junction Transistor (BJT) and has three legs:

Base (b)
Collector (c)
Emitter (e)


If you turn it ON, current can flow through it from the collector to the emitter. When it’s OFF, no current can flow.

In the example circuit below, the transistor is OFF. That means no current can flow through it, so the Light-Emitting Diode (LED) is also off.

To turn the transistor ON, you need a voltage of about 0.7V between the base and the emitter.

If you had a 0.7V battery, you could have connected it between the base and emitter, and the transistor would have turned ON.

Since most of us don’t have a 0.7V battery, how do we turn on the transistor?

Easy! The base-to-emitter part of a transistor works like a diode. A diode has a forward voltage that it “grabs” from the available voltage. If you add a resistor in series, the rest of the voltage drops across the resistor.

So you’ll automatically get around 0.7V by adding a resistor.

This is the same principle you use to limit the current through an LED to make sure it doesn’t blow up.

If you also add a pushbutton, you can control the transistor, and thereby the LED, ON and OFF with a button:

How a MOSFET Transistor Works
The MOSFET transistor is another very common type of transistor. It also has three pins:

Gate (g)
Source (s)
Drain (d)


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