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Intro to Transistors Page 1

Introduction to Transistors

So far we have covered devices that have only two legs: resistors, capacitors, batteries, LED's and switches. With the transistor we introduce the first 3 legged device. Transistors come in many varieties, shapes and sizes. For the most part, they all operate the same, with some subtle differences.

Some of the many types of transistors include Bipolar Junction Transistors (BJT) and Field Effect Transistors (FET). This guide will use FET transistors because they are easier to comprehend and generally more useful. Just about everything you learn here will apply to BJT transistors as well.

Schematic Symbol

Here is the schematic symbol for a transistor, and some examples of what they look like.

Field Effect Transistor (FET)
Real Life Transistors

Here is an FET schematic next to some various transistors.

Notice that the three legs on the schematic are labeled G, S and D. These stand for Gate, Source and Drain. We will talk about these in detail in just a minute.


One of the hard parts about using a transistor is knowing which of the three legs in real life is the corresponding leg in the schematic symbol. In the picture above, there are 3 different transistor packages. The package on the left is called a TO-92, the package in the middle is called a TO-220, and the package on the right is called a metal can. Metal can packages are almost never used anymore. TO-92 packages are very common for small signals, while TO-220 packages are common for larger things like motors and speaker drivers. Here we will show you the most common pin mappings for TO-92 and TO-220 FET transistors.

TO-92 FET Packages
TO-220 FET Packages

These are the most common pin to schematic mappings.
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Jason Bauer

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Jason Bauer is an owner and programmer for Portforward.com. He's allergic to twitter and facebook, but you can find more of his articles in the Guides section.
Sunday, 18-Feb-2018 16:15:19 PST