Transistors: Masters of Switching and Amplifying

By Cathleen Shamieh

Transistors basically do just two things in electronic circuits: switch and amplify. But those two jobs are the key to getting really interesting things done. Here’s why those functions are so important:

  • Switching: If you can switch electron flow on and off, you have control over the flow, and you can build involved circuits by incorporating lots of switches in the right places.

    Consider, for instance, the telephone-switching system: By dialing a 10-digit number, you can connect with any one of millions of people around the world. Or look at the Internet: Switching enables you to access a website hosted in, say, Sheboygan while you’re sitting on a train in, say, London. Other systems that rely on switching are computers, traffic lights, the electric power grid — well, you get the idea. Switching is pretty darn important.

  • Amplifying: If you can amplify an electrical signal, you can store and transmit tiny signals and boost them when you need them to make something happen.

    For instance, radio waves carry tiny audio signals over long distances, and it’s up to the amplifier in your stereo system to magnify the signal so it can move the diaphragm of a speaker so you can hear the sound.

Before the invention of the transistor, vacuum tubes did all the switching and amplifying. In fact, the vacuum tube was widely regarded as the greatest marvel in electricity in the early 20th century. Then, Bardeen, Brattain, and Shockley showed the world that tiny semiconductor transistors could do the same job — only better (and for less money). The trio was awarded the 1956 Nobel Prize in Physics for their invention of the transistor.

Transistors these days are microscopically small, have no moving parts, are reliable, and dissipate a heck of a lot less power than their vacuum-tube predecessors. (However, many audiophiles believe tubes offer superior sound quality compared to solid-state transistor technology.)

The two most common types of transistors are

  • Bipolar junction transistors

  • Field-effect transistors

The figure shows the circuit symbols commonly used for various types of transistors. The sections that follow provide a closer look at bipolar transistors and field-effect transistors.

Circuit symbols for bipolar junction transistors and field-effect transistors, with labeled leads.

Circuit symbols for bipolar junction transistors and field-effect transistors, with labeled leads.