Complete Electronics Self-Teaching Guide with Projects

Chapter 4

The Transistor Switch

Transistors are everywhere. You can't avoid them as you move through your daily tasks. For example, almost all industrial controls, and even your MP3 player, stereo, and television may use transistors as switches.

In Chapter 3, “Introduction to the Transistor,” you saw how a transistor can be turned ON and OFF, similar to a mechanical switch. Computers work with Boolean algebra, which uses only two logic states—TRUE and FALSE. These two states are easily represented electronically by a transistor that is ON or OFF. Therefore, the transistor switch is used extensively in computers. In fact, the logic portions of microprocessors (the brains of computers) consist entirely of transistor switches.

This chapter introduces the transistor's simple and widespread application—switching, with emphasis on the bipolar junction transistor (BJT).

When you complete this chapter, you will be able to do the following:

• Calculate the base resistance, which turns a transistor ON and OFF.
• Explain how one transistor turns another ON and OFF.
• Calculate various currents and resistances in simple transistor switching circuits.
• Calculate various resistances and currents in switching circuits, which contain two transistors.
• Compare the switching action of a junction field effect transistor (JFET) to a BJT.