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, you saw how a transistor can be turnedONand 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 isONorOFF. 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 andwidespread 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 will turn 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.