Complete Electronics Self-Teaching Guide with Projects

Summary

At this point, it's useful to compare the properties of a mechanical switch with the properties of both types of transistors, as summarized in the following table.

Switch	BJT	JFET
OFF (or open)
No current.	No collector current.	No drain current.
Full voltage across terminals.	Full supply voltage between collector and emitter.	Full supply voltage between drain and source.
ON (or closed)
Full current.	Full circuit current.	Full circuit current.
No voltage across terminals.	Collector to emitter voltage is 0 volts.	Drain to source voltage is 0 volts.

The terms ON and OFF are used in digital electronics to describe the two transistor conditions you just encountered. Their similarity to a mechanical switch is useful in many electronic circuits.

In Chapter 4 you learn about the transistor switch in more detail. This is the first step toward an understanding of digital electronics. In Chapter 8 you examine the operation of the transistor when it is biased at a point falling between the two conditions, ON and OFF. In this mode, the transistor can be viewed as a variable resistance and used as an amplifier.