Page 87 - Understanding Automotive Electronics

2735 | CH 3 Page 74 Tuesday, March 10, 1998 11:03 AM

3 ELECTRONICS FUNDAMENTALS

Transistors

Transistors are useful as Diodes are static circuit elements; that is, they do not have gain or store
amplifying devices. energy. Transistors are active elements because they can amplify or transform a
signal level. Transistors are three-terminal circuit elements that act like current
valves. There are two common bipolar (i.e., consisting of n-type and p-type
semiconductors) types. Figure 3.2a shows the schematic symbol for an NPN
transistor, and Figure 3.2b shows the schematic symbol for a PNP transistor. P
represents p-type semiconductor material; N represents n-type material. The
area where the p-type and n-type materials join is called a PN junction (or
simply a junction). Current flows into the base and collector of an NPN
transistor and out of the emitter. The currents in a PNP transistor flow exactly
opposite to those in the NPN transistor; that is, current flows into the emitter
and out of the base and collector. In fact, this is the only difference between the

Figure 3.2
Transistor
Schematic Symbols

FPO

74 UNDERSTANDING AUTOMOTIVE ELECTRONICS