The bipolar transistor

Chapter The bipolar transistor

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • 22CHAPTERThe bipolar transistorTHE WORD TRANSISTOR IS A CONTRACTION OF “CURRENT-TRANSFERRING RESIS-tor. “ This is an excellent description of what a bipolar transistor does.Bipolar transistors have two P-N junctions connected together. This is done in ei-ther of two ways: a P-type layer sandwiched between two N-type layers, or an N typelayer between two P-type layers.Bipolar transistors, like diodes, can be made from various semiconductor sub-stances. Silicon is probably the most common material used.NPN versus PNPA simplified drawing of an NPN transistor, and its schematic symbol, are shown in Fig.22-1. The P-type, or center, layer is called the base. The thinner of the N-type semi-conductors is the emitter, and the thicker is the collector. Sometimes these are labeledB, E, and C in schematic diagrams, although the transistor symbol alone is enough totell you which is which.A PNP bipolar transistor is just the opposite of an NPN device, having two P-typelayers, one on either side of a thin, N-type layer (Fig. 22-2). The emitter layer is thinner,in most units, than the collector layer.You can always tell whether a bipolar transistor in a diagram is NPN or PNP. Withthe NPN, the arrow points outward; with the PNP it points inward. The arrow is alwaysat the emitter.Generally, PNP and NPN transistors can do the same things in electronic circuits.The only difference is the polarities of the voltages, and the directions of the currents.In most applications, an NPN device can be replaced with a PNP device or vice versa,and the power-supply polarity reversed, and the circuit will still work as long as the newdevice has the appropriate specifications.400Copyright © 2002, 1997, 1993 by The McGraw-Hill Companies. Click here for terms of use.