Chapter 22

Bipolar Transistors

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• Objectives– After completing this chapter, the student

should be able to:• Describe how a transistor is constructed and

describe its two different configurations.• Draw and label the schematic symbol for an NPN

and a PNP transistor.• Identify the ways of classifying transistors.

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• Identify the function of a transistor using a reference manual and the identification number (2NXXXX).

• Identify commonly used transistor packages.

• Describe how to bias a transistor for operation.

• Explain how to test a transistor with both a transistor tester and an ohmmeter.

• Describe the process used for substituting a transistor.

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• Transistor construction– A bipolar transistor is produced when a third

layer is added to a semiconductor.– It can amplify power, current, or voltage.– Also called a junction transistor or– Transistor

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• A transistor:– Can be constructed of germanium or silicon.– Silicon is more popular.– Consists of three alternately doped regions.– The regions are arranged two ways.

• P-type material is sandwiched between two N-type materials, NPN transistor.

• N-type material is sandwiched between two P-type materials, PNP transistor.

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– Transistors are classified according to:• type (either NPN or PNP).

• material used (germanium or silicon).

• major use (high or low power, switching, or high frequency).

– Transistors are identified by a number.• Begins with 2N and up to four more digits.

• Identifies the device as a transistor.

• Indicates that it has two junctions.

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• The package of the transmitter:– Serves as protection.– Provides a means of making electrical

connections to the emitter, base, and collector.– Serves as a heat sink, removing excess heat

from the transistor.– Designed by size and configuration.

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• Basic functions of a transistor are:– to provide current amplification of a signal.– to switch a signal.

• A transistor must be properly biased.– The emitter junction is forward biased.– The collector junction is reverse biased.

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• The differences between NPN and PNP transistors:– The batteries have opposite polarities.– The direction of the electron flow is reversed.

• In a transistor, the barrier voltage is:– produced across the emitter junction.– determined by the type of semiconductor

material used.

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– the barrier voltage for a germanium transistor is .3.

– the barrier voltage for a silicon transistor is .7.

• The reverse-biased voltage applied to the collector-base junction is usually much higher than the forward-biased voltage across the emitter-base junction.

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• If a transistor fails:– It is generally caused by high temperature,

high current, or high voltage.– Failure can also be caused by extreme

mechanical stress.

• The results:– A transistor may open or short.– A transistor’s characteristics may alter enough

to affect its operation.

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• Two methods to determine functionality:– Use an ohmmeter.– Use a transistor tester.

• To use an ohmmeter to test a transistor:– Resistance tests are made between two

junctions in the following way:• emitter to base.• collector to base.• collector to emitter.

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– Connect any two terminals one way.

– Then reverse the leads.

– In one connection, the resistance should be high, 10,000 ohms or more.

– In the other connection, the resistance should be lower, less than 10,000 ohms.

• If a transistor fails this test, it is defective.• If a transistor passes this test, it can still be

defective.

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• A transistor tester is more reliable than an ohmmeter.– Designed specifically for testing transistors.– Two types:

• An in-circuit tester.

• An out-of-circuit tester.

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• Transistor substitution– Procedure for determining accurate replacement

selection of a transistor.• NPN or PNP?

• Germanium or silicon?

• Operating frequency range?

• Operating voltage?

• Collector current requirements?

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• Maximum power dissipation?

• Current gain?

• Case style?

• Lead configuration?

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• In Summary– A transistor:

• is a three-layer device used to amplify and switch power and voltage.

• is also called a junction transistor or bipolar transistor.

• Can be configured as NPN or PNP.– Middle region is called the base.

– Outer regions are called the emitter and collector.

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– The schematic symbols for NPN and PNP transistors are:

– Transistors are classified according to whether:• NPN or PNP• silicon or germanium• high power or low power• switching or high frequency

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– Transistors are identified with a prefix of 2N and up to four other digits.

– The transistor package provides:• protection.

• a heat sink.

• a support for leads.

– Transistor packages identified with the letters TO (transistor outline).

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– In a properly biased transistor:• the emitter-base junction is forward biased.

• the collector-base junction is reverse biased.

– PNP bias sources are the reverse of NPN bias sources.

– Internal barrier voltage for:• germanium transistors is .3 volt.

• silicon transistors is .7 volt.

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– When testing with an ohmmeter:• Each junction exhibits a low resistance when it is

forward biased.

• Each junction exhibits a high resistance when it is reverse biased.

– Two types of transistor testers:• in-circuit.

• out-of-circuit.