chapter 22 bipolar transistors. 2 objectives –after completing this chapter, the student should be...
TRANSCRIPT
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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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• 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.