Chapter 22

Chapter 22
Bipolar Transistors
• 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.
• 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 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
• 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.
– Transistors are classified according to:
• type (either NPN or PNP).
• material used (germanium or silicon).
• major use (high or low power, switching, or high
– 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.
• 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.
• 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.
• The differences between NPN and PNP
– 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.
– the barrier voltage for a germanium transistor is
– 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.
• 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.
• 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.
– 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
• A transistor tester is more reliable than an
– Designed specifically for testing transistors.
– Two types:
• An in-circuit tester.
• An out-of-circuit tester.
• Transistor substitution
– Procedure for determining accurate replacement
selection of a transistor.
Germanium or silicon?
Operating frequency range?
Operating voltage?
Collector current requirements?
Maximum power dissipation?
Current gain?
Case style?
Lead configuration?
• 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
• Can be configured as NPN or PNP.
– Middle region is called the base.
– Outer regions are called the emitter and collector.
– The schematic symbols for NPN and PNP
transistors are:
– Transistors are classified according to whether:
silicon or germanium
high power or low power
switching or high frequency
– 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).
– 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
– Internal barrier voltage for:
• germanium transistors is .3 volt.
• silicon transistors is .7 volt.
– 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.