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. 2 • 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. 3 • 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 4 • 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. 5 6 7 – 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. 8 • 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. 9 • 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. 10 • 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. 11 – 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. 12 • 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. 13 • 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. 14 – 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. 15 • 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. 16 • 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? 17 • • • • Maximum power dissipation? Current gain? Case style? Lead configuration? 18 • 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. 19 – 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 20 – 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). 21 – 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. 22 – 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. 23