Chapter 22 Bipolar Transistors

2 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.

3 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.

4 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

5 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.

6

7

8 –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.

9 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.

10 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.

11 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.

12 –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.

13 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.

14 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.

15 –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.

16 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.

17 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?

18 Maximum power dissipation? Current gain? Case style? Lead configuration?

19 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.

20 –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

21 –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).

22 –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.

23 –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.