x Th e instructional design of the text contains student-focused learning tools to help you succeed. Th e various features designed for the textbook are highlighted as follows. Features of the Textbook 336 Vacuum tubes led the way to the development of many electronic devices such as the radio receiver, radio transmitter, television, radar, and the fi rst computers. Vacuum tube applications have been replaced by the transistor. Transistor circuits can operate more quickly, are more sensitive to small currents, and allow for much cheaper and more compact designs. We will explore the transistor and the operation of common amplifi er circuits, as well as two common thyristors. Chapter 20 Transistors and Amplifi ers OBJECTIVES After studying this chapter, you will be able to: ■ Explain the operation of the bipolar transistor. ■ Explain the operation of fi eld- effect transistors. ■ Discuss different biasing techniques. ■ Identify various transistor circuit confi gurations. ■ List the components of amplifi er circuits and give the function for each component. ■ Explain amplifi er operation. ■ Compute the gain of amplifi er circuits. ■ Perform DC load line analysis on a transistor circuit. ■ Discuss the advantages and disadvantages of various methods of amplifi er coupling. ■ Describe several common thyristors. KEY TERMS amplifi er base biasing bipolar junction transistor (BJT) blocking capacitor cascading Class A amplifi er Class B amplifi er Class C amplifi er collector collector junction common base (CB) common collector (CC) common emitter (CE) cutoff decibel (dB) degeneration emitter emitter biasing emitter junction fi eld-effect transistor (FET) gain heat sink junction fi eld-effect transistor (JFET) load line analysis metal oxide semiconductor fi eld-effect transistor (MOSFET) NPN transistor PNP transistor Q-point saturation point self bias silicon controlled rectifi er (SCR) thermal runaway thyristor triac Chapter 20 Transistors and Amplifi ers 341 P-channel will exist. If a large potential is applied to the gate, a small current in the channel between the source the Notice how the potential applied at the gate pinches off the fl ow of electrical energy. is It is used extensively in digital circuits and memory circuits in computers. Look at Figure 20-9. Th e MOSFET is similar in construction to the JFET. Th e diff erence is that the MOSFET has a very thin fi (silicon dioxide) between the gate and the channel area. C A U T I O N The layer of insulation is so thin that it can easily be damaged by static electricity. Care must be taken when handling MOSFET transistors and devices. Th e thin layer of high resistance insulation prevents electron fl ow between the gate and the channel material. Th e resistance between the gate and channel area makes for a very high impedance input device. A high impedance input device is very desirable in circuits such as amplifi ers. Also, the channel consisting of the same material from the source to the drain a low low low through the channel is also very desirable for devices such as amplifi ers. Th ere are two main types of MOSFET, enhancement mode and depletion mode. In a depletion-mode through the source is by gate voltage. In an enhancement-mode MOSFET, current through the source drain circuit is increased by the gate voltage. Look closely at the four symbols used for the MOSFET, Figure 20-9. Th e symbols diff er for enhancement-mode and depletion-mode MOSFETs. SECTION 20.1 REVIEW Answer the following questions using information in this section. 1. True or False? A bipolar junction transistor has three junctions. 2. A PNP transistor has a(n) _____ material for the base. A. P-type B. N-type C. Both A and B. D. Neither A nor B. 3. True or False? A MOSFET is easily damaged by static electricity. 4. True or False? In enhancement mode, current through the source drain circuit is reduced by gate voltage.ge. 20.2 TRANSISTORS AS AMPLIFIERS An amplifi er is an electronic circuit that uses a small input signal to control a signal. In physics, weight can be amplifi ed using levers. A small lever is used to lift a larger weight. In electronics, amplifi early twentieth century. Amplifi cation can be using semiconductor devices such as transistors or integrated circuits. P-channel depletion-mode MOSFET P-channel enhancement-mode MOSFET N-channel enhancement-mode MOSFET N-channel depletion-mode MOSFET S G D S G D P Drain Gate Source S G D S G D Silicon dioxide layer Channel N Drain Gate Source Goodheart-Willcox Publisher Figure 20-9. The MOSFET is similar in construction to the JFET. Copyright Goodheart-Willcox Co., Inc. CHAPTER OPENING MATERIALS Each chapter opening provides a preview of topics to come. Objectives clearly identify the knowledge and skills to be gained while reading the content. Next, Key Terms list signifi cant vocabulary to be learned in the chapter. Finally, an introduction presents a brief synopsis of the topics to be covered, providing you with a framework on which to build your learning. SPECIAL FEATURES Special features add interest and extend learning and knowledge. Safety features alert you to potentially dangerous materials and practices. Cautions alert you to practices that could potentially damage equipment or instruments. Examples demonstrate the mathematical concept just presented, showing the math skills required to solve real-world problems. Section Reviews provide opportunities to review what you have learned before moving on to additional content. y LIF RS uit ght c ng rs. s le fi ectr have fiers accomplished s. Chapter 6 Energy 97 Example 6-1 A has an applied voltage of 100 volts. Th e mea- 2 much power is consumed? P = I × E = 2 A × 100 V = 200 W Example 6-2 An electric toaster, rated at 550 watts, is connected to a 117 volt source. How much current will this appliance use? I = P E = 550 W 117 V = 4.7 A 6.1.2 Ohm’s Law and Watt’s Law Ohm’s law and Watt’s law can be combined to produce simple formulas that permit you to solve for current, voltage, resistance, or power if any two of those quantities are known. In Figure 6-3, these equations are listed with an explanation of how they were created from the two laws. Th ese formulas can be arranged in a wheel-shaped memory device for ready reference. Refer to Figure 6-4. To use this device, fi nd your unknown quantity in the smaller center circle. Th is is the left half of your equation. Next, choose one of the three variable combinations in the outer circle that falls in the same quarter of the chart as your unknown quantity. Choose the variable combination that uses two known quantities in your circuit. Th is becomes the right half of your equation. S A F E T Y An electric current produces heat when it passes through a resistance. Heaters and resistors will remain hot for some time after the power is removed. Handle them carefully. Burns should be treated immediately. See your instructor. 1. E = I × R Ohm’s law 2. E = Watt’s law 3. E = By transposing equation 12 and taking the square root. 4. I = Ohm’s law 5. I = Watt’s law 6. I = By transposing equation 9 and taking the square root. 7. R = Ohm’s law 8. R = By transposing equation 12. 9. R = By transposing equation 11. 10. P = I × E Watt’s law 11. P = I2 × R By substituting I × R from equation 1, for E. 12. P = By substituting from equation 4, for I. P I E R E R P E P I2 E I E2 R E2 P P R PR Goodheart-Willcox Publisher Figure 6-3. This table states a variety of basic formulas that are needed to solve problems. P R P E E R P I P I2 E2 P E I E2 R E I I2R IR PR P I ER Goodheart-Willcox Publisher Figure 6-4. A memory device combining Ohm’s law and the power formulas. It can be a great help when solving problems. Copyright Goodheart-Willcox Co., Inc.