Complete Electronics Self-Teaching Guide with Projects
Page 6
Expected Results
Figure 2.34 shows the breadboarded circuit for this project.
Figure 2.34
Figure 2.35 shows the test setup for this project.
Figure 2.35
Compare your measurements with the ones shown in the following table. You should see a similar trend in the measured values, but not exactly the same values.
As you can see in this data, even though the supply voltage dropped by approximately 15 percent, the lamp current stayed roughly constant, showing less than a 1 percent drop.
Summary
Semiconductor diodes are used extensively in modern electronic circuits. Following are the main advantages of semiconductor diodes:
They are small.
They are rugged and reliable if properly used. You must remember that excessive reverse voltage or excessive forward current could damage or destroy the diode.
Diodes are easy to use because there are only two connections to make.
They are inexpensive.
They can be used in all types of electronic circuits, from simple DC controls to radio and TV circuits.
They can be made to handle a wide range of voltage and power requirements.
Specialized diodes (which have not been covered here) can perform particular functions, which no other components can handle.
Finally, as you see in Chapter 3, “Introduction to the Transistor,” diodes are an integral part of transistors.
All the many uses of semiconductor diodes are based on the fact they conduct in one direction only. Diodes are often used for the following:
Protecting circuit components from voltage spikes
Converting AC to DC
Protecting sensitive components from high-voltage spikes
Building high-speed switches
Rectifying radio frequency signals
Self-Test
The following questions test your understanding of this chapter. Use a separate sheet of paper for your diagrams or calculations. Compare your answers with the answers that follow the test.
1. Draw the circuit symbol for a diode, labeling each terminal. _____
2. What semiconductor materials are used in the manufacture of diodes? _____
3. Draw a circuit with a battery, a resistor, and a forward-biased diode. _____
4. What is the current through a reverse-biased perfect diode? _____
5. Draw a typical V-I curve of a forward-biased diode. Show the knee voltage. _____
6. What is the knee voltage for silicon? _____
Germanium? _____
7. In the circuit shown in Figure 2.36, VS = 10 volts and R = 100 ohms. Find the current through the diode, assuming a perfect diode. _____
Figure 2.36
8. Calculate question 7 using these values: VS = 3 volts and R = 1 kΩ. _____
9. In the circuit shown in Figure 2.37, find the current through the diode.
VS = 10 volts
R1 = 10 kΩ
R2 = 1 kΩ
_____
Figure 2.37
10. In the circuit shown in Figure 2.38, find the current through the zener diode.
VS = 20 volts
VZ = 10 volts
R1 = 1 kΩ
R2 = 2 kΩ
_____
Figure 2.38
11. If the supply voltage for question 10 increases to 45 volts, what is the current in the zener diode? _____
12. What is the maximum power dissipated for the diode in questions 10 and 11? _____
Answers to Self-Test
If your answers do not agree with those given here, review the problems indicated in parentheses before you go to Chapter 3, “Introduction to the Transistor.”
1. See Figure 2.39.
Figure 2.39
(problem 3)
2. Germanium and silicon. (problem 1)
3. See Figure 2.40.
Figure 2.40
(problem 4)
4. There is zero current flowing through the diode. (problem 6)
5. See Figure 2.41.
Figure 2.41
(Project 2.1)
6. Si = 0.7 volt; Ge = 0.3 volt (These are approximate.) (Project 2.1)
7. ID = 100 mA. (problem 12)
8. As VS = 3 volt, do not ignore the voltage drop across the diode. Thus, ID = 2.7 mA. (problem 12)
9. Ignore VD in this case. Thus, ID = 0.3 mA. If VD is not ignored, ID = 0.23 mA. (problem 19)
10. IZ = 5 mA. (problem 29)
11. IZ = 30 mA. (problem 29)
12. The maximum power will be dissipated when IZ is at its peak value of 30 mA. Therefore, PZ(MAX) = 0.30 watt. (problem 31)
Chapter 3
Introduction to the Transistor
The transistor is undoubtedly the most important modern electronic component because it has enabled great and profound changes in electronics and in your daily lives since its discovery in 1948.
This chapter introduces the transistor as an electronic component that acts similarly to a simple mechanical switch, and it is actually used as a switch in many modern electronic devices. A transistor can be made to conduct or not conduct an electric current—exactly what a mechanical switch does.
Most transistors used in electronic circuits are bipolar junction transistors (BJTs), commonly referred to as bipolar transistors junction field effect transistors (JFETs) or metal oxide silicon field effect transistors (MOSFETs). This chapter (along with Chapter 4, “The Transistor Switch,” and Chapter 8, “Transistor Amplifiers”) illustrates how BJTs and JFETs function and how they are used in electronic circuits. Because JFETs and MOSFETs function in similar fashion, MOSFETs are not covered here.
Projects in this chapter can help you to build a simple one-transistor circuit. You can easily set up this circuit on a home workbench. You should take the time to obtain the few components required, and actually build and operate the circuit.
In Chapter 4, you continue to study transistor circuits and the operation of the transistor as a switch. In Chapter 8, you learn how a transistor can be made to operate as an amplifier. In this mode, the transistor produces an output that is a magnified version of an input signal, which is useful because many electronic signals require amplification. These chapters taken together present an easy introduction to how transistors function, and how they are used in basic electronic circuits.
When you complete this chapter, you can do the following:
Describe the basic construction of a BJT.
Describe the basic construction of a JFET.
Specify the relationship between base and collector current in a BJT.
Specify the relationship between gate voltage and drain current in a JFET.
Calculate the current gain for a BJT.
Compare the transistor to a simple mechanical switch.
Understanding Transistors
1 The diagrams in Figure 3.1 show some common transistor cases (also called packages). The cases protect the semiconductor chip on which the transistor is built and provide leads that can be used to connect it to other components. For each transistor, the diagrams show the lead designations and how to identify them based on the package design.
Transistors can be soldered directly into a circuit, inserted into sockets, or inserted into breadboards. When soldering, you must take great care because transistors can be destroyed if overheated. A heat sink clipped to the transistor leads between the solder joint and the transistor case can reduce the possibility of overheating. If you use a socket, you can avoid exposing the transistor to heat by soldering the connections to the socket before inserting the transistor.
Figure 3.1
Questions
A. How many leads are there on most transistors? _____
B. Where there are only two leads, what takes the place of the third lead? _____
C. What are the three leads or connections called? _____
D. Why should you take care when soldering transistors into a circuit? _____
Answers
/>
A. Three.
B. The case can be used instead, as indicated in the diagram on the right side of Figure 3.1. (This type of case is used for power transistors.)
C. Emitter, base, and collector.
D. Excessive heat can damage a transistor.
2 You can think of a bipolar junction transistor as functioning like two diodes, connected back-to-back, as illustrated in Figure 3.2.
Figure 3.2
However, in the construction process, one important modification is made. Instead of two separate P regions, as shown in Figure 3.2, only one thin region is used, as shown in Figure 3.3.
Figure 3.3
Question
Which has the thicker P region, the transistor shown in Figure 3.3 or two diodes connected back-to-back? _____
Answer
Two diodes. The transistor has a thin P region.
3 Because two separate diodes wired back-to-back share two thick P regions, they cannot behave like a transistor.
Question
Why don't two diodes connected back-to-back function like a transistor? _____
Answer
The transistor has one thin P region, whereas the diodes share two thick P regions.
4 The three terminals of a transistor (the base, the emitter, and the collector) connect, as shown in Figure 3.4.
Figure 3.4
When talking about a transistor as two diodes, you refer to the diodes as the base-emitter diode and the base-collector diode.
Figure 3.5 shows the symbol used in circuit diagrams for the transistor, with the two diodes and the junctions shown for comparison. Because of the way the semiconductor materials are arranged, this is known as an NPN transistor.
Figure 3.5
Question
Which transistor terminal includes an arrowhead? _____
Answer
The emitter
5 It is also possible to make transistors with a PNP configuration, as shown in Figure 3.6.
Figure 3.6
Both NPN and PNP type transistors can be made from either silicon or germanium.
Questions
A. Draw a circuit symbol for both an NPN and a PNP transistor. (Use a separate sheet of paper for your drawings.)
B. Which of the transistors represented by these symbols might be silicon? _____
C. Are silicon and germanium ever combined in a transistor? _____
Answers
A. See Figure 3.7.
B. Either or both could be silicon. (Either or both could also be germanium.)
C. Silicon and germanium are not mixed in any commercially available transistors. However, researchers are attempting to develop ultra-fast transistors that contain both silicon and germanium.
Figure 3.7
6 Take a look at the simple examples using NPN transistors in this and the next few problems.
If a battery is connected to an NPN transistor, as shown in Figure 3.8, then a current will flow in the direction shown.
Figure 3.8
The current flowing through the base-emitter diode is called base current and is represented by the symbol IB.
Question
Would base current flow if the battery were reversed? Give a reason for your answer. _____
Answer
Base current would not flow because the diode would be back-biased.
7 In the circuit shown in Figure 3.9, you can calculate the base current using the techniques covered in Chapter 2, “The Diode.”
Figure 3.9
Question
Find the base current in the circuit shown in Figure 3.9. (Hint: Do not ignore the 0.7-volt drop across the base-emitter diode.)
IB = _____
Answer
Your calculations should look something like this:
8 For the circuit shown in Figure 3.10, because the 10 volts supplied by the battery is much greater than the 0.7-volt diode drop, you can consider the base-emitter diode to be a perfect diode and thus assume the voltage drop is 0 volts.
Figure 3.10
Question
Calculate the base current.
IB = _____
Answer
9 Look at the circuit shown in Figure 3.11.
Figure 3.11
Question
Will current flow in this circuit? Why or why not? _____
Answer
It cannot flow because the base-collector diode is reverse-biased.
10 Examine the circuit shown in Figure 3.12. Batteries are connected to both the base and collector portions of the circuit.
Figure 3.12
When you connect batteries to both the base and the collector portions of the circuit, currents flowing through the circuit demonstrate a key characteristic of the transistor. This characteristic is sometimes called transistor action—if base current flows in a transistor, collector current will also flow.
Examine the current paths shown in Figure 3.13.
Figure 3.13
Questions
A. What current flows through the base-collector diode? _____
B. What current flows through the base-emitter diode? _____
C. Which of these currents causes the other to flow? _____
Answers
A. IC (the collector current).
B. IB and IC. Both of them flow through the base-emitter diode.
C. Base current causes collector current to flow.
No current flows along the path shown by the dotted line in Figure 3.14 from the collector to the base.
Figure 3.14
11 Up to now, you have studied the NPN bipolar transistor. PNP bipolar transistors work in the same way as NPN bipolar transistors and can also be used in these circuits.
There is, however, one important circuit difference, which is illustrated in Figure 3.15. The PNP transistor is made with the diodes oriented in the reverse direction from the NPN transistor.
Figure 3.15
Questions
Compare Figure 3.15 with Figure 3.13. How are the circuits different relative to the following?
A. Battery connections: _____
B. Current flow: _____
Answers
A. The battery is reversed in polarity.
B. The currents flow in the opposite direction.
12 Figure 3.16 shows the battery connections necessary to produce both base current and collector current in a circuit that uses a PNP transistor.
Figure 3.16
Question
In which direction do these currents circulate—clockwise or counterclockwise? _____
Answer
Base current flows counterclockwise.
Collector current flows clockwise.
As stated earlier, NPN and PNP bipolar transistors work in much the same way: Base current causes collector current to flow in both. The only significant difference in using a PNP versus an NPN bipolar transistor is that the polarity of the supply voltage (for both the base and collector sections of the circuit) is reversed. To avoid confusion, bipolar transistors used throughout the rest of this book are NPNs.
13 Consider the circuit shown in Figure 3.17. It uses only one battery to supply voltage to both the base and the collector portions of the circuit. The path of the base current is shown in the diagram.
Figure 3.17
Questions
A. Name the components through which the base current flows. _____
B. Into which terminal of the transistor does the base current flow? _____
C. Out of which transistor terminal does the base current flow? _____
D. Through which terminals of the transistor does base current not flow? _____
Answers
A. The battery, the resistor RB, and the transistor
B. Base
C. Emitter
D. Collector
14 Take a moment to recall the key physical characteristic of the transistor.
Question
When base current flows in the circuit shown in Figure 3.17, what other
current can flow, and which components will it flow through? _____
Answer
Collector current will flow through the resistor RC and the transistor.
15 In Figure 3.18 the arrows indicate the path of the collector current through the circuit.
Figure 3.18
Questions
A. List the components through which the collector current flows. _____
B. What causes the collector current to flow? _____
Answers
A. The resistor RC, the transistor, and the battery.
B. Base current. (Collector current doesn't flow unless base current is flowing.)
16 It is a property of the transistor that the ratio of collector current to base current is constant. The collector current is always much larger than the base current. The ratio of the two currents is called the current gain of the transistor, and is represented by the symbol β, or beta. Typical values of β range from 10 to 300.
Questions
A. What is the ratio of collector current to base current called? _____
B. What is the symbol used to represent this? _____
C. Which is larger—base or collector current? _____
D. Look back at the circuit in problem 13. Will current be greater in RB or in RC? _____
Answers
A. Current gain.
B. β.
C. Collector current is larger.
D. The current is greater in RC because it is the collector current.