Complete Electronics Self-Teaching Guide with Projects
Page 11
0.4 10.7
0.6 9.8
0.8 8.9
1.0 8.1
1.3 6.8
1.5 6.0
1.8 4.9
2.0 4.1
2.3 3.1
2.5 2.5
2.7 1.9
3.0 1.1
3.3 0.5
3.5 0.2
3.7 0.1
4.0 0
Figure 4.27 is the V-I curve generated using the measurements shown in the preceding table. This graph is called the transfer curve for the JFET.
Figure 4.27
With the potentiometer set to 0 ohms (point A in Figure 4.22), the voltage from the gate to the source is zero (VGS = 0). The current that flows between the drain and source terminals of the JFET at this time is at its maximum value and is called the saturation current (IDSS).
Note One property of the saturation current is that when VGS is set at zero, and the transistor is fully ON, the current doesn't drop as long as the value of VDS is above a few volts. If you have an adjustable power supply, you can determine the value of VDS at which ID starts to drop by starting with the power supply set at 12 volts. Watch the value of ID as you lower the power supply voltage until you see ID start to decrease.
38 Refer to the transfer curve shown in Figure 4.28 to answer the following questions.
Figure 4.28
Questions
Using the transfer curve shown in Figure 4.28, answer the following:
A. With VGS = 0, what is the value of the drain current? _____
B. Why is this value called the drain saturation current? _____
C. What is the gate to source cutoff voltage for the curve shown? _____
D. Why is this called a cutoff voltage? _____
Answers
A. 12 mA on the graph.
B. The word “saturation” is used to indicate that the current is at its maximum.
C. Approximately −4.2 V on the graph.
D. It is termed a “cutoff voltage” because at this value, the drain current goes to 0 ampere.
39 Now, look at the circuit shown in Figure 4.29. Assume that the JFET has the transfer characteristic shown by the curve in problem 38.
Figure 4.29
When the gate is connected to ground, the drain current will be at 12 mA. Assuming that the drain to source resistance is negligible, you can calculate the required value for RD using the following formula:
If you know the drain to source voltage, then you can include it in the calculation.
Question
What should the value of RD be for the IDSS shown at point A in the curve? _____
Answer
40 For the JFET circuit shown in Figure 4.29, assume that VDS = 1 volt when the ID is at saturation.
Questions
A. What is the required value of RD? _____
B. What is the effective drain to source resistance (rDS) in this situation? _____
Answers
A.
B.
Note You can see from this calculation that RD is 19 times greater than rDS. Thus, ignoring VDS and assuming that rDS = 0 does not greatly affect the value of RD. The 1.67 kΩ value is only about 5 percent higher than the 1583 ohm value for RD.
41 Now, turn the JFET OFF. From the curve shown in Figure 4.28, you can see that a cutoff value of −4.2 volts is required. Use a gate to source value of −5 volts to ensure that the JFET is in the “hard OFF” state. The purpose of resistor RG is to ensure that the gate is connected to ground while you flip the switch between terminals, changing the gate voltage from one level to the other. Use a large value of 1 MΩ here to avoid drawing any appreciable current from the gate supply.
Question
When the gate is at the −5 V potential, what is the drain current and the resultant output voltage? _____
Answer
ID = 0 ampere and Vout = VDS = 20 volts, which is VDD
Summary
In this chapter, you learned about the transistor switch and how to calculate the resistor values required to use it in a circuit.
You worked with a lamp as the load example because this provides an easy visual demonstration of the switching action. All the circuits shown in this chapter work when you build them on a breadboard, and the voltage and current measurements are close to those shown in the text.
You have not yet learned all there is to transistor switching. For example, you haven't found out how much current a transistor can conduct before it burns out, what maximum voltage a transistor can sustain, or how fast a transistor can switch ON and OFF. You can learn these things from the data sheet for each transistor model, so these things are not covered here.
When you use the JFET as a switch, it does not switch as fast as a BJT, but it does have certain advantages relating to its large input resistance. The JFET does not draw any current from the control circuit to operate. Conversely, a BJT will draw current from the control circuit because of its lower input resistance.
Self-Test
These questions test your understanding of the concepts introduced in this chapter. Use a separate sheet of paper for your diagrams or calculations. Compare your answers with the answers provided.
For the first three questions, use the circuit shown in Figure 4.30. The objective is to find the value of RB that turns the transistor ON. As you may know, resistors are manufactured with “standard values.” After you have calculated an exact value, choose the nearest standard resistor value from Appendix D, “Standard Resistor Values.”
Figure 4.30
1. RC = 1 kΩ, β = 100
RB = _____
2. RC = 4.7 kΩ, β = 50 RB = _____
3. RC = 22 kΩ, β = 75
RB = _____
For questions 4–6, use the circuit shown in Figure 4.31. Find the values of R3, R2, and R1 that ensure that Q2 is ON or OFF when the switch is in the corresponding position. Calculate the resistors in the order given. After you find the exact values, again choose the nearest standard resistor values.
Figure 4.31
Note Rounding off throughout a problem, or rounding off the final answer, could produce slightly different results.
4. R4 = 100 ohms, β1 = 100, β2 = 20.
R3 = _____
R1 = _____
R2 = _____
5. R4 = 10 ohms, β1 = 50, β2 = 20.
R3 = _____
R1 = _____
R2 = _____
6. R4 = 250 ohms, β1 = 75, β2 = 75.
R3 = _____
R1 = _____
R2 = _____
For questions 7–9, find the values of the resistors in the circuit shown in Figure 4.32 that ensure that Q3 will be ON or OFF when the switch is in the corresponding position. Then, select the nearest standard resistor values.
Figure 4.32
7. RC = 10 ohms, β3 = 20, β2 = 50, β1 = 100.
R4 = _____
R2 = _____
R3 = _____
R1 = _____
8. RC = 28 ohms, β3 = 10, β2 = 75, β1 = 75.
R4 = _____
R2 = _____
R3 = _____
R1 = _____
9. RC = 1 ohm, β3 = 10, β2 = 50, β1 = 75.
R4 = _____
R2 = _____
R3 = _____
R1 = _____
Questions 10–12 use the circuit shown in Figure 4.33. Find values for R1 and R2 that ensure that the transistor turns ON when the switch is closed and OFF when the switch is open.
Figure 4.33
10. RC = 1 kΩ, β = 100.
R1 = _____
R2 = _____
11. RC = 22 kΩ, β = 75.
R1 = _____
R2 = _____
12. RC = 100 Ω, β = 30.
R1 = _____
R2 = _____
13. An N-channel JFET has a transfer curve with the following characteristics. When VGS = 0 volt, the saturation current (IDSS) is 10.5 mA, and the cutoff voltage is −3.8 volts. With a drain supply of 20 volts, design a biasing circuit
that switches the JFET from the ON state to the OFF state.
Answers to Self-Test
The exercises in this Self-Test show calculations that are typical of those found in practice, and the odd results you sometimes get are quite common. Thus, choosing a nearest standard value of resistor is a common practice. If your answers do not agree with those given here, review the problems indicated in parentheses before you go on to Chapter 5.
1. 100 kΩ (problem 8)
2. 235 kΩ. Choose 240 kΩ as a standard value. (problem 8)
3. 1.65 MΩ. Choose 1.6 MΩ as a standard value. (problem 8)
4. R3 = 2 kΩ; R1 = 200 kΩ; R2 = 200 kΩ. Use these values. (problem 22)
5. R3 = 200 ohms; R1 = 10 kΩ; R2 = 10 kΩ. Use these values. (problem 22)
6. R3 = 18.8 kΩ. Choose 18 kΩ as a standard value. (problem 22)
R1 = 1.41 MΩ. Choose 1.5 MΩ as a standard value.
Select 1 MΩ for R2.
7. R4 = 200 ohms; R3 = 10 kΩ; R2 = 1 MΩ; R1 = 1 MΩ. Use these values. (problem 26)
8. R4 = 280 ohms. Choose 270 ohms as a standard value. (problem 26)
R3 = 21 kΩ. Choose 22 kΩ as a standard value.
R2 = 1.56 MΩ. Choose 1.5 or 1.6 MΩ as a standard value.
R1 = 1.56 MΩ. Choose 1.5 or 1.6 MΩ as a standard value.
9. R4 = 10 ohms. Choose 10 ohms as a standard value. (problem 26)
R3 = 500 ohms. Choose 510 ohms as a standard value.
R2 = 37.5 kΩ. Choose 39 kΩ as a standard value.
R1 = 37.5 kΩ. Choose 39 kΩ as a standard value.
10. R2 = 700 ohms. Choose 680 or 720 ohms as a standard value. (problems 31–33)
R1 = 8.45 kΩ. Choose 8.2 kΩ as a standard value.
If 0.7 is ignored, then R1 = 9.1 kΩ.
11. R2 = 11.7 kΩ. Choose 12 KΩ as a standard value. (problems 31–33)
R1 = 141 kΩ. Choose 140 or 150 kΩ as a standard value.
12. R2 = 21 ohms. Choose 22 ohms as a standard value. (problems 31–33)
R1 = 273 ohms. Choose 270 ohms as a standard value.
13. Use the circuit shown in Figure 4.29. Set the gate supply at a value slightly more negative than −3.8 volts. A value of −4 V would work. Make resistor RG = 1 MΩ. Set RD at a value of (20 volts)/(10.5 mA), which calculates a resistance of 1.9 kΩ. You can wire a standard resistor of 1 kΩ in series with a standard resistor of 910 ohms to obtain a resistance of 1.91 kΩ. (problems 39 and 41)
Chapter 5
AC Pre-Test and Review
You need to have some basic knowledge of alternating current (AC) to study electronics. To understand AC, you must understand sine waves.
A sine wave is simply a shape, like waves in the ocean. Sine waves in electronics are used to represent voltage or current moving up and down in magnitude. In AC electronics, some signals or power sources (such as the house current provided at a wall plug) are represented by sine waves. The sine wave shows how the voltage moves from 0 volts to its peak voltage and back down through 0, its negative peak voltage, at 60 cycles per second, or 60 Hertz (Hz).
The sound from a musical instrument also consists of sine waves. When you combine sounds (such as all the instruments in an orchestra), you get complex combinations of many sine waves at various frequencies.
The study of AC starts with the properties of simple sine waves and continues with an examination of how electronic circuits can generate or change sine waves.
This chapter discusses the following:
Generators
Sine waves
Peak-to-peak and root mean square voltages
Resistors in AC circuits
Capacitive and inductive reactance
Resonance
The Generator
1 In electronic circuits powered by direct current (DC), the voltage source is usually a battery or solar cell, which produces a constant voltage and a constant current through a conductor.
In electronic circuits or devices powered by alternating current (AC), the voltage source is usually a generator, which produces a regular output waveform, such as a sine wave.
Question
Draw one cycle of a sine wave.
Answer
Figure 5.1 shows one cycle of a sine wave.
Figure 5.1
2 A number of electronic instruments are used in the laboratory to produce sine waves. For purposes of this discussion, the term generator means a sine wave source. These generators enable you to adjust the voltage and frequency by turning a dial or pushing a button. These instruments are called by various names, generally based on the method of producing the sine wave, or the application as a test instrument. The most popular generator at present is called a function generator. It provides a choice of functions or waveforms, including a square wave and a triangle wave. These waveforms are useful in testing certain electronic circuits.
The symbol shown in Figure 5.2 represents a generator. Note that a sine wave shown within a circle designates an AC sine wave source.
Figure 5.2
Questions
A. What is the most popular instrument used in the lab to produce waveforms? _____
B. What does the term AC mean? _____
C. What does the sine wave inside a generator symbol indicate? _____
Answers
A. Function generator.
B. Alternating current, as opposed to direct current.
C. The generator is a sine wave source.
3 Figure 5.3 shows some key parameters of sine waves. The two axes are voltage and time.
Figure 5.3
The zero axis is the reference point from which all voltage measurements are made.
Questions
A. What is the purpose of the zero axis? _____
B. What is the usual point for making time measurements? _____
Answers
A. It is the reference point from which all voltage measurements are made.
B. Time measurements can be made from any point in the sine wave, but usually they are made from a point at which the sine wave crosses the zero axis.
4 The three most important voltage or amplitude measurements are the peak (p), peak-to-peak (pp), and the root mean square (rms) voltages.
The following equations show the relationship between p, pp, and rms voltages for sine waves. The relationships between p, pp, and rms voltages differ for other waveforms (such as square waves).
Note the following:
Question
If the pp voltage of a sine wave is 10 volts, find the rms voltage. _____
Answer
5 Calculate the following for a sine wave.
Question
If the rms voltage is 2 volts, find the pp voltage. _____
Answer
6 Calculate the following for a sine wave.
Questions
A. Vpp = 220 volts. Find Vrms. _____
B. Vrms = 120 volts. Find Vpp. _____
Answers
A. 77.77 volts
B. 340 volts (This is the common house current supply voltage; 340 Vpp = 120 Vrms.)
7 There is a primary time measurement for sine waves. The duration of the complete sine wave is shown in Figure 5.4 and referred to as a cycle. All other time measurements are fractions or multiples of a cycle.
Figure 5.4
Questions
A. What is one complete sine wave called? _____
B. What do you call the time it takes to complete one sine wave? _____
C. How is the frequency of a sine wave related to this time? _____
D. What is the unit for frequency? _____
E. If the period of a sine wave is 0.5 ms, what is its frequency? What is the frequency of a sine wave with a period of 40 μsec? _____
F. If the frequency of a sine wave is 60 Hz, what is its period? What is the period of sine waves with frequencies of 12.5 kHz and 1 MHz? _____
Answers
A. Cycle
B. The period, T
C. f = 1/T
D. Hertz (
Hz) is the standard unit for frequency. One Hertz equals one cycle per second.
E. 2 kHz, 25 kHz
F. 16.7 ms, 80 μsec, 1 μsec
8 Choose all answers that apply.
Questions
Which of the following could represent electrical AC signals?
A. Simple sine wave
B. Mixture of many sine waves, of different frequencies and amplitudes
C. Straight line
Answer
A and B
Resistors in AC Circuits
9 Alternating current is passed through components, just as direct current is. Resistors interact with alternating current just as they do with direct current.
Question
Suppose an AC signal of 10 Vpp is connected across a 10-ohm resistor. What is the current through the resistor? _____
Answers
Use Ohm's law:
Because the voltage is given in pp, the current is a pp current.
10 An AC signal of 10 Vrms is connected across a 20-ohm resistor.
Question
Find the current. _____
Answer
Because the voltage was given in rms, the current is in rms.
11 You apply an AC signal of 10 Vpp to the voltage divider circuit, as shown in Figure 5.5.
Figure 5.5
Question
Find Vout. _____
Answer
Capacitors in AC Circuits
12 A capacitor opposes the flow of an AC current.
Questions
A. What is this opposition to the current flow called? _____
B. What is this similar to in DC circuits? _____