by Earl Boysen
Figure 8.37
Figure 8.38
Carefully check your circuit against the diagram.
After you check your circuit, follow these steps, and record your measurements in the blank table following the steps.
1. Connect the oscilloscope probe for channel 2 to a jumper wire connected to Vin. Connect the ground clip to a jumper wire attached to the ground bus.
2. Connect the oscilloscope probe for channel 1 to a jumper wire connected to Vout, and then connect the ground clip to a jumper wire attached to the ground bus.
3. Set the function generator to generate a 10 kHz sine wave with approximately 0.2 Vpp.
4. Measure and record Vout and Vin.
5. Change the feedback resistor to the value shown in the next row of the table (labeled 100 k in this instance). Each time you change the resistor, it's advisable to disconnect the batteries to avoid shorting wires.
6. Measure and record Vout and Vin.
7. Repeat steps 5 and 6 until you have recorded Vout and Vin for the last row of the table.
8. Determine the calculated AV and the measured AV, and record these values in each row of the table.
Expected Results
Figure 8.39 shows the breadboarded circuit for this project.
Figure 8.39
Figure 8.40 shows a function generator and oscilloscope attached to the circuit.
Figure 8.40
The input signal is represented by the upper sine wave shown in Figure 8.41, and the output signal is represented by the lower sine wave. Count the number of divisions for the peak-to-peak output sine wave, and multiply that number by the corresponding VOLTS/DIV setting to determine Vout and Vin.
Figure 8.41
As you measure Vin and Vout, you may need to adjust the TIME/DIV control, the VOLTS/DIV control, and vertical POSITION controls on the oscilloscope. The controls shown in Figure 8.42 are adjusted to measure Vout when RF = 380 k.
Figure 8.42
Your values should be close to those shown in the following table.
The measured values of AV are quite close to the calculated values of AV, well within variations that could be caused by the ± 5 percent tolerance specified for resistor values.
Summary
This chapter introduced the most common types of amplifiers in use today: the common emitter BJT, the common source JFET, and the op-amp. At best, this chapter has scratched only the surface of the world of amplifiers. Actually, there are many variations and types of amplifiers. Still, the terminology and design approach you learned here should give you a basic foundation for further study.
Following are the key skills you gained in this chapter:
How to design a simple amplifier when the bias point and the gain are specified
How to do the same for an emitter follower
How to analyze a simple amplifier circuit
Self-Test
These questions test your understanding of the material presented in this chapter. Use a separate sheet of paper for your diagrams or calculations. Compare your answers with the answers provided following the test.
1. What is the main problem with the amplifier circuit shown in Figure 8.1? _____
2. What is the gain formula for that circuit? _____
3. Does it have a high or low gain? _____
Use the circuit shown in Figure 8.43 for questions 428.
Figure 8.43
4. Design an amplifier so that the bias point is 5 volts, and the AC voltage gain is 15. Assume β = 75, Rin = 1.5 kΩ, VS = 10 volts, and RC = 2.4 kΩ. Add capacitor CE to the circuit and calculate a suitable value to maintain maximum AC voltage gain at 50 Hz. What is the approximate value of this gain? _____
5. Repeat question 4 with these values: VS = 28 volts, β = 80, Rin = 1 kΩ, and RC = 10 kΩ. The bias point should be 14 volts and the AC voltage gain 20. _____
6. Repeat question 4 with these values: VS = 14 volts, β = 250, Rin = 1 kΩ, and RC = 15 kΩ. The bias point should be 7 volts and the AC voltage gain 50. _____
7. Design an emitter follower amplifier given that VS = 12 volts, RE = 100 ohms, β = 35, VE = 7 volts, and RC = 0 ohms. _____
8. Design an emitter follower amplifier given that VS = 28 volts, RE = 100 ohms, β = 35, VE = 7 volts, and RC = 0 ohms. _____
In questions 9211, the resistance and β values are given. Analyze the circuit to find the bias point and the gain.
9. R1 = 16 kΩ, R2 = 2.2 kΩ, RE = 100 ohms, RC = 1 kΩ, β = 100, and VS = 10 volts _____
10. R1 = 36 kΩ, R2 = 3.3 kΩ, RE = 110 ohms, RC = 2.2 kΩ, β = 50, and VS = 12 volts _____
11. R1 = 2.2 kΩ, R2 = 90 kΩ, RE = 20 ohms, RC = 300 kΩ, β = 30, and VS = 50 volts _____
12. The circuits from questions 4 and 5 are connected to form a two-stage amplifier. What is the gain when there is an emitter bypass capacitor for both transistors? When the capacitor is not used in either of them? _____
13. Design a JFET amplifier using the circuit shown in Figure 8.32. The characteristics of the JFET are IDSS = 20 mA and VGS(off) = 24.2 volts. The desired value of VDS is 14 volts. Find the value of RD. _____
14. If the transconductance of the JFET used in question 13 is 0.0048 mhos, what is the voltage gain? _____
15. If the desired output is 8 Vpp for the JFET of questions 13 and 14, what should the input be? _____
16. Design a JFET amplifier using the circuit in Figure 8.33. The JFET characteristics are IDSS = 16 mA and VGS(off) = 22.8 volts. Using a VDS of 10 volts, find the values of RS, CS, and RD. _____
17. If the input to the JFET in question 16 is 20 mVpp, what is the AC output voltage, and what is the gain? _____
18. For the op-amp circuit shown in Figure 8.35, what is the output voltage if the input is 50 mV and the feedback resistor is 750 kΩ? _____
Answers to Self-Test
If your answers do not agree with those provided here, review the problems in parentheses before you go on to Chapter 9, “Oscillators.”
1. Its bias point is unstable, and its gain varies with temperature. Also, you cannot guarantee what the gain will be. (problem 10)
2. (problem 10)
3. Usually the gain is quite high. (problem 10)
For Numbers 426, suitable values are given. Yours should be close to these.
4. R1 = 29 kΩ, R2 = 3.82 kΩ, RE = 160 ohms, CE = 200 μF, AV = 120 (problems 17222)
5. R1 = 138 kΩ, R2 = 8 kΩ, RE = 500 ohms, CE = 64 μF, AV = 800 (problems 17222)
6. R1 = 640 kΩ, R2 = 45 kΩ, RE = 300 ohms, CE = 107 μF, AV = 750 (problems 17222)
7. R1 = 8 kΩ; R2 = 11.2 kΩ (problem 27)
8. R1 = 922 ohms; R2 = 385 ohms (problem 27)
9. VC = 5 volts, AV = 10 (problems 28230)
10. VC = 6 volts, AV = 20 (problems 28230)
11. VC = 30 volts, AV = 15 (problems 28230)
12. When the capacitor is used, AV = 120 × 800 = 96,000. (problems 17222)
When the capacitor is not used, AV = 15 × 20 = 300.
13. Use VGS = 22.1 volts, then ID = 5 mA, RD = 2 kΩ. (problems 31233)
14. Av = 29.6 mVpp (problem 39)
15. Vin = 83 mVpp (problem 38)
16. Use VGS = 21.4 volts, then ID = 4 mA. (problem 42)
RS = 350 ohms
CS = 4.5 μF (assume f = 1 kHz)
RD = 3.15 kΩ
17. VGS varies from 21.39 to 21.41 volts, ID varies from 4.06 to 3.94 mA, Vout will be 400 mVpp,
(problem 42)
18. Av = 275, Vout = 3.75 volts (problem 45)
Chapter 9
Oscillators
This chapter introduces you to oscillators. An oscillator is a circuit that produces a continuous output signal. There are many types of oscillator circuits used extensively in electronic devices. Oscillators can produce a variety of different output signals, such as sine waves, square waves, or triangle waves.
When the output signal of an oscillator is a sine wave of constant frequency, the circuit is called a sine wave oscillator. Radio and telev
ision signals are sine waves transmitted through the air, and the 120-volts AC from the wall plug is a sine wave, as are many test signals used in electronics.
This chapter introduces three basic sine wave oscillators. They all rely on resonant LC circuits as described in Chapter 7, “Resonant Circuits,” to set the frequency of the sine wave.
When you complete this chapter, you will be able to do the following:
Recognize the main elements of an oscillator.
Differentiate between positive and negative feedback.
Specify the type of feedback that causes a circuit to oscillate.
Specify at least two methods of obtaining feedback in an oscillator circuit.
Understand how resonant LC circuits set the frequency of an oscillator.
Design a simple oscillator circuit.
Understanding Oscillators
1 An oscillator can be divided into three definite sections:
An amplifier
The feedback connections
The components that set frequency
The amplifier replaces the switch in the basic oscillator circuit, introduced in problem 35 of Chapter 7 (Figure 7.46).
Question
Draw an oscillator circuit, and label the parts. Use a separate sheet of paper for your drawing.
Answer
See Figure 9.1.
Figure 9.1
2 When you connect the output of an amplifier to its input, you get feedback. If the feedback is “out of phase” with the input, as shown in Figure 9.2, then the feedback is negative.
Figure 9.2
When the signal from the collector is fed back to the base of the transistor through a feedback resistor (Rf), as in the circuit shown in Figure 9.3, the feedback signal is out of phase with the input signal. Therefore, the feedback is negative.
Figure 9.3
Negative feedback is used to stabilize the operation of an amplifier by doing the following:
Preventing the DC bias point and gain of an amplifier from being affected by changes in temperature
Reducing distortion in amplifiers, thereby improving the quality of the sound
Questions
A. Why would feedback signals be used in quality audio amplifiers? _____
B. What kind of feedback do they have? _____
Answers
A. To reduce distortion
B. Negative feedback
3 If the feedback from the output is in phase with the input, as shown in Figure 9.4, the circuit's feedback is positive.
Figure 9.4
In the circuit shown in Figure 9.5, the collector of the second transistor is connected to the base of the first transistor. Because the output signal at the collector of the second transistor is in phase with the input signal at the base of the first transistor, this circuit has positive feedback.
Figure 9.5
Positive feedback can cause an amplifier to oscillate even when there is no external input.
Questions
A. What type of feedback is used to stabilize an amplifier? _____
B. What type of feedback is used in oscillators? _____
C. What parts of an amplifier do you connect to produce feedback? _____
Answers
A. Negative feedback.
B. Positive feedback.
C. Connect the output of an amplifier to its input.
4 The amplifier shown in Figure 9.6 is the same type of amplifier that was discussed in problems 11–18 of Chapter 8, “Transistor Amplifiers.” It is called a common emitter amplifier.
Figure 9.6
Questions
A. What effect would negative feedback have on this amplifier? _____
B. What effect would positive feedback have on this amplifier? _____
Answers
A. Stabilize it, reduce gain, and reduce distortion
B. Cause it to oscillate
5 In the circuit shown in Figure 9.6, an input signal applied to the base will be amplified.
Questions
A. What is the basic formula for an amplifier's voltage gain? _____
B. What is the voltage gain formula for the amplifier circuit shown in Figure 9.6? _____
Answers
A.
B.
C. (as discussed in problem 12 of Chapter 8)
6 In the circuit shown in Figure 9.7, an input signal is applied to the emitter of the transistor instead of the base. This circuit is called a common base amplifier.
Figure 9.7
Note When you apply a signal to the emitter, it changes the voltage drop across the base-emitter diode, just as an input signal applied to the base does. Therefore, a signal applied to the emitter changes the base current and the collector current, just as if you had applied a signal to the base.
The voltage gain formula for this type of amplifier can be simplified because the input impedance to the amplifier is so low when the signal is fed into the emitter that you can discount it. This results in the following voltage gain formula for the common base amplifier:
RS is the output resistance or impedance of the source or generator. It is also called the internal impedance of the source.
Question
What is the voltage gain formula for the circuit shown in Figure 9.7? _____
Answer
7 Notice that the input and output sine waves in Figure 9.7 are in phase. Although the signal is amplified, it is not inverted.
Questions
A. What happens to the input signal to the amplifier when you apply it to the emitter instead of the base? _____
B. Is the input impedance of the common base amplifier high or low compared to the common emitter amplifier? _____
C. What is the gain formula for the common base amplifier? _____
Answers
A. Amplified and not inverted
B. Low
C.
8 Figure 9.8 shows an amplifier circuit with a parallel inductor and capacitor connected between the collector of the transistor and ground. A parallel inductor and capacitor circuit is sometimes called a tuned (or resonant) load.
Figure 9.8
In this circuit, the inductor has a small DC resistance, which could pull the collector DC voltage down to near 0 volts. Therefore, you include capacitor CC in the circuit to allow AC signals to pass through the LC circuit while preventing the collector DC voltage from being pulled down to 0 volts.
Questions
A. What term would you use to describe the load in this circuit? _____
B. Does the circuit contain all three components of an oscillator at this point? _____
Answers
A. Resonant or tuned.
B. No, the feedback connections are missing.
Note The circuit shown in Figure 9.8 does not have an input signal either to the emitter or to the base. By adding a feedback connection to a parallel LC circuit, you provide an input signal to the emitter or base, as explained later in this chapter.
9 Write the voltage gain formulas for the following circuits. Refer to the circuits and voltage gain formulas in problems 4–6, if necessary.
Questions
A. Common emitter circuit _____
B. Common base circuit _____
Answers
A.
B.
10 You can use common emitter and common base amplifier circuits in oscillators, and in each case, you would usually also include an extra capacitor.
In a common emitter amplifier, you can add a capacitor (CE) between the emitter and ground, as discussed in problems 19 and 20 of Chapter 8.
In a common base circuit, you can add a capacitor (CB) between the base and the ground, as is shown in Figure 9.7.
Question
What is the general effect in both cases? _____
Answer
An increase in the gain of the amplifier
The gain is increased to the point where you can consider it “large enough” to use the amplifier as an oscillator. When these capa
citors are used in either a common emitter or common base amplifier, it is not usually necessary to calculate the gain of the amplifier.
11 An LC circuit has a resonance frequency that you can determine using the methods discussed in problems 6–12 of Chapter 7. When you use an LC circuit in an oscillator, the output signal of the oscillator will be at the resonance frequency of the LC circuit.
Question
What is the formula for the oscillation (or resonant) frequency? _____
Answer
In practice, the actual measured frequency is never quite the same as the calculated frequency. The capacitor and inductor values are not exact, and other stray capacitances in the circuit affect the frequency. When you need to set an exact frequency, use an adjustable capacitor or inductor.
12 Figure 9.9 shows the parallel LC circuit connected between the collector and the supply voltage, rather than between the collector and ground (as in Figure 9.8).
Figure 9.9
You can use this circuit and the circuit shown in Figure 9.8 to selectively amplify one frequency far more than others.
Questions
A. What would you expect this one frequency to be? _____