제18장 학습문제

제18장 Operational Amplifiers
Self Test
1. What usually controls the open-loop cutoff frequency of an op amp?
  1. Stray-wiring capacitance
  2. Base-emitter capacitance
  3. Collector-base capacitance
  4. Compensating capacitance

2. A compensating capacitor prevents

  1. Voltage gain
  2. Oscillation
  3. Input offset current
  4. Power bandwidth

3. At the unity-gain frequency, the open-loop voltage gain is

  1. 1
  2. Av(mid)
  3. Zero
  4. Very large

4. The cutoff frequency of an op amp equals the unity-gain frequency divided by

  1. The cutoff frequency
  2. Closed-loop voltage gain
  3. Unity
  4. Common-mode voltage gain

5. If the cutoff frequency is 20Hz and the midband open-loop voltage gain is 1,000,000, the unity-gain frequency is

  1. 20 Hz
  2. 1 MHz
  3. 2 MHz
  4. 20 MHz

6. If the unity-gain frequency is 5 MHz and the midband open-loop voltage fgain is 100,000, the cutoff frequency is

  1. 50 Hz
  2. 1 MHz
  3. 1.5 MHz
  4. 15 MHz

7. The initial slope of a sine wave is directly proportional to

  1. Slew rate
  2. Frequency
  3. Voltage gain
  4. Capacitance

8. When the initial slope of a sine wave is greater than the slew rate,

  1. Distortion occurs
  2. Linear operation occurs
  3. Voltage gain is maximum
  4. The op amp works best

9. The power bandwidth increases when

  1. Frequency decreases
  2. Peak value decreases
  3. Initial slope decreases
  4. Voltage gain increases

10. A 741C contains

  1. Discrete resistors
  2. Inductors
  3. Active-load resistors
  4. A large coupling capacitor

11. A 741C cannot work without

  1. Discrete resistors
  2. Passive loading
  3. DC return paths on the two bases
  4. A small coupling capacitor

12. The input impedance of a BIFET op amp is

  1. Low
  2. Medium
  3. High
  4. Extremely high

13. An LF157A is a

  1. Diff amp
  2. Source follower
  3. Bipolar op amp
  4. BIFET op amp

14. If the two supply voltages are ±12V, the MPP value of an op amp is closest to

  1. 0
  2. +12V
  3. -12V
  4. 24V

15. The open-loop curoff frequency of a 741C is controlled by

  1. A coupling capacitor
  2. The output short circuit current
  3. The power bandwidth
  4. A compensating capacitor

16. The 741C has a unity-gain frequency of

  1. 10 Hz
  2. 20 kHz
  3. 1 MHz
  4. 15 MHz

17. The unity-gain frequency equals the product of closed-loop voltage gain and the

  1. Compensating capacitance
  2. Tail current
  3. Closed-loop cutoff frequency
  4. Load resistance

18. If funity is 10 MHz and midband open-loop voltage gain is 200,000, then the open-loop curoff frequency of the op amp is

  1. 10 Hz
  2. 20 Hz
  3. 50 Hz
  4. 100 Hz

19. The initial slope of a sine wave increases when

  1. Frequency decreases
  2. Peak value increase
  3. Cc increases
  4. Slew rate decreases

20. If the frequency of the input signal is greater than the power bandwidth,

  1. Slew-rate distortion occurs
  2. A normal output signal occurs
  3. Output offset voltage increases
  4. Distortion may occur

21. An op amp has an open base resistor. The output voltage will be

  1. Zero
  2. Slightly different from zero
  3. Maximum positive or negative
  4. An amplified sine wave

22. An op amp has a voltage gain of 200,000. If the output voltage is 1V, the input voltage is

  1. 2 uV
  2. 5 uV
  3. 10 mV
  4. 1 V

23. A 741C has supply voltages of ±15V. If the load resistance is large, the MPP value is approximately

  1. 0
  2. +15V
  3. 27V
  4. 30V

24. Above the cutoff frequency, the voltage gain of a 741C decreases approximately

  1. 10 dB per decade
  2. 20 dB per octave
  3. 10 dB per octave
  4. 20 dB per decade

25. The voltage gain of an op amp is unity at the

  1. Cutoff frequency
  2. Unity-gain frequency
  3. Generator frequency
  4. Power bandwidth

26. When slew-rate distortion of a sine wave occurs, the output

  1. Is larger
  2. Appears triangular
  3. Is normal
  4. Has no offset

27. A 741C has

  1. A voltage gain of 100,000
  2. An input impedance of 2 MΩ
  3. An output impedance of 75 Ω
  4. All of the above

28. The closed-loop voltage gain of an inverting amplifier equals

  1. The ratio of the input resistance to the feedback resistance
  2. The open-loop voltage gain
  3. The feedback resistance divided by the input resistance
  4. The input resistance

29. The noninverting amplifier has a

  1. Large closed-loop voltage gain
  2. Small open-loop voltage gain
  3. Large closed-loop input impedance
  4. Large closed-loop output impedance

30. The voltage follower has a

  1. Closed-loop voltage gain of unity
  2. Small open-loop voltage gain
  3. Closed-loop bandwidth of zero
  4. Large closed-loop output impedance

31. A summing amplifier can have

  1. No more than two input signals
  2. Two or more input signals
  3. A cclosed-loop input impedance of infinity
  4. A small open-loop voltage gain


Self Test Answers
문제번호answer 문제번호answer 문제번호answer 문제번호answer 문제번호answer
1d 2b 3a 4b 5d
6a 7b 8a 9b 10c
11c 12d 13d 14d 15d
16c 17c 18c 19b 20a
21c 22b 23c 24d 25b
26b 27d 28c 29c 30a
31b            


Problems
1. Assume that negative saturation occurs at 1V less than the supply voltage with an 741C. How much inverting input voltage does it take to drive the op amp of Fig.19-29 into negative saturation

Fig. 18-29

2. what is thecommon-mode rejection ratio of an LF157A at low frequencies? Convert this decibel value to an ordinary number.

3. What is the open-loop voltage gain of an LF157A when the input frequency is 1kHz? 10kHz? 100kHz? (Assume a first-order response, that is, 20dB per decade rolloff.)

4. The input voltage to an op amp is a large voltage step. The output is an exponential waveform that changes 2.0V in 0.4us. What is the slew rate of the op amp?

5. An LM318 has a slew rate of 70V/us. What is the power bandwidth for a peak output voltage of 7V?

6. Use the equation fmax = SR / 2 πVp to calculate the power bandwidth for each of the following:

  1. SR = 0.5V/us and Vp=1V
  2. SR = 3V/us and Vp=5V
  3. SR = 15V/us and Vp=10V

7. What are closest-loop voltage gain and bandwidth in Fig.18-30? What is the output voltage at 1kHz? At 10MHz? Draw the ideal Bode plot of closest-loop voltage gain.

Fig. 18-30

8. What is the output voltage in Fig.18-31 whaen vin is zero? Use the typical values of Table 18-1.

Fig. 18-31

Table 18-1 : Typical Op-Amp Characteristics
QuantitySymbol Ideal LM741C LF157A
Open-loop voltage gainAVOL Infinite100,000200,000
Unity-gain frequencyfunity Infinite1MHz20MHz
Input resistanceRin Infinite2MΩ1012Ω
Output resistanceRout Zero75Ω100Ω
Input bias currentIin(bias) Zero80nA30pA
Input offset currentIin(off) Zero20nA3pA
Input offset voltageVin(off) Zero2mV1mV
common-mode rejenction ratioCMRR Infinite90dB100dB

9. The data sheet of an LF157A lists the following worst-case parameters: Iin(base)=50pA, Iin(off)=10pA, and Vin(off)=2mV. Recalculate the output voltage when Vin is zero in Fig. 18-31.


10. In Fig. 18-32, what are the closest-loop voltage gain and bandwidth? The ac output voltage at 100kHz?

Fig. 18-32

11. What is the output voltage when Vin is reduced to zero in Fig. 18-32? Use the worst-case parameters given in Prob. 9.


12. In Fig. 18-33a, what is the ac output voltage? If a vcompensating resistor needs to be added to the noninverting input, what size should it be?

Fig. 18-33

13. What is the output voltage in Fig. 18-33b? The bandwidth?


Critical Thinking

14. The adjustable resistor of Fig. 18-34 can be varied from 0 to 100kΩ. Calculate the minimum and maximum closest-loop voltage gain and bandwidth.

Fig. 18-34

15. Calculate the minimum and maximum closest-loop voltage gain and bandwidth in Fig. 18-35.

Fig. 18-35

16. In Fig. 18-33b, the ac output voltage is 49.98mV. What is the closest-loop output impedance?

17. What is the initial slope of a sine wave with a frequency of 15kHz and a peak value of 2V? What happens to the initial slope if the frequency increases to 30kHz?


18. Which op amp in Table 18-2 has the following:
  1. Minimum input offset voltage
  2. Minimum input offset current
  3. Maximum output-current capability
  4. Maximum bandwidth
  5. Minimum drift

Table 18-2

19. What is the CMRR of a 741C at 100kHz? The MPP value when the load resistance is 500Ω? The open-loop voltage gain at 1kHz?

20. If the feedback resistor in fig. 18-33a is changed to a 100kΩ variable resistor, what is the maximum output voltage? The minimum?

21. In Fig. 18-36, what is the closest-loop voltage gain for each switch position?

Fig. 18-36

22. What is the closest-loop voltage gain for each switch position of Fig. 18-37? The bandwidth?

Fig. 18-37

23. In wiring the circuit of Fig. 18-37, a technician leaves the ground off the 6kΩ resistor. What is the closest-loop voltage gain in each switch position?

24. If the 120kΩ resistor opens in Fig.18-37, what is the output voltage most likely to do?

25. What is the closest-loop voltage gain for each switch position of Fig. 18-38? The bandwidth?

Fig. 18-38

26. If the input resistor opens in Fig.18-37, what is the closest-loop voltage gain for each switch position??

27. If the feeback resistor opens in Fig.18-38, what is the output voltage most likely to do?

28. The worst-case parameters for a 741C are Iin(base)=500nA, Iin(off)=200nA, and Vin(off)=6mV. What is the total output error voltage in Fig. 18-39?

Fig. 18-39

29. In Fig. 18-39, the input signal has a frequency of 1kHz. What is the ac output voltage?

30. If the capacitor is shorted in fig. 18-39, what is the total output error voltage? Use the worst-case parameters given in prob. 28.


Up-Down Circuit Analysis

Use Fig. 18-40 for the remaining problems. A circuit like this is impractical for mass production because it has no feedback. The input offset positive or negative saturation. But assume that we have hand-selected a 741C to get a zero output error voltage for this theoritical exercise.

Fig. 18-40

31. Predict the responses for each input base current.

32. Predict the responses for supply-voltage variations.

33. Predict the responses for slew-rate changes.

34. Predict the responses for peak-voltage changes.

Up-down analysis
increase V1 V2 Vin Vout MPP fmax
IB1       
IB2       
±VCC       
SR       
Vp        


Job Interview Questions
1.What is an ideal op amp? Compare the properities of a 741C to those of an ideal op amp.
2.Draw an op amp with an input voltage step. What is slew rate, and why is it important?
3.Draw an inverting amplifier using an op amp with component values. Now, tell me where the virtual ground is. What are the properties of a virtual ground? What is the closest-loop voltage gain, input impedance, and bandwidth?
4.Draw a noninverting amplifier using an op amp with component values. Now, tell me where the virtual short is. What are the properties of a virtual short? What is the closest-loop voltage gain and bandwidth?
5.Draw a summing amplifier and tell me the theory of operation.
6.Draw a voltage follower. What are the closest-loop voltage gain and bandwidth? Describe the closest-loop input and output impedances. What good is this circuit if its voltage gain is so low?
7.What are the input and output impedances of a typical op amp? What advantage do these values have?
8.How does the frequency of the input signal to an op amp affect voltage gain?
9.The LM318 is a much faster op amp than the LM741C. In what applications might the 318 be preferred to the 741C? What are some possible disadvantages of using the 318?
10.With zero input voltage to an ideal op amp, why is there exactly zero output voltage?
11.Name a few linear ICs besides the op amp.
12.What condition is needed for an LM741 to produce maximum voltage gain?
13.Draw an inverting op amp and derive the formula for voltage gain?
14.Draw a noninverting op amp and derive the fomula for voltage gain.
15.Why is a 741C thought of as a dc or low-frequency amplifier?