55:041 Electronic Circuits. The University of Iowa. Fall 2013.
Homework Assignment 13
Question 1
Short Takes – 2 points each.
1.
Classify the type of feedback uses in the circuit below (i.e., shunt-shunt, series-shunt, …)
2.
True or false: an engineer uses series-shunt negative feedback to extend the bandwidth of a
voltage amplifier—this will also increase the input resistance.
3.
What type of negative feedback (series-shunt, series-series,…) is used in the following
amplifier?
4.
True or false: voltage regulators use negative feedback to stabilize/regulate their output
voltages: a side effect is that their output resistances are high.
5.
An amplifier has gain of 800. After adding negative feedback, the gain is measured as 25.
Find the loop gain.
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55:041 Electronic Circuits. The University of Iowa. Fall 2013.
6.
An amplifier has gain of 800. After adding negative feedback, the gain is measured as 25.
Find the feedback factor.
7.
An amplifier with gain of 200 has a 10% variation in gain over a certain frequency range.
Using negative feedback, what value of 𝛽 should one use to reduce the gain variation to
1%?
8.
An amplifier has gain of 100,000, and a 20% variation in gain over a certain temperature
range. Negative feedback is used to reduce the gain to 10. What is the variation in gain
with temperature of the feedback amplifier?
9.
An op-amp has an open-loop gain of 120 dB and an input resistance of 50 MΩ. An
engineer wants to use negative feedback to obtain an amplifier with input resistance of
5 GΩ. What is the gain (in dB) of the feedback amplifier? (2 points)
10.
A single-pole op-amp has an open-loop low-frequency gain of 𝐴 = 105 and an open
loop, 3-dB frequency of 4 Hz. If an inverting amplifier with closed-loop low-frequency
gain of �𝐴𝑓 � = 50 uses this op-amp, determine the closed-loop bandwidth.
11.
A single-pole op-amp has an open-loop gain of 100 dB and a unity-gain bandwidth
frequency 5 MHz. What is the open-loop bandwidth of the amplifier? The amplifier is
used as a voltage follower. What is the bandwidth of the follower?
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55:041 Electronic Circuits. The University of Iowa. Fall 2013.
Question 2 A certain audio power amplifier with a signal gain of 10 V/V is found to produce a
2-V peak-to-peak 60-Hz hum. We wish to reduce the output hum to less than 1 mV peak-topeak without changing the signal gain. To this end, we precede the power stage with a
preamplifier stage with gain 𝑎1 and then apply negative feedback around the composite
amplifier. What are the required values of 𝑎1 and 𝛽? Provide 𝛽 to four significant digits.
(6 points)
Original amplifier with 60-Hz hum
problem.
Preamplifier and negative feedback to fix hum
problem.
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55:041 Electronic Circuits. The University of Iowa. Fall 2013.
Question 3 The amplifier below has an open-loop gain 𝐴𝑂𝐿 = 80 dB. What is 𝛽, the loop gain
𝑇, and the closed-loop gain 𝐴𝑓 (6 points)
𝑅1 = 1K
𝑅2 = 47K
𝑅𝐿 = 4.7K
Question 4 For the non-inverting op-amp circuit below, the parameters are 𝐴 = 105 , 𝐴𝑣𝑓 =
20, 𝑅𝑖 = 100K, and 𝑅𝑜 = 100 Ω. Determine 𝑅𝑖𝑓 and 𝑅𝑜𝑓 respectively (6 points)
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55:041 Electronic Circuits. The University of Iowa. Fall 2013.
Question 5 The parameters of the ideal shunt-series amplifier below are 𝐼𝑖 = 20 𝜇A, 𝐼𝑓𝑏 =
19 𝜇A, 𝑅𝑖 = 500 Ω, 𝑅𝑜 = 20K, and 𝛽𝑖 = 0.0095 A/A.. The open-loop gain is 𝐴𝑖 = 2,000 A⁄A.
Determine the values and units for 𝐼𝜀 , 𝐼𝑜 , 𝐴𝑖𝑓 , 𝑅𝑖𝑓 , and 𝑅𝑜𝑓 . (8 points)
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55:041 Electronic Circuits. The University of Iowa. Fall 2013.
Question 6 The open-loop gain and input resistance of the
op-amp below is 106 and 1 MΩ respectively. Further,
𝑅1 = 99K, 𝑅2 = 1K. What is the closed-loop gain and input
resistance? (5 points)
Question 7 An op-amp having a single-pole at 100 Hz, and a low-frequency gain of 105 is
operated is a feedback loop with 𝛽 = 0.01.
(a) What is the factor which feedback shifts the pole? (2 points)
(b) To what frequency? (2 points)
(c) If 𝛽 is changed to a value that results in a closed loop gain of +1, to what frequency does
the pole shift? (2 points)
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55:041 Electronic Circuits. The University of Iowa. Fall 2013.
Question 8 Part (a) of the figure below shows a non-feedback amplifier with gain A that
delivers 5 W into a 5 Ω speaker when the amplifier input is 50 mV rms. The nonlinear distortion
in the amplifier output is 1% of the total signal. In part (b) negative feedback is employed to
reduce the nonlinear distortion. A preamplifier is used to compensate for changes in gain the
feedback introduces
(a) Find the numerical value of the voltage gain A. (3 points).
(b) Find the value of β required to reduce the distortion to 0.1% with the same output signal
amplitude. Find the value of the preamplifier voltage gain 𝐴𝑃𝑅 . You can assume the
preamplifier nonlinear distortion is negligible. (5 points)
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55:041 Electronic Circuits. The University of Iowa. Fall 2013.
Question 9 Consider an op-amp having a single-pole open-loop response with 𝐴𝑜 = 105 and an
open-loop 3-dB bandwidth of 10 Hz. The amplifier is ideal otherwise. The amplifier is
connected in the non-inverting configuration with a nominal low-frequency closed-loop gain of
100. (a) Find the feedback factor 𝛽. (2 points) (b) Make neat Bode plots showing the open-loop
gain and the phase of the open-loop amplifier. (6 points) (c) Add a plot of the loop gain 𝑇 to
you figure and find the frequency at which 𝑇 = 1. (3 points) (e) Find the phase margin of the
closed loop amplifier. (3 points) (f) Is the amplifier stable? (1 point)
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55:041 Electronic Circuits. The University of Iowa. Fall 2013.
Question 10 The open-loop voltage gain of an amplifier is given by
𝐴𝑣 =
�1 + 𝑗
105
𝑓
𝑓
� �1 + 𝑗 5 �
103
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An engineer used the amplifier to design a feedback amplifier with closed-loop gain 𝐴𝑓𝑣 = 100.
Will the amplifier be stable? If so, what is the phase margin? (15 points)
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55:041 Electronic Circuits. The University of Iowa. Fall 2013.
Question 11 Consider a feedback amplifier with loop gain transfer function
𝑇(𝑠) =
𝛽(100)
3
𝑠
�1 +
�
3
5 × 10
Determine the stability with 𝛽 = 0.2 (12 points)
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