Download Assignment 05 - The University of Iowa

55:041 Electronic Circuits. The University of Iowa. Fall 2013.
Homework Assignment 05
Question 1 (Short Takes). 2 points each unless otherwise noted.
1.
Pick the word/ phrase that best completes the following sentence. To obtain a frequency
response plot of a circuit in SPICE, one must perform _________ analysis
(a) an AC
2.
(b) Harmonic Distortion
(c) Transient
What is the 3-dB bandwidth of the circuit below?
(a)
(b)
(c)
(d)
3.
(b) Transfer function
≈ 8 kHz
31.83 kHz
15.92 kHz
100 kHz
For the current source in the circuit below, 𝐼𝑆 (𝑡) = (0.5)u(𝑡) mA, where u(𝑡) is the unit
step function. What is 𝑉𝑂 at 𝑡 = 22 𝜇s? The capacitor is initially uncharged. (3 points)
(a)
(b)
(c)
(d)
≈ 4.5 V
≈5V
≈9V
≈ 10 V
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55:041 Electronic Circuits. The University of Iowa. Fall 2013.
4.
What is the 3-dB bandwidth of the amplifier shown below if 𝑟𝜋 = 2.5K, 𝑟𝑜 = 100K,
𝑔𝑚 = 40 mS, and 𝐶𝐿𝐿 = 1 nF?
(a)
(b)
(c)
(d)
5.
What is the 3-dB bandwidth of the amplifier shown below if 𝑟𝜋 = 2.5K, 𝑟𝑜 = 100K,
𝑔𝑚 = 40 mS, and 𝐶𝐿𝐿 = 𝐶𝐹 = 1 nF? (3 points)
(a)
(b)
(c)
(d)
6.
65.25 kHz
10 kHz
1.59 kHz
10.4 kHz
795.8 Hz
1.59 kHz
5 kHz
4.71 kHz
Pick the word/ phrase that best completes the following sentence. The mobility of holes is
_____ the mobility of electrons in semiconductor materials.
(a) always the same as
7.
(b) always greater than
(c) always less than
Pick the word/ phrase that best completes the following sentence. The diffusion
capacitance 𝐶𝑑 of a forward-biased a pn junction is generally_______ the junction
capacitance 𝐶𝑗 .
(a) much larger than
(b) much smaller than
2
(c) about the same as
55:041 Electronic Circuits. The University of Iowa. Fall 2013.
8.
Pick the word/ phrase that best completes the following sentence. The diffusion
capacitance 𝐶𝑑 of a pn junction is ________when the junction is reverse-biased.
(a) negligible
(b) large
(c) same as a forward-biased diode
9.
True or false: a silicon diode is biased so that 𝑉𝐷 = 0.7 at 25 oC. VD changes with 2 mV/
o
C, so that at 125 oC, 𝑉𝐷 will be 0.7 + 100×0.002 = 0.9 V.
10.
True or false: a diode, forward biased at ID = 1 mA, has a small-signal or incremental
resistance 𝑟𝑑 of about 260 Ω.
11.
True or false: the turn-on voltages of Schottky diodes are less than that of Si diodes.
However, their reverse leakage/saturation currents are also higher.
12.
True or false: The turn-on voltage (𝑉𝛾 ) of red LEDs is larger than the turn-on voltage of
blue LEDs.
13.
Which of the following depicts the correct current direction? Circle one.
14.
Consider the small-signal equivalent model for a diode below. A typical value for the
series resistance 𝑟𝑆 for small silicon diode is (circle one).
(𝑎) 20 𝜇Ω— 20 mΩ
(b) 20 mΩ— 20 Ω
3
(c) 20 Ω— 200 Ω
55:041 Electronic Circuits. The University of Iowa. Fall 2013.
15.
The reverse saturation current for a Si diode is 𝐼𝑆 = 8 × 10−14 A at room temperature
(25 ℃ ). Estimate the value of 𝐼𝑆 at −5 ℃. (2 points)
16.
In the context of diodes, the term “PIV” means: (1 point)
17.
Consider a linear power supply consisting of a transformer, a full-wave, 4-diode bridge
rectifier, smoothing capacitor, and a load current 1.2 A. By what percentage will the ripple
voltage increase if the load current increases to 1.5 A? (4 points)
a) 100 %
18.
(b) 25%
(c) Stay the same
(d) 50%
Consider a linear power supply consisting of a transformer, a full-wave, a bridge rectifier
and a smoothing capacitor. Increasing the smoothing capacitor by 50% will (4 points)
(a)
Reduce ripple voltage by 50% and increase maximum inrush current by 50%
(b) Reduce both ripple voltage and maximum inrush current by 50%
(c)
Reduce ripple voltage by 50% and leave maximum inrush current unaffected
(d) Reduce ripple voltage by 50% and increase maximum inrush current by 100%
19.
Consider a linear power supply consisting of a transformer, a half-wave, 1-diode rectifier,
and a smoothing capacitor. The rectifier diode is now replaced with a bridge (4-diode)
rectifier. Neglecting the diodes’ turn-on voltages, the ripple voltage will (1 points)
a)
b)
c)
d)
20.
Decrease by a factor 4
Decrease by a factor 2
Stay the same
Increase by a factor 4
A filtered full-wave rectifier voltage has a smaller ripple than does a half-wave rectifier
voltage for the same load resistance and capacitor values because:
(a)
(b)
(a)
(b)
There is a shorter time between peaks
There is a longer time between peaks
The larger the ripple, the better the filtering action
None of the above
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55:041 Electronic Circuits. The University of Iowa. Fall 2013.
21.
The PIV across a nonconducting diode in a bridge rectifier equals approximately:
(a) half the peak
(b) 2 × the peak
value of the transformer secondary voltage.
(c) the peak
(d) 4 × the peak
Problem 2 An engineer designs a power supply that consists of a transformer, a full-wave, 4diode bridge rectifier and a smoothing capacitor. She designed the supply to operate in the U.S.
where the power line (mains) frequency and voltage is 60 Hz and 120 V respectively. The ripple
voltage at full load is 20 mV. Estimate the ripple voltage when the unmodified supply is used in
regions of Japan where the corresponding values are 50 Hz and 100 V respectively. Assume that
the equivalent load resistance stays the same. (5 points)
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55:041 Electronic Circuits. The University of Iowa. Fall 2013.
Problem 3 An engineer designs a power supply that consists of a transformer, a full-wave, 4diode bridge rectifier and a smoothing capacitor. The nominal load current is 1.2 A. By what
percentage will the ripple voltage increase/decrease if the supply is used with an actual load of
1.5 A? Assume the transformer and rectifier diodes are capable of handling the increase in load
current. (5 points)
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55:041 Electronic Circuits. The University of Iowa. Fall 2013.
Question 4 Consider a battery-operated consumer electronics device that uses an embedded
microcontroller that will accept a 2.7–5.5 V power. The device can also be powered from a
power supply that plugs into a mains outlet. Consumers’ expectation is that the switchover
between battery- and mains power is transparent. That is, the instant a user inserts the power
supply connector, the device switches to the power supply, and the instant the user unplugs the
device, it switched to battery power.
Draw a block diagram/schematic that shows how to implement this functionality using diode(s),
linear regulator(s), battery, etc. Explain how the circuit works. Provide as much details as you
can. For example, specify the type of diodes, indicate voltages on the diagram, and include
critical capacitors, and so on. You can assume that unregulated 9 V dc power is available.
(8 points)
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55:041 Electronic Circuits. The University of Iowa. Fall 2013.
Question 5 Determine the resistance
that the 1K load resistance 𝑅𝑅𝐿𝐿 sees.
(8 points)
𝑅𝑅𝐿𝐿
Hint: Use the open circuit voltage,
short circuit current technique.
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55:041 Electronic Circuits. The University of Iowa. Fall 2013.
Question 6 What is the voltage 𝑉𝑂 in the circuit below? Assume an ideal op-amp. (6 points)
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55:041 Electronic Circuits. The University of Iowa. Fall 2013.
Question 7 Find the expression for 𝑉𝑜 for the
differential amplifier circuit below. Assume an ideal
op-amp. (6 points)
Question 8 Using nodal analysis, find 𝐼𝑜 in the circuit
below as follows. Write a KCL equation at the node that
connects the 4K, 2K, and other 4K resistor. Use the
convention that currents flow away from the node.
(6 points)
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55:041 Electronic Circuits. The University of Iowa. Fall 2013.
Question 9 (Principles) Design an RC circuit to delay a 1-kHz sine wave voltage by 62.5 𝜇s.
Provide a schematic that shows where the input and output connections are, and specify values
for 𝑅𝑅 and 𝐶. (8 points)
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55:041 Electronic Circuits. The University of Iowa. Fall 2013.
Question 10 (Varactor) For the circuit shown, 𝑉𝑃𝑆 = 5 V and = 10K . The varactor
characteristics are shown in the graph. What is the bandwidth of the circuit? (5 points)
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55:041 Electronic Circuits. The University of Iowa. Fall 2013.
Question 11 For the circuit shown, 𝑉𝑃𝑆 = 5 V, 𝑅𝑅 = 5K,
𝑣𝛾 = 0.6 V. The input is 𝑣𝑖 = 0.1 sin(𝜔𝑡) V. (8 points)
Draw a small-signal ac model and provide numerical values
for the model parameters. Then determine and expression
for the time-varying component of the output voltage 𝑣𝑂 .
Next determine an expression for the complete (timevarying and dc) solution.
Provide your answer to 3 significant digits.
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55:041 Electronic Circuits. The University of Iowa. Fall 2013.
Question 12 Determine the time-constant for the circuit shown.
𝑅𝑅 = 10 Ω and 𝐶 = 0.01 𝜇F. Start by calculating the resistance
the capacitor sees. That is, the Thevenin resistance 𝑅𝑅𝑇𝐻 of the
controlled source and resistor. (6 points)
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