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ECE 3455 -- Exam #2
April 16, 2005
Keep this exam closed
until you are told to
begin.
1. This exam is closed book, closed notes. You may use one 8.5” x 11” crib
sheet, or its equivalent.
2. Show all work on these pages. Show all work necessary to complete the
problem. A solution without the appropriate work shown will receive no credit. A
solution that is not given in a reasonable order will lose credit. Clearly indicate
your answer (for example by enclosing it in a box).
3. It is assumed that your work will begin on the same page as the problem
statement. If you choose to begin your work on another page, you must indicate
this on the page with the problem statement, with a clear indication of where the
work can be found. If your work continues on to another page, indicate clearly
where your work can be found. Failure to indicate this clearly will result in a
loss of credit.
4. Show all units in solutions, intermediate results, and figures. Units in the exam
will be included between square brackets.
5. Do not use red ink. Do not use red pencil.
6. You will have 90 minutes to work on this exam.
1. ________________/35
2. ________________/35
3. ________________/35
Total = 105
ECE 3455 Exam 2 April 16, 2005 – Page 2
Room for extra work
ECE 3455 Exam 2 April 16, 2005 – Page 3
1. {35 Points} Assume ideal op amps for all of the op amps in this problem. The
power supply values are shown. Find the voltage gain, vo/vi.
7.5[k]
5.6[k]
+15[V]
2.2[k]
+15[V]
-
+
+
-
12[k]
-15[V]
+
-15[V]
vi
3.3[k]
4.7[k]
2.7[k]
+15[V]
+
82[k]
vo
+
-
-15[V]
-15[V]
ECE 3455 Exam 2 April 16, 2005 – Page 4
Room for extra work
ECE 3455 Exam 2 April 16, 2005 – Page 5
2. {35 Points} Assume that we have diodes that can be modeled with a piecewise
linear diode model, with Is = 1[mA], Vf = 1[V], and rd = 1[k]. Find VX.
You are expected to be able to perform at least three complete sets of guesses,
clearly stating your guesses, and explicitly showing how you tested your guesses,
in the time period allowed.
+15[V]
680[]
VX
1.5[mA]
-15[V]
ECE 3455 Exam 2 April 16, 2005 – Page 6
Room for extra work
ECE 3455 Exam 2 April 16, 2005 – Page 7
3. {35 Points} Give your answers to this question using complete sentences,
along with equations, diagrams, plots, or combinations of these, as needed to make
your answers clear.
The goal of each of the circuits shown below is to build an ac voltmeter, that
measures the zero-to-peak value of the voltage vi(t). Assume that the diodes that
can be modeled with a piecewise linear diode model, with Is = 0, Vf = 0.7[V], and
rd = 100[]. Assume that the op amp is ideal. The ac voltmeter works by using a
dc voltmeter for the display.
a) What are the advantages and disadvantages of using the op amp based
circuit in Figure 2, as compared to the circuit in Figure 1. Consider in
your answer the possibility of having very small values for vi(t), and very
large values for vi(t).
b) What are the advantages and disadvantages of having the 200[] resistor
in this circuit? Consider practical effects and concerns in making
measurements, and reasonable expected values of the voltmeter
resistance, based on your experience with meter circuits in Circuit
Analysis, and from the values you encountered in the laboratory.
+
200[]
CX =
10,000[F]
vi
dc voltmeter
Figure 1
+15[V]
+
+
-15[V]
vi
CX =
10,000[F]
Figure 2
200[]
dc voltmeter
ECE 3455 Exam 2 April 16, 2005 – Page 8
Solution:
1. {35 Points} Assume ideal op amps for all of the op amps in this problem. The
power supply values are shown. Find the voltage gain, vo/vi.
ECE 3455 Exam 2 April 16, 2005 – Page 9
ECE 3455 Exam 2 April 16, 2005 – Page 10
2. {35 Points} Assume that we have diodes that can be modeled with a piecewise
linear diode model, with Is = 1[mA], Vf = 1[V], and rd = 1[k]. Find VX.
You are expected to be able to perform at least three complete sets of guesses,
clearly stating your guesses, and explicitly showing how you tested your guesses,
in the time period allowed.
ECE 3455 Exam 2 April 16, 2005 – Page 11
ECE 3455 Exam 2 April 16, 2005 – Page 12
3. {35 Points} Give your answers to this question using complete sentences,
along with equations, diagrams, plots, or combinations of these, as needed to make
your answers clear.
The goal of each of the circuits shown below is to build an ac voltmeter, that
measures the zero-to-peak value of the voltage vi(t). Assume that the diodes that
can be modeled with a piecewise linear diode model, with Is = 0, Vf = 0.7[V], and
rd = 100[]. Assume that the op amp is ideal. The ac voltmeter works by using a
dc voltmeter for the display.
a) What are the advantages and disadvantages of using the op amp based
circuit in Figure 2, as compared to the circuit in Figure 1. Consider in
your answer the possibility of having very small values for vi(t), and very
large values for vi(t).
b) What are the advantages and disadvantages of having the 200[] resistor
in this circuit? Consider practical effects and concerns in making
measurements, and reasonable expected values of the voltmeter
resistance, based on your experience with meter circuits in Circuit
Analysis, and from the values you encountered in the laboratory.
+
200[]
CX =
10,000[F]
vi
dc voltmeter
Figure 1
+15[V]
+
+
-15[V]
vi
CX =
10,000[F]
Figure 2
See next page for solution.
200[]
dc voltmeter
ECE 3455 Exam 2 April 16, 2005 – Page 13
ECE 3455 Exam 2 April 16, 2005 – Page 14
ECE 3455 Exam 2 April 16, 2005 – Page 15