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Name: _____________________________ (please print) Signature: __________________________ 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