Examination 3. November 13, 2003

Transcription

1 Color: University of Arizona Department of Electrical & Computer Engineering ECE 0 Basic Circuits Examination 3 November 3, 003 Closed book/notes, calculators allowed up to level of TI89, HP48. Part I: 7 questions Part II: 7 questions. Part II is worth twice as much as Part I. On the SCANTRON write and bubblein your: POINTS TOTAL. Name (Last, first). 3 digit ID number which was given to you at the first exam. Write this leftjustified under columns AC under IDENTIFICATION NUMBER. 3. Write the color of your exam paper (IVORY or GREEN) on the top left margin of the SCANTRON. Place your UA picture ID card on the adjacent desk where it can be easily seen. When the 9:30 bell rings, begin the examination. All work should be done on the examination paper. Allow for reasonable amounts of roundoff error, and carefully mark one choice for each problem on the SCANTRON answer sheet. e.g. A B C D E 4 5 if the answer for Question is C. All SCANTRON forms and examinations will be collected at or before 0:30. You will be asked to stop writing and hand in your papers/answer sheets. Failure to comply promptly may result in disqualification from the exam. NAME: SIGNATURE: Write your ECE 0 ID number here and on the SCANTRON:

2 Part I (0 min) ALL OPAMPS IDEAL UNLESS STATED OTHERWISE Which LED(s) will be ON in the following ckt? (a) LED I (b) LED II (c) Both will be ON (d) Both will be OFF 00 µa 0kΩ LED I 500Ω 5V LED II 0kΩ 500Ω What is the value of load current i L at t = 7.5ms? (a) 900 µa (b) 0.9mA (c) 800 µa (d) 0.8mA vin kω 5kΩ 8kΩ 9V 9V i L 0kΩ v in V V ms Find the opamp output current i o. The opamp has ±5V power rails (not shown). (a) 0 (b) ma (c) ma (d) ma 5kΩ ma 0kΩ i o 5kΩ

3 Part I (0 min) ALL OPAMPS IDEAL UNLESS STATED OTHERWISE Which LED will be ON a moment after the switch is closed? (a) red LED (b) yellow LED (c) both LED's (d) no LED's will be ON t=0 0V 500 Ω red yellow 0V How much energy is stored in the capacitor, how much in the inductor? (Assume the circuit is in steady state.) (a) Cap stores J; Inductor stores 0J (b) Cap stores J; Inductor stores 0J (c) Cap stores 0J; Inductor stores J (d) Cap stores J; Inductor stores J V H F The smaller inductor on the left has an inductance of 50mH. The big inductor on the right has four times as many coil turns as the small one. Both inductors are wound on the same core material with identical permeance. What is the value of the big inductor? (a) 50 mh (b) 00 mh (c) 400 mh (d) 800 mh

4 Part I (0 min) Given the resistor voltage waveform v R (t) shown below, which is the correct waveform for the coil voltage v L (t)? (Your choices for v L (t) are shown in (a)(e) below.) v R Waveform generator v L v R t (a) (b) (c) (d) are close

5 Part II 40 min Before being sworn in as Governor of California, Arnold must pass a physical exam. The physical includes an EKG test. A very small voltage is produced between electrodes A and B. The amplifier below is designed to amplify the voltage across the electrodes by a factor of 00. Which set of resistor values should be used? A R R All resistances are in kω R R R 3 R 4 R 5 R 6 (a) (b) (c) (d) (e) V 5 V R 3 R 4 v out B R 6 R 5 A B

6 The following amplifier ckt is based on the practical model for the opamp. Which value of i o is closest to the value that would be computed from the ckt below? (Assume no saturation occurs.) (a) 60 µa (b) 300 µa (c) 0 µa (d) 00 µa (e) 50 µa (v and v are with respect to ground) 75kΩ 5kΩ v 500 Ω MΩ 0 6 (v v ) i o 50kΩ v kω 500mV

7 How much power does the lamp consume in this ideal opamp ckt? (a) 60mW (b) 00mW (c) 90mW (d) 0mW 8.kΩ V 4V V 300Ω lamp 00Ω

8 Part II 40 min Find the value of i o in this ckt. (a) 500µA (b).75ma (c).75ma (d) 400µA 4kΩ kω V 5 V i o ma 5kΩ 5 V 7 kω

9 Which of the following are valid mesh equations for the ckt shown below? The ckt contains a pair of magnetically coupled coils with mutual inductance H. i 5H Ω Ω 4V 3H A i (a) i i 3i i di 5 di 3 = = 4 (b) i i 3i i di 5 di 3 = = 4 (c) i i i i di 5 di 3 = = 4 (d) i i 3i i di 5 di 3 = = 4

10 Assume both switches were closed for a long time before being opened at t = 0. Find the energy stored in the capacitor at t = sec. (a) 0.5mJ (b).mj (c).8mj (d) 3.mJ kω t=0 kω 0V 50µA t=0 µf

11 Part II 40 min This DC ckt is in steady state. How much power is being delivered by the voltage source? (a) 8 W (b) 8.05 W (c) 4 W (d) 4. W v s H 4.5 Ω 0. F 9V