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PHYS245
Electricity and Electronics Syllabus - Classes - Assignments - Laboratory - Resources Final Examination, Spring 2005 |
PHYS245 Final Exam May 23, 2005
This is a closed book exam,
but one 3”x5” note card is permitted.
Programmable calculators and graphing calculators may be used during this exam.
Show ALL work on problem sheet and only on that sheet.
Please read questions carefully.
Credit may be lost inadvertently if solutions are not neat and orderly.
Be careful with units, signs, and significant figures.
1. (20 points)
a. A tachometer transducer is producing a signal of 25 mV at 12,000 rpm; the tachometer has a resistance of 10 k. A signal amplifier is being used to amplify the signal for display on an inexpensive oscilloscope; the amplifier has a gain of 30 dB and an input impedance of 50 k. What is the voltage of the signal at the oscilloscope?
b. It turns out that there is a noise source of 2.0 mV at 60 Hz from the tachometer arising from pickup from a neighboring power transformer. What is the ratio of the signal voltage to the noise voltage at the oscilloscope in dB?
c. A technician decides to ac couple the tachometer to the amplifier with a capacitor of 0.010 microfarad to suppress the 60 Hz noise, inserted between points A and B. What is the new signal-to-noise ratio at the oscilloscope?
2. (20 points)
a. A young MacGyver enthusiast is attempting to design a simple switched RC circuit to use as a fuse timer. The child has a 5.0 F capacitor and one fresh 9.0 V alkaline battery with an internal resistance of 18 ohm. If the fuse will ignite when the capacitor is charged to a voltage of 1.0 V, how much time does the youngster have to vacate the premises?
b. How much current is flowing through the battery at ignition?
c. With a never ending enthusiasm for adding batteries to a circuit, the youngster connects a fresh 1.5 V AAA cell (0.60 ohm internal resistance) as shown. Now how much time expires after switch closure until the fuse is ignited?
d. How much current is flowing through the 9.0 V battery at ignition in this circuit?
3. (20 points)
a. MacGyver finds himself trapped in the main post office in Lancaster, PA. He decides to signal his buddy Pete with a digital beacon that cycles through the sequence of five digits that represents the zip code 17602. Design a three-bit counter using edge-triggered D-type flip-flops to accomplish this sequence.
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By judicious choice of ‘don’t care’ substitutions, your design can be implemented with only two dual-input OR gates and two dual-input AND gates – make it so! [A bit of Boolean algebra could reduce the design further to one AND gate and two OR gates, but this is not necessary here.]
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b. Indicate the behavior of the excluded states on the following state diagram.
c. MacGyver is able to locate one 7400 IC package of four dual-input NAND gates. Design the combinational logic for the three data inputs above to use solely NAND gates.
4. (20 points)
a. From the following parts, design an op amp circuit with a gain of 20 dB: LF356 op amp, collection of 1.0 k, 2.0 k, 5.0 k, and 10.0 k resistors. Sketch!
In the circuit above, connect a 2.5 microfarad capacitor in parallel with the feedback resistor.
- What is the breakpoint (-3.0 dB) frequency?
- Is this a low-pass or high-pass filter? Sketch the Bode plot.
- What is the gain in dB at 60 Hz?
- What is the gain in dB at 600 Hz?
5. (20 points)
- What is the equivalent impedance seen by the voltage source?
- What is the current through the voltage source?
- What is the power provided by the voltage source?
- What is the power dissipated by the resistor?
- What is the voltage across the capacitor?
Last updated May 23, 2005.
Copyright George Watson, Univ. of Delaware, 2005.