PHYS345 Final Exam December 17, 1998
1. Motors and dimming lights (20 points)
What is the voltage at the distribution point A?
- 465 V \/ -4.2 deg
What is the voltage at point A with the starting coil engaged?
- 402 V \/ -6.7 deg
2. Digital Comparator (15 points)
Design a gate combination that is true if input A is larger than input B.
Design a gate combination that is true if a two-bit input A is larger than a two-bit input B.
- (one answer) - AH.BH' + (AL.BL').(AH+BH')
3. Too many batteries! (20 points)
One battery: What is the current in the inductor a long time later?
- 0.20 A
What is the power delivered by the battery to the rest of the circuit?
- 1.08 W
Two batteries: What is the current in the inductor a long time later?
- 0.063 A
Thevenin equivalent of the two batteries has emf of 1.74 V and
a resistance of 0.58 ohm.
How long does it take for the inductor to change to its new current value?
- 0.18 sec
4. Gray counter (15 points)
Determine the state diagram for this two-bit counter.
- 00 -> 10 -> 11 -> 01 and repeat
How would you reconfigure the counter to reverse the sequence determined above?
- D1 = Q2' and D2 = Q1
5. Real-world differentiator (20 points)
Inverting: At what frequency is the magnitude of the gain 40dB?
- 160 Hz
Input impedance at this frequency?
Non-inverting: Range of frequencies for accurate differentiation?
- above 1.6 Hz to a good approximation
Stabilized non-inverting: Rrange of frequencies for accurate differentiation?
- between 1.6 Hz and 1.6 kHz, roughly 4 decades
What is the high frequency gain in dB?
- 60dB, ignoring rolloff in the op amp's open loop gain.
6. The 555 Timer (15 points)
Determine a formula for the frequency of the clock.
- f(Hz) would be the reciprocal of ln(2).(RA+2RB)C
Check its validity against the timing diagram.
- Expected period is 1.14 ms (874 Hz), reasonably close to the sketch.
Last updated Dec. 17, 1998.
Copyright George Watson, Univ. of Delaware, 1998.