PHYS345 Final Exam December 17, 1998
1. Motors and dimming lights (20 points)

What is the voltage at the distribution point A?
(starter bypassed)?
 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.
 A^{.}B'

Design a gate combination that is true if a twobit input A is larger than a twobit input B.
 (one answer)  A_{H}^{.}B_{H}' + (A_{L}^{.}B_{L}')^{.}(A_{H}+B_{H}')
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 twobit counter.
 00 > 10 > 11 > 01 and repeat
How would you reconfigure the counter to reverse the sequence determined above?
 D_{1} = Q_{2}' and D_{2} = Q_{1}
5. Realworld differentiator (20 points)

Inverting: At what frequency is the magnitude of the gain 40dB?
 160 Hz
 Input impedance at this frequency?  
j 1.0k

Noninverting: Range of frequencies for accurate differentiation?
 above 1.6 Hz to a good approximation
 Stabilized noninverting: Rrange of frequencies for accurate differentiation?
 between 1.6 Hz and 1.6 kHz,
roughly three 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)^{.}(R_{A}+2R_{B})C
Check its validity against the timing diagram.
 Expected period is 1.14 ms (874 Hz), reasonably close to the sketch.
"http://www.physics.udel.edu/~watson/phys345/exams/fin98fans.html"
Last updated May 18, 2005.
Copyright George Watson, Univ. of Delaware, 1998.