PHYS345 Final Exam          December 17, 1998

1.  Motors and dimming lights (20 points)

1. What is the voltage at the distribution point A? (starter bypassed)?   -   465 V \/ -4.2 deg

2. What is the voltage at point A with the starting coil engaged?   -   402 V \/ -6.7 deg

2.  Digital Comparator (15 points)

1. Design a gate combination that is true if input A is larger than input B.   -   A.B'

2. 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)

1. One battery: What is the current in the inductor a long time later?   -   0.20 A

2. What is the power delivered by the battery to the rest of the circuit?   -   1.08 W

3. 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.

4. 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)

1. Inverting: At what frequency is the magnitude of the gain 40dB?   -   160 Hz

2. Input impedance at this frequency?   -   1.0k

3. Non-inverting: Range of frequencies for accurate differentiation?   -   above 1.6 Hz to a good approximation

4. Stabilized non-inverting: Rrange of frequencies for accurate differentiation?   -   between 1.6 Hz and 1.6 kHz, roughly 4 decades

5. 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.

"http://www.physics.udel.edu/~watson/phys345/exams/fin-98f-ans.html"
Last updated Dec. 17, 1998.
Copyright George Watson, Univ. of Delaware, 1998.