PHYS245 Electricity and Electronics
Syllabus - Classes - Assignments - Laboratory - Resources
Second Examination, Spring 2005

PHYS245 Second Midterm Exam April 29, 2005

This is a closed book exam.
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)

Consider the passage of a 5.0 V, 1.0 GHz, sinusoidal signal through the filter below, with resistance of 2.0 k and capacitance of 1.0 pF.
  1. What is the -3.0 dB frequency? (that is, the breakpoint frequency)?
  2. What is the gain in dB at 1.0 GHz?
  3. What voltage will appear at the output if 5.0 V at 1.0 GHz is applied to the input?

2. (20 points)

In the laboratory you encounter a rudimentary 2-bit counter, assembled without any supporting combinational logic. Which sequence of states is accessed by this counter?


Sketch a complete state diagram. Please show your analysis by using the timing diagram.

3. (20 points)

The 2-bit counter in part 2 has been modified with a bit of supporting combinational logic, as shown. (The clocking connection has been suppressed for simplicity in the sketch.)


This counter was designed to count down in a rational sequence (unlike the previous counter in part 2). Unfortunately there is an excluded state, 00.

Sketch a complete state diagram, indicating what happens if the counter starts in the excluded state. Please show your analysis by using the timing diagram.

4. (20 points)

You have decided that the behavior of the excluded state in the 2-bit counter in part 3 is unacceptable. Your preference is to have the excluded 00 state enter the counting sequence at 11 instead. The combination logic that establishes D0 is fine, but new logic is needed for D1. Please complete the design, determine the correct gate(s) and connections, and complete the schematic sketch.

5.  (20 points)

We can use our study of transient circuits to model the effects of RC circuits on fast digital signals. Consider the pulsing signal below, 5.0 V in amplitude, and width 0.5 ns. (This signal might represent a 1.0 GHz clock.)


Consider the passage of this digital signal through the circuit below, with resistance of 2.0 k and capacitance of 1.0 pF. (You may imagine that the capacitor represents a MOSFET in a simple gate that we wish to switch on and off.)
  1. Evaluate the time constant of this circuit. You should get a number in units of nanoseconds.
  2. If the capacitor is initially uncharged, what voltage is reached across it in 0.50 ns if 5.0 V (logic TRUE) is suddenly applied to the input?
  3. Next a voltage of 0.0 V is suddenly applied (logic False). What voltage is present across the capacitor after it discharges for 0.50 ns from the resulting voltage in part b?
  4. Sketch the resulting output on the timing diagram above.

"http://www.art-sci.udel.edu/ghw/phys245/05S/exams/mid2-05s.html"
Last updated May 23, 2005.
Copyright George Watson, Univ. of Delaware, 2005.