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UNIVERSITY OF UTAH
ELECTRICAL AND COMPUTER ENGINEERING DEPARTMENT
ECE 3110
Electronics II
Midterm 1
October 6, 2004
NAME: _
(Please print)
- Do not open the exam until instructed.
- Draw a circle or box around your final answers.
- All answers should include units (e.g., V, mA, kΩ) where appropriate.
- If you want partial credit on incorrect answers, show your work on the pages you turn in! If you choose to turn in any sheets of scratch paper, write your name on those sheets!
- Don’t spend all of your time on one difficult problem. Don’t be afraid to skip ahead if you get stuck. You don’t have to work the problems in order.
- The use of wireless devices is strictly prohibited during the exam.
- (25 points) An amplifier has the following transfer function:
( ) ( / 10 1 )( / 100 1 )( / 10 1 )
5
2
s s s
s Hs
List the pole frequencies (if any) in rad/s : ____________________________________
List the zero frequencies (if any) in rad/s : ____________________________________
What is the midband gain in dB? AM = ______________________________
Sketch the magnitude (not phase) Bode plot for this system, labeling frequencies, gains, and slopes of interest.
H dB
What would you expect the phase of this transfer function to be at very, very high
frequencies? _________________________
- (20 points) Short questions:
(a) Suppose we wish to select an amplifier (voltage in; voltage out) that is most like an ideal op-amp. In each row, circle the characteristic that is closest to that of an ideal op- amp.
Differential gain: -100 dB +60 dB +80 dB
Common-mode gain: -40 dB 0 dB +40 dB
CMRR: -100 dB 0 dB +100 dB
Input bias current: -1μA +1 nA +1 mA
Input resistance 0 O 10 kO 1 MO
Output resistance 10 O 100 O infinite
(b) A circuit has a voltage gain of 0.25 V/V. Convert this to dB: __________________
(c) A circuit has a voltage gain of 26 dB. Convert this to V/V: ____________________
(d) What is the name of the effect that makes capacitors acts larger than they actually are
if they are connected across negative-gain amplifiers? _______________________
(e) As a general rule of thumb (discussed in lecture), the dominant-pole approximation
holds as long as poles are separated by at least a factor of __________.
- (15 points) Resistance reflection rule. Assume the transistors below have a finite ß and an infinite Early voltage.
Write an expression for the input resistance Rin in the circuit shown below. Your expression should include only real resistances ( R 1 , R 2 , R 3 , or a subset of these) and possibly ß , re 1 , and re 2. (Assume both transistors have the same ß .) Circle your answer.
vO
-VCC
VCC
R 1
R 3
R 2
Rin
Q 1
Q 2
