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UNIVERSITY OF UTAH
ELECTRICAL AND COMPUTER ENGINEERING DEPARTMENT
ECE 3110
Electronics II
Midterm 2
November 22, 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. For frequencies, use Hz (or kHz, or MHz), not radians/second.
- 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 prohibited during the exam.
- (20 points) (a) We have an amplifier with a dc gain of 100 dB and (normal) poles at 100 Hz and 1 MHz. Draw the Bode magnitude and phase plots of this amplifier below.
1 Hz 1 kHz 1 MHz f
A dB
120 dB
-40 dB
0 dB
40 dB
80 dB
-180° 1 Hz 1 kHz 1 MHz
f
∠ A
(b) What is the phase margin for β = 0.001? _______________
(c) What is the lowest closed- loop gain Af for which this amplifier has acceptable stability (having a phase margin of at least 45°)? Af = _________
- (22 points) A voltage amplifier (voltage input, voltage output) having an open-circuit gain of 200, an input resistance of 10 kΩ, and an output resistance of 100 Ω is connected in a negative-feedback loop.
(a) Which feedback topology (e.g., series-shunt, series-series, etc.) would be used in this situation?
The feedback network has an R 11 and R 22 of 5 kΩ and provides a feedback factor β = 0.15. The amplifier is fed by a voltage source having RS = 1 kΩ, and a load resistance RL = 1 kΩ is connected at the output.
(b) What is A? (Hint: It is not 200!)
(c) What is the closed-loop gain Af of the feedback amplifier?
(d) What is the feedback amplifier’s input resistance Rin?
- (20 points) The datasheet of the PN2222A bipolar transistor contains the following information:
Maximum allowable junction temperature TJmax = 150°C Maximum power dissipation PDmax = 625 mW at ambient temperature TA = 25°C Junction-to-case thermal resistance θJC = 80°C/W
(a) Draw an equivalent electrical circuit for this thermal system. Label all components as well as the junction temperature TJ , the case temperature TC , and the ambient temperature TA. (Note that we are not using a heat sink in this problem.)
(b) Find the case-to-ambient thermal resistance θCA.
(c) Assuming an ambient temperature of 25°C, what is the case temperature TC when the transistor dissipates 500 mW?
