ECE231S Introductory Electronics Midterm Lecturers A. Liscidini, O. Trescases, B. Wang Date Thursday, Feb. 26, 2015 Duration: 110 minutes 1. Answers should be written in pen. Answers written in pencil may be considered ineligible for remarking. 2. The marks for each question are indicated within brackets [ ]. Use the back side of sheets for your rough work. Place your final answers in the boxes where given. 3. Show your work: answers without justification will not receive full marks! 4. Aids: calculator only. (Only non-programmable calculators are allowed). Last Name: First Name: Student #: (write on the back of this cover page) Lecture Section: LEC101 LEC102 LEC103 (Liscidini) (Trescases) (Wang) Question 1 /12 Question 2 /5 Question 3 /8 Question 4 /6 Question 5 /4 Question 6 /10 Total /45 Page 1 of 14
Question 1 [12 marks]: This question has four unrelated parts. Part I. [3 marks] For each statement, circle TRUE or FALSE: a) Mobile carriers in a semiconductor will drift according to a concentration gradient:... a) TRUE (:ALS:J b) Leakage current in a PN junction diode increases at high temperature due to increased thermal generation of electron-hole pairs: c) The depletion region gets wider as the forward voltage is increased in a PN junction. d) An n-type semiconductor can be converted to p-type by doping it with a sufficient concentration of boron atoms. e) A p-type semiconductor has some free electrons in the conduction band. t) Some diodes are intentionally designed to operate in the breakdown region. c) TRUE e FALSE FALSE FALSE FALSE Part II. The following circuit was used in Lab # 1. R12 10k!) a) [1 mark] Based on your observations during the lab, which of the passive components is physically the largest on the PCB (circle your answer)? R" SW, 0 c, c, c, Page 2 of 14
Question 1 (cont.) Part IV. [2 marks] How does the 'virtual short' concept apply in this circuit (clearly explain your reasoning)?..rla., e 1\)+,,('r h e k.. Vs) a "'-.ol tide,; VLCf Q Page 4 of 14
Question 2 [5 marks] The circuit shown below is used as a basic over-temperature protection alarm, where the diode, D, is used as a temperature sensor. V DD = 3.3 V, R2 = 10 kq, 1/ = 300 faa. [5 marks] Design the value of R/ so that the alarm is activated (ie: V a/arm = 3.3 V) when the diode temperature exceeds the trip-point of 75 C. An excerpt from the diode datasheet is shown below. Assume both op amps to be ideal. Clearly show your steps and reasoning. [3 marks] Bonus: Based on your design, estimate the maximum percentage error in the temperature trip-point if the op amps have an offset voltage of up to +/- 10 my. 1000m........ 1u 10n 1n 10u l OOp......1._... _......1._-'-_......1_... o 0.2 o 10 20 30 40 50 60 70 80 Forward voltage (V ) Reverse voltage (V) Page 5 of 14
Question 2 (cont.) A \S -::;.. C \ +- ) VD y :. \f CD,(«teL.I o,b V + h) (0:0 \I OD (2.\... - O,l./;Lb4 \C --- \ - \<::: Answer for Question 2: Bonus: Error = <j 5" % Page 6 of 14
(! + i2.,) (VOJ, f V u ) = YK f V + t Df am..(' '2. \I b l \J f -+ \J os J. I, - V (J S ( VJ) :=. o.s9:."12..v Yt) - o'b :;: -(4,:L5M V 9.r% erro-r
Question 4 [6 marks]: Consider the circuit shown below. V B = 5V (DC source), R = 1kΩ. a) [2 marks] For each diode, circle the operating region and determine the DC current and voltage. You may assume the Constant-Voltage-Drop model for the diodes, where V D = 0.7 V in the on-state. D 1 : ON / OFF V D1 = I D1 = D 2 : ON / OFF V D2 = I D2 = Page 9 of 14
Question 4 (cont.) b) [4 marks] Draw the small-signal circuit and evaluate the small-signal gain, A v = v out /v in. A v = [V/V] Page 10 of 14
Question 5 [4 marks]: An op amp with a gain-bandwidth product of 10 MHz and a slew-rate of 10 V/μs is used in a noninverting configuration with a closed-loop gain of 6 db. a) [2 marks] For a sinusoidal input with an amplitude of 100 mv, find the frequency that results an output amplitude of 141 mv. b) [2 marks] Under the conditions of part a), determine if the output is slew-rate limited. Bonus: [3 marks] If the input is a step from 0 to V step, determine the maximum value of V step such that the op amp does not experience slew-rate limiting. Answers for Question 5 Part a) Frequency = Part b) Slew-rate limiting? Bonus: V step = khz YES / NO V Page 11 of 14
Question 6 [10 fsl 1- S Consider the circuit shown below, where both op amps are ideal. R = 1 kq. c R The measured bode plot for G(s) = vo(s) IvJ(s) is shown below. IGI o 0.1 100/ a) [3 marks] Calculate the value of c. 3oo kh e- - 'h"j i+ nf.. - Page 12 of 14
Question 6 (cont.) b) [4 marks] Accurately sketch the output voltage, vo(t) on the provided axis for the provided input waveform. What is the value of the output voltage after 2 5V o -5 V v c) [3 marks] Show how the circuit can be modified such that the closed-loop dc gain of G(s) is limited to 60 db (indicate the component values), and sketch the new magnitude response of G(s) on the same bode plot. - ArJ.. Page 13 of 14-6v R P J) C G,,) I + R - 10<.:)0
d) [1 mark] What is the main motivation for limiting the DC gain in the integrator circuit? -rw Q.lAy. y/6y\ - t;c-(o ;:Pc.. eft }l.. (Aflt} S ww ch LSo notre> c h u...q Page 14 of 14