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ECE 201 – Fall 2014

Exam 3

November 20, 2014

Section (circle below)

Bermel – 002 Peleato-Inarrea – 004 Qi – 005

Gray – 006 Allen – 007 Lin - 010

Name ____________________________ PUID____________

Instructions

1. DO NOT START UNTIL TOLD TO DO SO.

2. Write your name, section, professor, and student ID# on your Scantron sheet. We may check PUIDs.

3. This is a CLOSED BOOKS and CLOSED NOTES exam.

4. The use of a simple scientific calculator without memory or communication capabilities (such as TI-

30Xa or TI-30X IIS) is allowed, but not necessary.

5. If extra paper is needed, use the back of test pages.

6. Cheating will not be tolerated. Cheating in this exam will result in, at the minimum, an F grade for the

course. In particular, continuing to write after the exam time is up is regarded as cheating. 7. If you cannot solve a question, be sure to look at the other ones, and come back to it if time permits.

8. Exam #3 provides evidence for satisfaction of this ECE 20100 Learning Objective:

iii) An ability to analyze 2nd order linear circuits with sources and/or passive elements.

The minimum score needed to satisfy this objective will be posted on Blackboard after the exam has

been graded. Remediation options will be posted in Blackboard if you fail to satisfy any of the course

outcomes.

9. The last two pages of the exam contain potentially useful formulas.

Honor Pledge: I have neither given nor received unauthorized assistance on this exam

Signature:________________________________

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Question 1:

Use this circuit for Questions 1, 2, and 3.

Find R so that the circuit is critically damped.

(1) R = 0 Ohm

(2) R = 0.25 Ohm

(3) R = 0.5 Ohm

(4) R = 1 Ohm

(5) R = 2 Ohm

(6) R = 4 Ohm

(7) R = 10 Ohm

(8) R = 50 Ohm

(9) It cannot be found with the given data

(10) None of the above

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Question 2:

For the same circuit from the previous question (copied above for your convenience), find iL(∞), the

current through the inductor once all the transient behavior has died out. The value of the resistance R

may be different from the solution of the previous question.

(1) iL(∞) = - 2 Amp, regardless of R

(2) iL(∞) = - 1.5 Amp, regardless of R

(3) iL(∞) =−1

2𝑅 Amp

(4) iL(∞) = 0 Amp, regardless of R

(5) iL(∞) = 1 Amp, regardless of R

(6) iL(∞) = 2 Amp, regardless of R

(7) iL(∞) = 2 −3

𝑅 Amp

(8) iL(∞) = 3 −1

𝑅 Amp

(9) iL(∞) = 2 +𝑅

2 Amp

(10) iL(∞) = 5

𝑅 Amp

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Question 3:

For the same circuit from the previous question (copied above for your convenience) find the derivative

of the current through the inductor at t=0+.

(1) iL’(0+) = - 5 Amp/sec

(2) iL’(0+) = - 3 Amp/sec

(3) iL’(0+) = - 1 Amp/sec

(4) iL’(0+) = 0 Amp/sec

(5) iL’(0+) = 1 Amp/sec

(6) iL’(0+) = 3 Amp/sec

(7) iL’(0+) = 5 Amp/sec

(8) iL’(0+) = 10 Amp/sec

(9) The current through an inductor is continuous, so its derivative is always zero

(10) The answer depends on the value of resistance R.

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Question 4:

In the above circuit, the capacitor voltage at time t=0 is 10V. Which statement is true for any non-

negative (i.e., zero, positive, or infinite) value of R? Mark only one answer. (Hint: Consider the range of

the equivalent resistance first.)

(1) The circuit is overdamped

(2) The circuit is critically damped

(3) The circuit is underdamped with angular frequency equal to 50 rad/sec

(4) The circuit is undamped (oscillator) with angular frequency equal to 50 rad/sec

(5) The circuit can be overdamped or critically damped, depending on R

(6) The circuit oscillates (underdamped or undamped) with angular frequency depending on R

(7) Since there are no sources, all the voltages in the circuit remain constant

(8) The capacitor behaves like an open circuit, so no current can flow through it or the inductor

(9) The inductor behaves like a short circuit, so there can be no voltage drop across it

(10) None of the above

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Question 5:

In the circuit above, the switch has been closed for a long time and opens at t=0. Find Vc(t) for t>0 (in

Volts).

(1) Vc(t) = 10 – 1.75*exp(-500t) – 0.25*exp(-1500t)

(2) Vc(t) = 8 + exp(-500t) + exp(-1000*t)

(3) Vc(t) = 8*exp(-1000*t)

(4) Vc(t) = 10 – 2.25*exp(-500t) + 0.25*exp(-1500t)

(5) Vc(t) = (8 - 2*t)*exp(-1000t)

(6) Vc(t) = 10 + (2 - 2*t)*exp(-1000t)

(7) Vc(t) = 8 + 2*t*exp(-1500t)

(8) Vc(t) = 10 – 2*cos(1500t)*exp(-500t)

(9) Vc(t) = 8 + 3*cos(1000t) - sin(100t)

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Question 6:

Find R so that Vout/Vin = -4. Assume that the Op-Amp is ideal.

(1) R = 0 Ohms

(2) R = 0.1 kOhms

(3) R = 0.25 kOhms

(4) R = 0.5 kOhms

(5) R = 0.75 kOhms

(6) R = 1 kOhms

(7) R = 5 kOhms

(8) R = 7.5 kOhms

(9) R = 12 kOhms

(10) None of the above

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Question 7:

Assume that the Op-Amp in the circuit below is ideal. Find the output current Iout in the following circuit:

(1) Iout = - 40 mA

(2) Iout = -35 mA

(3) Iout = -25 mA

(4) Iout = -15 mA

(5) Iout = 0 mA

(6) Iout = 15 mA

(7) Iout = 25 mA

(8) Iout = 35 mA

(9) Iout = 40 mA

(10) None of the above

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Question 8:

Assume ideal op-amp. The Thevenin equivalent of the circuit on the left when looking into

terminals A and B is:

(1) 2 , 6s thV V R

(2) 2 , 6s thV V R

(3) 2 , 6s thV V R

(4) 2 , 6s thV V R

(5) 3 , 10s thV V R

(6) 3 , 10s thV V R

(7) 5 , 25s thV V R

(8) 2 , 15s thV V R

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Question 9:

Assume ideal op-amp. The initial voltage of the 0.1F capacitor at time 0 is 0V. Find Vout(t) in

Volts.

(1) -0.5 sin(t)

(2) 0.5 sin(t)

(3) -0.5 cos(t)

(4) 0. 5 cos(t)

(5) -5 sin(t)

(6) 5 sin(t)

(7) -5 cos(t)

(8) 5 cos(t)

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Question 10:

In the circuit below, Vin(t) = 5cos(120t) V, V1(t) = 2 2 cos(120 45 )ot V. Find the phasor for

the voltage V2 in Volts. Assume sinusoidal steady-state. (Hint: Be careful of the reference

directions of V1 and V2.)

(1) 3 − 𝑗2

(2) 3 cos(120𝑡) − 𝑗2sin(120𝑡)

(3) 3 + 𝑗2

(4) 3 cos(120𝑡) + 𝑗2sin(120𝑡)

(5) 7 − 𝑗2

(6) 7 cos(120𝑡) − 𝑗2sin(120𝑡)

(7) 7 + 𝑗2

(8) 7 cos(120𝑡) + 𝑗2sin(120𝑡)

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Question 11:

In the circuit below, find the phasor for the current IL through the 0.5H inductor (in Amps).

Assume sinusoidal steady-state.

(1) −6 − 𝑗6

(2) −6 + 𝑗6

(3) −1.2 − 𝑗2.4

(4) −1.2 + 𝑗2.4

(5) 1.2 − 𝑗2.4

(6) 1.2 + 𝑗2.4

(7) 6 − 𝑗6

(8) 6 + 𝑗6

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Question 12:

Assume sinusoidal steady state. The phasor voltage V and phasor current I for the device on the

left are shown on the right. The angle between the two phasors is 60o. The magnitudes of the two

phasors are 2 and 6, respectively, as shown in the figure on the right. Find the impedance Z of

the device in Ohms. (Hint: 1

cos602

, 3

sin 602

.)

(1) 1 3

6 6j

(2) 1 3

6 6j

(3) 1 3

6 6j

(4) 1 3

6 6j

(5) 3 3 3

2 2j

(6) 3 3 3

2 2j

(7) 3 3 3

2 2j

(8) 3 3 3

2 2j

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Potentially Useful Formulas

First order circuit: ot t /

ox(t) x( ) x(t ) x( ) e , = L/R for LR circuit, = RC for RC circuit

Series RLC: 2 R 1

s s 0L LC

Parallel RLC: 2 1 1

s s 0RC LC

1 2s t s tx(t) x( ) Ae Be --- Over-damped,

2 4 0b c

tx(t) x( ) A Bt e --- Critically-damped, 2 4 0b c

td dx(t) x( ) Acos t Bsin t e --- Under-damped,

2 4 0b c

22

1 2b b 4c

s ,s for s bs c 02

, where

1c LC

R / 2L (series)b

12 (parallel)

2RC

o1

LC

2 2

1,2 os

22 2

d o4c b

2

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