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5
0
1
~l
(lpt)
Prelim 2
ECE 2100, spring 2011, Professor Molnar
The exam will last 2.5 hours, you are permitted 2 sides
of 8.5"x11" crib sheet to be
turned in for Spts. You may also use a calculator. Scrap paper is provided and should
be attached: show your work for partial credit. However, answers should be written in
the space provided:
if we have to go looking for your answer, you will lose 1 point
off
that answer!
If you get stuck on any given part, move on, and come back later:
REALLY make sure you look at every part
of this exam.
1)
(2pts) What is the magnitude and phase
of the complex number 1+j
~r:
17
/
Mag =
U
2.. ,phase =
'/4
2)
(4 pts)
Ifa component driven by voltage Vet) = 14Vcos(rot), absorbs a current
of
I(t) = 1Acos(rot
1[14),
what is the instantaneous power delivered? What is the
average power?
pet)
=
5
wCI
f(6'72w0
t
5' 7"
ote
lwt)
,Pavg=
So
C\l
3)
(4 pts) Your friend from Harvard
claims that this is a noninverting
amplifier with gain
RF/R1.
Is it?
Vout
,
Vin
Circle one: Y
1
~
If not, then, what is the gain?
(assume the opamp is ideal)
!L
Gain = Vout/Vin =
_
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View Full Document4)
When measuring from a coaxial cable with one end disconnected, and the other
driven by a source with open circuit voltage Vs(t), and thevenin resistance
of
son,
you observe that the input voltage take the form Yin =
12
(Vs(t) + Vs(t+T)). (the
signal propagates down the cable to the end, and is reflected, with a roundtrip
time
ofT)
Vs(t)


a.
(2pts) 1fVs(t) =1 Vcos(wt), what will lin(t) be in terms
ofT and w? (hint: what
is lin in terms
ofVs and Yin?)
_)\')
i
A((6'}
~r
(O'j(\J.Jt
","wi
I)
lin(t)
=_0_""
_
b.
(4pts) Describe the impedance, Z=Vinl1in
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 Spring '05
 KELLEY/SEYLER

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