Application of Laplace to Circuits
Transcript of Application of Laplace to Circuits
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APPLICATION OF THE LAPLACE TRANSFORM
TO CIRCUIT ANALYSISLEARNING GOALS
Laplace circuit solutionsShowing the usefulness of the Laplace transform
Circuit Element ModelsTransforming circuits into the Laplace domain
Analysis Techniques
All standard analysis techniques, KVL, KCL, node,loop analysis, Thevenins theorem are applicable
Transfer FunctionThe concept is revisited and given a formal meaning
Pole-Zero Plots/Bode PlotsEstablishing the connection between them
Steady State Response
AC analysis revisited
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LAPLACE CIRCUIT SOLUTIONS
We compare a conventional approach to solve differential equations with atechnique using the Laplace transform
)()()( tdt
diLtRitvS +=:KVL
tCC
CC eKtit
dt
diLtRi ==+ )(0)()(
equationaryComplement
pC iii +=
LeLKeRK
tC
tC ==+
0)(
pp Kti =)(
casethisforsolutionParticular
pS RKv ==1
tL
R
CeKti
+=1)(
0000 =
=
teR
tit
L
R
Take Laplace transform of the equation
+=dt
diLsRIsVS L)()(
)()0()( ssIissIdt
di==
L
)()(1 sLsIsRIs
+=)(
1)(LsRs
sI+
=
LRs
K
s
K
sLRs
LsI
/)/(
/1)( 21
++=
+=
RsILRsK
RssIK
LRs
s
1|)()/(
1
|)(
/2
01
=+=
==
=
=
0;11)( >
=
teR
tit
L
R
)()()( tdt
diLtRitvS +=
LInitial conditionsare automatically
included
No need tosearch forparticularor comple-mentarysolutions
Only algebrais needed
P
ar
ticul
ar
Complementary
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LEARNING BY DOING 0),( >ttvFind
KCLusingModel
dt
dvC
R
vv S
Sv
0=
+vv
dt
dvC S
Svvdt
dv
RC=+
L)()( sVsV
dt
dvRC S=+
L
)()0()( ssVvssVdt
dv==
L
ssVtuv SS1)()( ==
0)0(0,0)( =
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CIRCUIT ELEMENT MODELS
The method used so far follows the steps:
1. Write the differential equation model2. Use Laplace transform to convert the model to an algebraic form
For a more efficient approach:1. Develop s-domain models for circuit elements
2. Draw the Laplace equivalent circuit keeping the interconnections and replacing
the elements by their s-domain models3. Analyze the Laplace equivalent circuit. All usual circuit tools are applicable and all
equations are algebraic.
Resistor
)()()()( sRIsVtitv ==
)()(
)()(
sIti
sVtv
SS
SS
sourcestIndependen
...
)()()()()()()()(
sBVsItBvtisAIsVtAitv
CDCD
CDCD
==
==
sourcesDependent
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Capacitor: Model 1
svsI
CssV )0()(1)( +=)0()(1)(
0
vdxxiC
tvt
+=
Capacitor: Model 2
)0()(1)(0
vdxxiC
tvt
+=
Cs
)0()()( CvsCsVsI =
Source transformation
)0(1
)0(Cv
Cs
sv
Ieq ==
Impedance in serieswith voltage source
Impedance in parallelwith current source
s
sIdxxi
t)(
)(
0
=
L
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Inductor Models
))0()(()()()( issILsVtdt
diLtv ==
)0()( issIdtdi =L
s
i
Ls
sVsI
)0()()( +=
s
i
Ls
sVsI
)0()()( +=)0()()( LisLsIsV =
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)0()()0()()(
)0()()0()()(
22112
2211111
LisLsIMisMsIsV
MisMsIiLssILsV
+=
+=
Mutual Inductance
)()()(
)()()(
22
12
2111
tdt
diLt
dt
diMtv
tdt
diMt
dt
diLtv
+=
+=
Combine into a single source in the primary
Single source in the secondary
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LEARNING BY DOING
Ai 1)0( =
Determine the model in the s-domain and the expression forthe voltage across the inductor
Inductor with
initial current
Equivalent circuit ins-domain
1
1
)()(1)( +== ssVsIsV
LawsOhm'
)()1(1 sIs+=:KVL
Steady state for t
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ANALYSIS TECHNIQUES
All the analysis techniques are applicable in the s-domain
LEARNING EXAMPLE Draw the s-domain equivalent and find the voltage in boths-domain and time domain
0)0(0,0)( =
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LEARNING EXAMPLE Write the loop equations in the s-domain
=+ )0()0()0()( 1121 iLs
vs
vsVA
1Loop
))()(())()((1
)(1
)( 211212
1
1
11 sIsIsLsIsIsC
sIsC
sIR +++
=+ )()0()0(
)0( 222
11 sViLs
viL B
2Loop
)()())()((
1
))()(( 222122
121 sIRsLsIsIsCsIsIsL +++
Do not increasenumber of loops
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LEARNING EXAMPLE Write the node equations in the s-domain
=+s
ivC
s
isIA
)0()0(
)0()( 2111
1VNode
)(1)(11 212
11
21
1 sVsCsL
sVsCsLsL
G +
+++
)(1
)(1
1
2
12
2
122 sVsL
sCsVsL
sCsCG
+
+++
=+s
ivCvCsIB
)0()0()0()( 21122
2VNode
Do not increase number
of nodes
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LEARNING EXAMPLE
theorem.sNorton'andsThevenin'tion,transformasource
ion,superpositanalysis,loopanalysis,nodeusingFind )(tvo
Assume all initial conditions are zero
Node Analysis
01
)()(
12)(
4 11
=
+
+
s
sVsV
s
ssV
s
o
1V@KCL
01
)()(
2
)( 1=
+
s
sVsVsV oo
oKCL@V
s
2
0)()21()(2
124)()()1(
1
21
2
=++
+=+
sVsssV
s
ssVssVs
o
o
)1(2
s+
s2
2)1(
)3(8)(
s
ssVo
+
+=
Could haveused voltage
divider here
s
sV
12
)(1
+
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Loop Analysis
s
sI4)(1 =
1Loop
ssIsI
ssIsIs
12)(2)(
1))()(( 2212 =++
2Loop
22)1()3(4)(
++=
sssI
22)1(
)3(8)(2)(
+
+==
s
ssIsVo
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Source Superposition Applying current source
Current divider
'2I
s
s
ssVo
4
212
2)('
++
=
Applying voltage source
Voltage divider
sss
sVo12
12
2)("
++
=2
"'
)1(
)3(8)()()(
+
+=+=
s
ssVsVsV
ooo
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Source Transformation
The resistance is redundant
Combine the sources and use currentdivider
+
++
=2124
21
2)(ss
ss
ssVo
2)1(
)3(8)(
+
+
=s
ssVo
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Using Thevenins Theorem
Reduce this part
s
s
ss
ssVOC
124412)(
+=+=
Only independent sources
s
ss
sZ
Th
112+
=+=
s
s
s
ssVo124
12
2
)( 2+
++
=
Voltagedivider
2)1(
)3(8)(
+
+=
s
ssVo
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Using Nortons Theorem
2
124/124)(
s
s
s
s
ssISC
+=+=
Reduce this part
sZTh =
2
124
21
2)(s
s
s
s
ssVo
+
++
=
2)1(
)3(8)(
+
+=
s
ssVo
Currentdivision
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LEARNING EXAMPLE zerobetoconditionsinitialallAssumevoltagetheDetermine ).(tvo
. Three loops, three non-reference nodes
. One voltage source between non-referencenodes - supernode
. One current source. One loop current knownor supermesh
. If v_2 is known, v_o can be obtained with avoltage divider
Selecting the analysis technique:
Transforming the circuit to s-domain
ssVsV12)()( 12 =:constraintSupernode
01
)()(2
/2
)(
2
)( 211=
+++
s
sVsI
s
sVsV:supernodeKCL@
2
)()( 1
sVsI =:variablegControllin
)(11)( 20 sV
ssV
+=:dividerVoltage
ssVsV /12)()( 21 =:algebratheDoing
ssVsI /62/)()( 2 +=
( )
0)1/()(
)/62/)((2/12)()1)(2/1(
2
22
=++
++
ssV
ssVssVs
)54(
)3)(1(12)(
22++
++=
sss
sssV
)54(
)3(12)(
2++
+=
sss
ssVo
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Continued ... theoremsThevenin'usingCompute )(sVo
-keep dependent source and controlling
variable in the same sub-circuit
-Make sub-circuit to be reduced as simpleas possible
-Try to leave a simple voltage divider afterreduction to Thevenin equivalent
sVOC /12
2
/12'
0'2/2
/12
2
/12
sVI
Is
sVsV
OC
OCOC
=
=
+
ssVOC12)( =
0'0'2)/2/()'2(' ==+ IIsII
0)/2/("2""2 = sIIIISC
+
s/12
sI /6"=
s
sISC
)3(6 +=
3
2
)(
)(
+==
ssI
sVZ
SC
OCTH
s
ss
sVo 12
3
21
1)(
+++
=
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Continued Computing the inverse Laplace transform
Analysis in the s-domain has established that the Laplace transform of the
output voltage is
)54(
)3(12)(
2++
+=
sss
ssVo
)12)(12(542 jsjsss +++=++
)12()12()12)(12(
)3(12)(
*11
js
K
js
K
s
K
jsjss
ssV oo
+++
++=
+++
+=
)()cos(||2)()(
11
*11 tuKteK
js
K
js
K t+
+++
+
536|)( 0== =soo ssVK
==
=
+
+=
+=+=
57.16179.343.19879.3)902(43.1535
45212
)2)(12(
)11(12
12|)()12(
1
jj
jjs
so
VjsK
)(57.161cos(59.75
36)(
2 tutetv to
++=
1)2(2++= s
( )[ ] 1)2(1)2()2(
12
)3(12)(
2
2
2
1
2++
+++
++=
++
+=
s
C
s
sC
s
C
ss
ssV oo
)(]sincos[)()(
)(21222221 tutCtCes
C
s
sCt
+
+++
++
+ 5/36|)( 0== =soo ssVC
])2([)1)2(()3(12 212
CsCssCs o +++++=+ 5/12610/362122 22 ==== CCCs o
5/360: 11 =+= CCCo2softscoefficienEquating
)(sin512)cos1(
536)( 22 tutetetv tto
=
One can also use
quadratic factors...
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LEARNING EXTENSION equationsnodeusingFind )(tio
supernode
oVSo VV +
Assume zero initial conditionsImplicit circuit transformation to s-domain
SV
KCL at supernode
02
)()(2))()(( =+++
sV
Ls
sV
ssVsVCs ooSo
2
)()(,
12)(
sVsI
ssV ooS ==
1615
41
61
15.0
61)(
22
+ +
=
++
=
s
s
ss
ssIo
algebratheDoing
++
+
+
=
++
+
=
4
15
4
1
4
15
4
1
4
15
4
1
4
15
4
1
61)(
*
1 1
js
K
js
K
jsjs
ssIo
)()cos(||2)()(
11
*11 tuKteK
js
K
js
K t+
+++
+
4
152
4
15
4
161
|)(415
41
4
15
4
11
j
j
sIjsKjs
o
+
=
+=
+=
=
= 72.15653.6
9097.0
72.6633.61K
=
72.1564
15cos06.13)( 4 teti
t
o
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LEARNING EXTENSION equationsloopusingFind )(tvo
supermesh
12
2II
s=
sourcetodueconstraint
0212
21
2211 =+++ Is
sIIIs
supermeshonKVL
Solve for I2
)73.3)(27.0(216
)14(216)(
22++
+=++
+=sss
ssss
ssI
73.327.0)( 2102
++
++=
s
K
s
K
s
KsI
2|)( 020==
=sssIK
48.2)73.327.0)(27.0(
2)27.0(16|)()27.0( 27.021 =
+
+=+=
=ssIsK
47.4)27.073.3)(73.3(
2)73.3(16
|)()73.3( 73.322=
+
+=+=
=ssIsK
( ))(2)(
)(47.448.22)(
2
73.327.02
titv
tueeti
o
tt
=
+=
Determine inverse transform
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TRANSIENT CIRCUIT ANALYSIS USING LAPLACE TRANSFORM
For the study of transients, especially transients due to switching, it is importantto determine initial conditions. For this determination, one relies on the properties:
1. Voltage across capacitors cannot change discontinuously2. Current through inductors cannot change discontinuously
LEARNING EXAMPLE 0),( >ttvoDetermine
AiVv LC 1)0(,1)0( ==
itshortcircuareinductors
circuitopenarecapacitorscaseDCFor
Assume steady state for t0
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Circuit for t>0Laplace
Use mesh analysis
14
)1( 21 +=+s
sIIs
11
)21( 21 =+++
sI
sssI
Solve for I2s
)1( + s
232
12)(
22
++
=
ss
ssI
s
sI
s
sVo1)(
2)( 2 +=
232
72)(
2++
+=
ss
ssVo
Now determine the inverse transform
rootsconjugatecomplex
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LEARNING EXTENSION 0),(1 >ttiDetermine
Initial current through inductor
Aii LL 1)0()0(=+=
12
6 s2s1)(1 sI
ss
ssI
1
182
2)(1
+
=Current
divider
)()(
9
)(9
11 tueti
s
ssI t=
+
=
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LEARNING EXTENSION 0),( >ttvoDetermine
Determine initial current through inductor
)0(LiUse sourcesuperposition
Ai V 212=
Ai V3
24 =
AiL
3
4)0( =
s2
+
V38
+
)(sVo
divider)(voltage +
+=
3812
242)(
sssVo
=+
+=
)2(3
)368()(
ss
ssVo
2
21
++
s
K
s
K
6|)( 01 == =so ssVK
3
10|)()2( 22 =+= =so sVsK
)(3
86)(
2 tuetv t
o
=
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TRANSFER FUNCTION
)(sX )(sYSystem with allinitial conditionsset to zero
)(
)()(
sX
sYsH =
xadtdxa
dtxda
dtxda
ybdt
dyb
dt
ydb
dt
ydb
om
m
mm
m
m
on
n
nn
n
n
++++=
++++
11
1
1
11
1
1
...
...
equationaldifferentiaissystemtheformodeltheIf
)(sYs
dt
yd kk
k
=
L
zeroareconditionsinitialallIf
)()(...)(
)()(...)(
01
01
sXassXasXsa
sYbssYbsYsb
mm
nn
+++=
+++
)(...
...
)(01
01
sXasasa
bsbsb
sY mm
nn
+++
+++
=
01
01
...
...)(
asasa
bsbsbsH
mm
nn
+++
+++=
1)()()( == sXttx
functionimpulsetheFor
The inverse transform of H(s) is alsocalled the impulse response of the system
If the impulse response is known then onecan determine the response of the systemto ANY other input
H(s) can also be interpreted as the Laplace
transform of the output when the input isan impulse and all initial conditions are zero
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LEARNING EXAMPLE responseimpulsehasnetworkA u(t)eth t=)(
)(10)()(2 tuetvtv tio
=inputthefor,response,theDetermine
In the Laplace domain, Y(s)=H(s)X(s)
)()()( sVsHsV io =
1
1)()()(
+==
ssHtueth t
2
10)()(10)(
2
+==
ssVtuetv i
ti
)2)(1(
10
)( ++= sssVo
21
21
+++= s
K
s
K
10|)()1( 11 =+= =so sVsK
10|)()2( 22 =+= =so sVs
)(10)(2
tueetvtt
o
=
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Impulse response of first and second order systems
First order system
t
Keths
KsH
=
+
= )(1
)(
Normalized second order system
2
00
2
20
2
)(
++
=
ss
sH
12
002,1 = s:poles
tt
eKeKth
)1(
2
)1(
1
200
200
)(
++=
>
networkOverdamped:1:1Case
networkdUnderdampe:1:2Case
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LEARNING EXAMPLE
)(
)()(
sV
sVsH
i
o=functiontransfertheDetermine
Transform the circuit to the Laplacedomain. All initial conditions set to zero
Mesh analysis
212)( IIsVi =
21
110 I
sCsI
+++=
)(sVi
)(1)( 2 sI
sCsVo =
Css
C
sVo /1)2/1(
)2/1(
)( 2 ++=
25.025.02,1s == :poles8FCa)
25.02,1 == s:poles16FCb)
073.0,427.02,1 == s:poles32FCc)
D t i th t f f ti th t f d i d th
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LEARNING EXAMPLE Determine the transfer function, the type of damping and the
unit step response
Transform the circuit to the Laplacedomain. All initial conditions set to zero
0=+
V
01
)(
1
)(1=+
s
sVsV o
01
)()(
1
)(
1
)(
1
)()( 01111=
+++
sVsVsV
s
sVsVsV S
= )()(1 ssVsV o
16
1
2
132
1
)(
)(
2++
=
ss
sV
sV
S
o
25.002
=o
o2 1=
ssVS1)( =responsestepUnit
2
4
1
)32/1()(
+
=
ss
sVo 21211
)25.0(25.0 ++
++=
s
K
s
K
s
Ko
5.0|)( 0== =soo ssVK
125.0|)()25.0( 25.02
12 =+= =so sVsK[ ]
5.0)(
25.0
2
11 ==
=s
o
ds
sVsdK
( )( ) )(5.0125.05.0)( 25.0 tuettv to +=
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LEARNING EXTENSION
84
10)(2
++
+=
ss
ssH
Determine the pole-zero plot, the type of damping and theunit step response
10-z =:zero
22084 2,12
jsss ==++
:poles
x
x
2
2O
10
842
++ ss
2o
o22
2=
)84(
101)()( 2
++
+
==sss
s
ssHsY
2222
)(*221
js
K
js
K
s
KsY
++
+
+
+=
)22)(22(842 jsjsss +++=++
8
10|)( 01 == =sssYK
)4)(22(
28|)()22( 222
jjsVjsK jso
+
+=+=
+=
)()cos(||2)()(
11
*11 tuKteK
js
K
js
K t+
+++
+
=
= 21173.0
90413583.2
1425.82K
+= )2112cos(46.1
8
10)( ttvo
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Second order networks: variation of poles with damping ratio
Normalized second order system
200
2
20
2)(
++=
sssH
12
002,1 = s:poles
networkdUnderdampe:1:2Case
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LEARNING EXAMPLE The Tacoma Narrows Bridge Revisited
Previously the eventwas modeled as a
resonance problem.More detailed studiesshow that a modelwith a wind-dependentdamping ratio provides
a better explanation
022
2
2
=++
oodt
d
dt
d
(mph)speedwind=
=
U
U00013.00046.0
Torsional ResonanceModel
45mincollapsetotime
12twist
42mphspeedwindfailureatConditions
=
=
=
Problem: Develop a circuit that models this event
2 integrator
adder
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model02
2
2
2
=++
oodt
d
dt
d
)2(02 2...
2
...
oooo +==++
iv
( )
)(1
)(
01
)()(
sVsCR
sV
Cs
sV
R
sV
i
i
=
=+
integrator
1v
2v
adder
+=
=++
2
2
1
1
2
2
1
1 0
VR
RV
R
RV
R
V
R
V
R
V
ff
f
dt
d
2
2
dt
d
Simulation usingdependent sources
579.1)00013001156.0( =
U
Using numerical values
Simulation
buildingblocks
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Simulation results
Wind speed=20mph
initial torsion=1 degree
Wind speed=35mphinitial torsion =1degree
POLE ZERO PLOT/BODE PLOT CONNECTION
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37
LCs
L
Rs
LCsV
sVsG
in
o
1
1
)(
)()(
2+
+
==
POLE-ZERO PLOT/BODE PLOT CONNECTION
Bode plots display magnitude and phase information of jssG =|)(
They show a cross section of G(s)
52)(2
2
++=
ss
ssG
Cross sectionshown by Bode
If the poles get closer toimaginary axis the peaksand valleys are morepronounced
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Cross section
Front view
Due to symmetryshow only positivefrequencies
Amplitude Bode plot
Uses log scales
STEADY STATE RESPONSE
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STEADY STATE RESPONSE
)()()( sXsHsY = Response when all initial conditions are zero
Laplace uses positive time functions. Even for sinusoids the response containstransitory terms
EXAMPLE ))(][cos)(()(,1
1)(
22tuttx
s
ssX
ssH =
+=
+=
jsK
jsK
sK
jsjssssY
+
++
+=
++=
*221
1))()(1()(
)()cos(||2)( 22 tuKtKKetyt
++=
transient Steady state response
If interested in the steady state responseonly, then dont determine residuesassociated with transient terms
termstransient++
+
=
++
=
oo
x
o
M
o
M
js
K
js
K
js
X
js
XsHsY x
*
2
1)()(
)(2
1|)()( oMjsox jHXsYjsK o == =
termstransient++= )cos(||2)( 2KtKty ox
))(cos(|)(|)( oooMss HtHXty +=
( )
++
=+=
o
M
o
MtjtjMM
js
X
js
XsXee
XtutX
2
1)(
2)(cos
For the general case))(cos(|)(||)(
)()cos()(
++=
+=
oooMss
oM
jHtjHXty
tutXtxIf
LEARNING EXAMPLE Determine the steady state response
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40
LEARNING EXAMPLE Determine the steady state response
Transform the circuit to the Laplace domain.
Assume all initial conditions are zero
0
1222: 111 =
+
++
s
VVVV i
1
KCL@V
1
12
1V
s
Vo+
=:dividerVoltage
443)()(443)( 2
2
2
2
++=
++=
ss
s
sHsVss
s
sV io
10,2 == Mo X
=++
= 45354.04)2(4)2(3
)2()2(
2
2
jj
jjH
))(cos(|)(||)(
)()cos()(
++=
+=
oooMss
oM
jHtjHXty
tutXtxIf
Vttys )452cos(54.3)( +=
LEARNING EXTENSION 0),( >ttvossDetermine
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41
LEARNING EXTENSION )(oss
))(cos(|)(||)(
)()cos()(
++=
+=
oooMss
oM
jHtjHXty
tutXtxIf
12,2 == Mo X
Transform circuit to Laplace domain.Assume all initial conditions are zero
s
s
1
)(sVi
Thevenin
)(1
1
)(11
1
)( sVssV
s
ssV iiOC +=
+=
1
1
1
1||1,1||)(
2
+
++=
++=+=
s
ss
ss
sssZTh
)()(2
2)( sV
sZsV OC
Th
o+
=
)(33
2)(2
sVss
sV io++
=
)(sH
=
+=
++=
46.9908.6
2
61
2
364
2)2(
jjjH
)(
1
1
112
2)(
2sV
ss
ss
sV io
+
++++
=
)46.992cos(08.6
212)( = ttvoss
LEARNING BY APPLICATION De-emphasize bass
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G C O De-emphasize bass
Enhances bass to original level
)1(
)1)(1()( 21
p
zzvR
s
ssKsG
+
++=
filterrecordingRIAA
s
s
s
p
z
z
318
3180
75
2
1
=
=
=
1|)(| 10002 == jssKG
]500[/1346,3
]50[/46.313
],12.2[/33.13
Hzsr
Hzsr
kHzskr
=
=
=
p
z1
z1
:pole
:zeros
Pole-zero map for )(sGvR
The playback filter is the reciprocal
)(
1
)( sGsG rpvp
=
Pole/zero of playback filter cancelspole/zero of recording filter
LEARNING BY DESIGN Filtering noise in a data transmission line
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43
Data bits at 1000bps Noise source is 100kHz
SOLUTION: Insert a second order low-passfilter in the path. Should not affect datasignal and should attenuate noise
Proposed filter
1V
0=V
0)/1(
3
1
2
1
1
1
1
1=
+++
R
VV
R
V
sC
V
R
VV odata
0)/1( 22
1=+
sC
V
R
V o
2132131211
2
21321
3
1111
1
)(
)(
CCRRCRCRCRss
CCRRR
R
sV
sV
data
o
+
+++
=
22
2 ooss ++
Below 100k
Well below 100kabove 1k
Filter designcriteria 1>
1
2
21
3212
3,
1
C
C
CCRRRR o ====
Design equations
Selected pole location or filter
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Selected pole location or filter
nFCnFC
kR
33.1,75.0
.2,000,25,40
21
0
==
===
determineandequationsdesignUse
Select
Circuit simulating the filter
noise
5102 =
The filter eliminates noise
but smooths data pulse
25kbpsdata transferrate
Filtered signalis useless
REDESIGN!
N l l ti 1
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New pole-zero selection 1=
Simulation for 25kbps
1
2
210
2
31
000,40
1
000,150
C
C
CC
==
==
equationsDesign
APPLI CATI ONAPPLI CATI ONAPPLI CATI ONAPPLI CATI ON
LAPLACELAPLACELAPLACELAPLACE