ultimo 15 supch08.pdf

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CHAPTER 8 - SECOND-ORDER CIRCUITS List of topics for this chapter : Finding Initial Values The Source-Free Series RLC Circuit The Source-Free Parallel RLC Circuit Step Response of a Series RLC Circuit Step Response of a Parallel RLC Circuit General Second-Order Circuits Second-Order Op Amp Circuits FINDING INITIAL VALUES Problem 8.1 Given the circuit shown in Figure 8.1, which has existed for a long time, find ) 0 ( v 1 C , ) 0 ( v 2 C , ) 0 ( i 1 L , and ) 0 ( i 2 L . Figure 8.1 When a circuit reaches steady state, an inductor looks like a short circuit and a capacitor looks like an open circuit. So, use the following circuit to find the initial values. 15 20 + v C2 10 + v C1 5 10 10 A 10 i L1 i L2 L 1 L 2 5 15 20 + v C2 10 5 10 10 A 10 i L1 i L2 + v C1 5

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  • CHAPTER 8 - SECOND-ORDER CIRCUITS

    List of topics for this chapter :Finding Initial ValuesThe Source-Free Series RLC CircuitThe Source-Free Parallel RLC CircuitStep Response of a Series RLC CircuitStep Response of a Parallel RLC CircuitGeneral Second-Order CircuitsSecond-Order Op Amp Circuits

    FINDING INITIAL VALUES

    Problem 8.1 Given the circuit shown in Figure 8.1, which has existed for a long time, find)0(v 1C , )0(v 2C , )0(i 1L , and )0(i 2L .

    Figure 8.1

    When a circuit reaches steady state, an inductor looks like a short circuit and a capacitor lookslike an open circuit. So, use the following circuit to find the initial values.

    15

    20

    + vC2

    10 +

    vC1

    5

    10 10 A

    10

    iL1 iL2

    L1 L2

    5

    15

    20

    + vC2

    10 5

    10 10 A10

    iL1 iL2

    +

    vC1

    5

  • Use source transformations to simplify the circuit.

    Now, it is evident that

    11C v)0(v = 5v)0(i 21L =

    322C vv)0(v = 10v)0(i 32L =

    Use nodal analysis to find 1v , 2v , and 3v .

    At node 1 : 05vv

    778.744.44v 211

    =

    +

    0)vv(778.7)44.44v(5 211 =+2.222v778.7v778.12 21 =

    At node 2 : 05v

    5vv 212

    =+

    0v2v- 21 =+

    2v

    v1

    2 =

    At node 3 : 010v

    10v 33

    =+

    0v2 3 =0v3 = volts

    Substitute the equation from node 2 into the equation for node 1.

    2.2222v

    778.7v778.12 11 =

    2.222v889.8 1 =25v1 = volts

    Then, 5.12225

    2v

    v1

    2 === volts

    + vC27.778

    5 10 44.44 V

    10

    iL1 iL2

    +

    vC1

    +

    v3v1 v25

  • Therefore,=)0(v 1C volts25 =)0(i 1L amps5.2=)0(v 2C volts5.12 =)0(i 2L amps0

    Problem 8.2 Given the circuit shown in Figure 8.1, which has existed for a long time, find)0(iL and )0(vC .

    Figure 8.1

    =)0(iL amp1 =)0(vC volts5

    THE SOURCE-FREE SERIES RLC CIRCUIT

    Problem 8.3 Given the circuit in Figure 8.1, which has reached steady state before theswitch closes, find )t(i for all 0t > .

    Figure 8.1

    Use KVL to write a loop equation for 0t > .

    +

    10 V

    10 10 H30

    i(t) 1/20 Ft = 0

    +

    10

    10

    iL

    L

    +

    10i 5

    5

    +

    vC

    20 V C

    i

  • 0dt)t(iC1

    dt)t(di

    L)t(Ri =++ Multiply by L1 and differentiate with respect to time.

    0)t(iLC1

    dt)t(id

    dt)t(di

    LR

    2

    2

    =++

    Rearranging the terms and inserting the values for R, L, and C,

    0)t(i2dt)t(di

    3dt)t(id

    2

    2

    =++

    Assume a solution of stAe .0Ae2sAe3Aes ststst2 =++

    0Ae)2s3s( st2 =++

    Thus,0)2s)(1s( =++

    which gives real and unequal roots at 1-s1 = and 2-s2 = .

    Hence,-2t

    2-t

    1 eAeA)t(i +=

    At += 0t , the circuit is

    So,21 AA0)0(i +== or 12 -AA =

    Also,

    10-dtdi(0)

    10)0(vL ==+ volts or -1dt)0(di=

    and 210

    20

    1 A2-AeA2e-Adt)0(di

    ==

    So,11121 AA2-AA2-A1- =+==

    Hence,

    +0 V +10 V

    i(0) = 0

    + vL

  • -1A1 = and 1A2 =

    Therefore,=)t(i (((( )))) 0tampsee- -2t-t >>>>++++

    Problem 8.4 Given the circuit in Figure 8.1, which has reached steady state before theswitch closes, find )t(i for all 0t > .

    Figure 8.1

    At = 0t ,0)0(i)0(i == + amps and 10)0(v)0(v CC == + volts

    For 0t > ,

    0dt)t(i10/11

    dt)t(di

    10)t(i20 =++ Multiply by 101 , differentiate with respect to time, and rearrange the terms.

    0)t(idt)t(di

    2dt)t(id

    2

    2

    =++

    Again, using a solution of stAe ,0AesAe2Aes ststst2 =++

    0Ae)1s2s( st2 =++

    Thus,0)1s( 2 =+

    which gives a real and repeated root at 1-s 2,1 = .

    A repeated root gives the following solution,-t

    2-t

    1 teAeA)t(i +=

    At 0t = ,1

    02

    01 Ae)0(AeA0)0(i =+== or 0A1 =

    +

    10 V

    10 10 H20

    i(t) 1/10 Ft = 0

  • Also,

    -10dt)0(di

    10)0(vL == or -1dt)0(di=

    and 20

    20

    2 Ae)0(AeA0dt)0(di

    =+=

    Hence,A2 = 1

    Therefore,=)t(i (((( )))) 0tampste- -t >>>>

    Problem 8.5 Given the circuit in Figure 8.1, which has reached steady state before theswitch closes, find )t(i for all 0t > .

    Figure 8.1

    Writing a loop equation for 0t > gives,

    0dt)t(i20/11

    dt)t(di

    10)t(i20 =++ Multiply by 101 , differentiate with respect to time, and rearrange the terms.

    0)t(i2dt)t(di

    2dt)t(id

    2

    2

    =++

    Again, using a solution of stAe ,0Ae2sAe2Aes ststst2 =++

    0Ae)2s2s( st2 =++

    Thus,0)j1s)(j1s( =+++

    which gives complex roots at j1-s 2,1 #= .

    Hence, we have a solutiontj)(-1

    2tj)-(-1

    1 eAeA)t(i ++=

    +

    10 V

    10 10 H20

    i(t) 1/20 Ft = 0

  • At 0t = ,21

    02

    01 AAeAeA0)0(i +=+== or 12 -AA =

    Also,0

    20

    1 eAj)-1(eA)j1-(-1dt)0(di

    ++==

    Using 12 A-A = , we get)j)(-A-1(A)j-1(1- 11 ++=11 Aj-2Aj)1j-1(1- =+=

    or2j1

    A1 = and 2j1-

    A2 =

    Therefore,t)j-1(tj)(-1 ej2

    1-e

    2j1)t(i + +=

    = j2ee

    -e)t(i-jtjt

    t-

    =)t(i (((( )))) 0tamps)tsin(e- t- >>>>

    THE SOURCE-FREE PARALLEL RLC CIRCUIT

    Problem 8.6 Given the circuit in Figure 8.1, find )t(vC for all 0t > .

    Figure 8.1

    Carefully DEFINE the problem.Each component is labeled, indicating the value and polarity. The problem is clear.

    20 V+

    vC(t)

    1/10 F 10 H+

    10

    iL

    t = 0

  • PRESENT everything you know about the problem.The goal of the problem is to find )t(vC for all 0t > .

    There is a switch which opens at 0t = . So, there are two circuits. The first circuit, when theswitch is closed, is used to find the initial values of the capacitor and inductor. Note thatthere is a dc source. At dc, a capacitor is an open circuit and an inductor is a short circuit.Thus, we have the following circuit.

    Recall that the voltage of a capacitor cannot change instantaneously and the current throughan inductor cannot change instantaneously.

    )0(v)0(v)0(v CCC + == and )0(i)0(i)0(i LLL + ==

    The second circuit, after the switch opens, is used to find the final solution.

    Establish a set of ALTERNATIVE solutions and determine the one that promises thegreatest likelihood of success.The first circuit was simplified by applying the characteristics of capacitors and inductorswith a dc source. Ohms law should provide the answer you need when the circuit consists ofa dc voltage source and a resistor.

    For the second circuit, there is only one node or one loop. In this case, the use of KCL orKVL should provide the desired equation to find the solution to the problem. Because thecomponents are in parallel, the voltage across each component is the same. So, use KCL tofind the currents in terms of the voltage Cv .

    ATTEMPT a problem solution.

    Begin by finding the initial values of the capacitor and inductor.

    10 H

    iLiC

    vC

    1/10 F

    20 V +

    10

    iL(0)+vC(0)

  • At 0t = ,

    It is evident from the circuit that0)0(vC = volts.

    Using Ohms law,21020)0(iL == amps

    After the switch opens, the circuit becomes

    Using KCL,0ii LC =+

    0dt)0v(101

    dt)0v(d

    101

    CC

    =+

    Multiply both sides of the equation by 10 and differentiate both sides with respect to time.

    0vdtvd

    C2C

    2

    =+

    but stAe must be a solution.

    Substituting,( )

    0AedtAed

    st2

    st2

    =+

    0AeAes stst2 =+0Ae)1s( st2 =+

    Thus,

    10 H

    iLiC

    vC

    1/10 F

    20 V +

    10

    iL(0)+vC(0)

  • 0)js)(js()1s( 2 =+=+which gives complex roots at js,s 21 =

    So, we have a solutionjt-

    2jt

    1C eAeA)t(v +=

    Now, to solve for 1A and 2A .0AA)0(v 21C =+= or 12 A-A =

    Now,-jt

    1jt

    1C eAeA)t(v =

    -2)0(-i)0(i LC == amps

    )ejAejA(101

    dt)0(dv

    101)0(i 0101CC +==

    or -210A2j 1

    =

    Hence,

    10jj10-

    j220-

    A1 ===

    Therefore,jt-jt

    C e10je10j)t(v =[ ]jt-90j-jt90jC eeee10)t(v +=[ ])90t(j-)90t(jC ee10)t(v ++ += [ ])90tcos(210)t(vC +=

    0tvolts)90tcos(20)t(vC >+=

    EVALUATE the solution and check for accuracy.The circuit must satisfy the conservation of energy. In this case, the energy supplied fromone component is absorbed by the other component.

    Recall, from Chapter 6 Capacitors and Inductors, that the energy stored in a capacitor is2vC

    21

    w =

    and the energy stored in an inductor is2iL

    21

    w =

  • Thus,

    20)20(101

    21

    Cv21

    w 22 =

    == joules

    and

    20)2)(10(21

    Li21

    w 22 =

    == joules

    These two answers match. This circuit satisfies conservation of energy.

    Has the problem been solved SATISFACTORILY? If so, present the solution; if not,then return to ALTERNATIVE solutions and continue through the process again.This problem has been solved satisfactorily.

    =)t(vC (((( )))) 0tvolts90tcos20 >>>>++++

    Problem 8.7 [8.23] In the circuit in Figure 8.1, calculate )t(io and )t(vo for 0t > .

    Figure 8.1

    At 0t = ,

    24)30(828)0(vo =+= and 0)0(io =

    t = 0, note this is amake before break

    switch so theinductor current is

    not interrupted.

    +

    2

    8

    1 H

    +

    vo(t)

    1/4 F30 V

    io(t)

    +

    8 +

    vo(t)

    1/4 F30 V

    io(t)2 1 H

  • For 0t > , we have a source-free parallel RLC circuit.

    41

    )4/1)(8)(2(1

    RC21

    ===

    2)41)(1(1

    LC1

    o ===

    Since is less than o , we have an underdamped response with a damping frequency of

    9843.1)16/1(422od ===and the natural response is

    t-d2d1o e))tsin(A)tcos(A()t(v +=

    Use the initial values to find the unknowns.1o A24)0(v ==

    Also,

    0dt)0(dv

    C)0(i oo ==

    where t-d2dd1dt-

    d2d1o

    e))tcos(A)tsin(A-(e))tsin(A)tcos(A(-dtdv

    +++=

    So,

    2d1o AA-0dt

    )0(dv+==

    Thus,

    024.39843.1)24)(41(

    AA 1d

    2 ==

    =

    Therefore,=)t(vo [[[[ ]]]] 0tvoltse))tsin(024.3)tcos(24( 4t-dd >>>>++++

    8

    1 H

    +

    vo(t)

    1/4 F

    io(t)

  • Problem 8.8 Given the circuit in Figure 8.1, which has reached steady state before theswitch closes, find )t(v for all 0t > .

    Figure 8.1

    =)t(v (((( )))) 0tvolts90tcose40 -t >>>>++++

    STEP RESPONSE OF A SERIES RLC CIRCUIT

    Problem 8.9 [8.25] A branch voltage in a series RLC circuit is described by

    24v8dtdv

    4dtvd2

    2

    =++

    If the initial conditions are dt)0(dv

    0)0(v == , find )t(v .

    Recall the general loop equation for a series RLC circuit.

    SVviRdtdi

    L =++

    where dtdvCi = . Then,

    LCV

    LCv

    dtdv

    LR

    dtvd S2

    2

    =++

    The complete response of this circuit has a forced response and a natural response. To find theforced response, realize that 24V8 S = which means that 3VS = . This is the forced response.Now, to find the natural response, let 0VS = and

    0v8dtdv

    4dtvd2

    2

    =++

    0v)8s4s( 2 =++

    10 t = 0

    10

    10 H4 A 1/20 F

    v(t)

  • Thus,08s4s2 =++

    which gives complex roots at 2j2-2

    32164-s 2,1 =

    = .

    So, we have the solution-2t

    21S e))t2sin(A)t2cos(A(V)t(v ++=

    Solve for the unknowns.11s A3AV0)0(v +=+== or 3-A1 =

    2t-21

    2t-21 e))t2cos(A2)t2sin(-2A(e))t2sin(A)t2cos(-2(Adt

    dv+++=

    21 A2A-20dt)0(dv

    +== or -3AA 12 ==

    Therefore,=)t(v volts]e))t2sin()t2(cos(33[ -2t++++

    Problem 8.10 [8.31] Calculate )t(i for 0t > using the circuit in Figure 8.1.

    Figure 8.1Before 0t = , the capacitor acts like an open circuit while the inductor acts like a short circuit.

    0)0(i = amps and 20)0(v = volts

    For 0t > , the LC circuit is disconnected from the voltage source as shown below.

    +

    20 V5

    1/16 F

    + v i

    1/4 Ht = 0

    1/16 F

    + v i

    1/4 H

  • This is a lossless, source-free series RLC circuit.

    0L2

    R== 8)41()161(

    1LC1

    o =

    +==

    8jjj-j-s o22od ====

    Since is equal to zero, we have an undamped response. Hence,)t8sin(A)t8cos(A)t(i 21 +=

    where 1A0)0(i == .

    So,)t8sin(A)t(i 2=

    To solve for 2A , we know that

    80-)20)(4-()0(vL1-)0(v

    L1

    dt)0(di

    L ====

    However,

    )t8cos(A8dtdi

    2=

    and 2A8-80dt)0(di

    ==

    which leads to -10A2 = .

    Therefore,=)t(i 0tampssin(8t)10- >>>>

    STEP RESPONSE OF A PARALLEL RLC CIRCUIT

    Problem 8.11 Given the circuit in Figure 8.1, find )t(v for all 0t > .

    Figure 8.1

    20 4 A

    t = 0

    15 H 1/30 F20

    v(t)

  • =)t(v (((( )))) 0tvoltse120e120 -2t-t >>>>

    Problem 8.12 Given the circuit in Figure 8.1, find )t(v for all 0t > .

    Figure 8.1

    At = 0t ,0)0(vC = volts and 0)0(iL = amps.

    At += 0t ,0vvv RLC === volts

    So, the 4 amp current source must all go through the capacitor or4)0(iC =+ amps.

    Hence,

    4)0(idt)0(dv

    C C == ++

    or 1203014

    C4

    dt)0(dv

    ===

    +

    volts/sec

    For 0t > , the circuit becomes

    where the 20 ohm resistor in parallel with the 60 ohm resistor is equivalent to a 15 ohm resistor.

    20 4 A

    t = 0

    15 H 1/30 F

    v(t)

    60

    4 A 15 H 1/30 F

    v

    15

  • Using nodal analysis,

    0dt)0v(d

    301

    dt)0v(151

    150v

    4- =

    ++

    + Multiplying by 30 and differentiating with respect to time yields

    0v2dtdv

    2dtvd2

    2

    =++

    Substituting the solution of stAe produces0Ae2sAe2Aes ststst2 =++

    0Ae)2s2s( st2 =++

    Hence,0)j1s)(j1s(2s2s2 =+++=++

    which gives complex roots at j1-s 2,1 #= .

    So, we have the solutiont)j(1-

    2t)j(1-

    1 eAeA)t(v + +=

    At 0t = ,0AAeAeA)0(v 210201 =+=+= or 12 A-A =

    Also,

    [ ] [ ] 120Aj)-(1-Aj)(1-dt)0(dv

    11 =+=

    120A-2jAjAAjA- 11111 ==+j

    1 e60j60j2-120

    A === and -j12 e60-AA ==

    Therefore,t)j(1-j-t)j(1-j ee60ee60)t(v + +=

    { }jjtj-jt-t eee60 + += )]90t(cos2[e60 -t =

    =)t(v (((( )))) 0tvolts90tcose120 -t >>>>

    The answer can also be written as=)t(v (((( )))) 0tvoltstsine120 -t >>>>

  • Problem 8.13 Given the circuit in Figure 8.1, find )t(v for all 0t > .

    Figure 8.1At 0t = ,

    0)0(iL = amps and 0)0(vC = volts

    For complicated circuits that can be simplified using either a Thevenin or Norton equivalent, doso immediately!

    Solving for ocV ,

    Using nodal analysis,

    080i80V

    16080V ococ

    =

    +

    0)i80V(2)80V( ococ =+0i16080V3 oc =

    where 160

    V80i oc

    = .

    Hence,

    +=

    160V80

    16080V3 ococ

    80 80 V

    t = 0

    30 H 1/60 F40 +

    i

    160

    +

    80i

    v(t)

    80 V +

    i

    160

    +

    Voc

    80

    +

    80i

  • ( )ococ V8080V3 +=

    160V4 oc =40Voc = volts

    Solving for scI ,

    Using KCL,

    800i80

    160080

    Isc

    +

    =

    where 21

    160080

    i =

    = amp

    Hence,

    121

    21

    i21

    80i80

    16080

    Isc =+=+=+= amp

    Finding a Norton equivalent circuit,1II scN == amp

    and 40140

    IV

    RRsc

    oc

    Neq ==== ohms

    Now, we can work with the following simplified circuit,

    The two 40 ohm resistors in parallel convert to a 20 ohm resistor, so we can simplify the circuitfurther to become

    80 80 V +

    i

    160

    +

    80iIsc

    1 A 30 H 1/60 F40 40

    v

  • Writing a single node equation results in

    0dt)0v(d

    601

    dt)0v(301

    200v

    1- =

    ++

    + Multiplying by 60 and differentiating with respect to time leads to

    0v2dtdv

    3dtvd2

    2

    =++

    Substituting the solution stAe , we get0Ae2sAe3Aes ststst2 =++

    0Ae)2s3s( st2 =++Hence,

    0)2s)(1s(2s3s2 =++=++which gives real and unequal roots at -1s1 = and -2s2 = .

    So, we have the solution-2t

    2-t

    1 eAeA)t(v +=

    At 0t = ,0)0(v = volts and 0)0(iL = amps.

    Since 0)0(v = volts,0vR = volts and 0iR = amps.

    Hence, the entire 1 amp of current flows through the capacitor. So, we have

    1)0(iC = amp and dt)0(dv

    C)0(iC =

    Thus,

    60)1(6011)0(iC

    1dt

    )0(dvC === volts/sec

    but 60A2A-eA2eA-dt)0(dv

    210

    20

    1 ===

    1 A 30 H 1/60 F

    v

    20

  • Also,0AAeAeA)0(v 210201 =+=+=

    implies that 12 -AA = .

    So,60A2A- 21 =

    or 60AA2A- 111 ==+

    Hence,60A1 = and -60A2 = .

    Therefore,=)t(v (((( )))) 0tvoltse60e60 -2t-t >>>>

    The power of using a Norton equivalent circuit is clearly demonstrated by this problem.

    GENERAL SECOND-ORDER CIRCUITS

    Problem 8.14 Given the circuit in Figure 8.1, find )t(vC for all 0t > .

    Figure 8.1

    This is a second-order circuit with a forced response and a natural response.

    Solve for initial conditions.At 0t = ,

    )0(v)0(v CC + = = 0 and 13030)0(i)0(i LL === + amp.

    Hence,

    1dt

    )0(dvC)0(i CC ==+

    + amp

    +

    30 V

    30 10 H

    1/20 F+

    vc(t)

    t = 0

  • or 20)1(2011)0(i

    C1

    dt)0(dv

    CC

    ===+

    +

    volts/sec

    Solving for final values,0)(iC = amps and 30)(vC = volts

    which is also the forced response,30v

    fC = volts.

    Solving for the natural response,

    The loop equation is

    0vdtdi

    LRi C =++

    where dtdv

    Ci C= .

    So, we now have,

    0vdtvd

    LCdtdv

    RC C2C

    2C

    =++

    Substituting for the values of R, L, and C

    0vdtvd

    2010

    dtdv

    2030

    C2C

    2C

    =++

    Simplifying and rearranging the terms,

    0v2dtdv

    3dtvd

    CC

    2C

    2

    =++

    Substituting a solution stC Aev = yields0Ae2sAe3Aes ststst2 =++

    0Ae)2s3s( st2 =++

    Thus,0)2s)(1s(2s3s2 =++=++

    which gives real and unequal roots at 1-s1 = and -2s2 = .

    30 10 H

    1/20 F+

    vc

    i

  • Hence, the natural response is-2t-t

    C BeAev n +=

    and the complete response is-2t-t

    CCC BeAe30vvv nf ++=+=

    Now, we need to solve for the unknowns, A and B.0BeAe30)0(v 00C =++=

    -30BA =+ or 30-AB =

    Also,

    202B-Ae-2B)(e)A-(0dt)0(dv 00C

    ==++=

    Now,20-B2A =+

    which leads to20-30)-A)(2(A =+

    Hence,-2060A- = or -40A =

    Then,1030-(-40)B ==

    Therefore,=)t(vC (((( )))) 0tvoltse10e4030 -2t-t >>>>++++

    Problem 8.15 Given the circuit shown in Figure 8.1, find )t(iL for all 0t > .

    Figure 8.1

    Carefully DEFINE the problem.Each component is labeled, indicating the value and polarity. The problem is clear.

    5 A 15 15 H 1/30 F

    iL(t)

    t = 0

  • PRESENT everything you know about the problem.The goal of the problem is to find )t(iL for all 0t > .

    There is a switch in the circuit. So, we will need to look at two different circuits. The firstcircuit, when the switch is open, is used to find the initial values of the capacitor andinductor. Note that there is a dc source. At dc, a capacitor is an open circuit and an inductoris a short circuit. Thus, we have the following circuit.

    Recall that the voltage of a capacitor cannot change instantaneously and the current throughan inductor cannot change instantaneously.

    )0(v)0(v)0(v CCC + == and )0(i)0(i)0(i LLL + ==

    The second circuit, after the switch opens is used to find the final solution.

    Establish a set of ALTERNATIVE solutions and determine the one that promises thegreatest likelihood of success.This is a second-order circuit with a forced response and a natural response. The forcedresponse is the response due to the current source at steady state. The natural response is theresponse of the parallel RLC circuit without the current source.

    For simple resistive circuits such as the one used to find the initial values and final values, wewill use observation and Ohms law. For the parallel RLC circuit, use nodal analysis. Therewill be only one equation with this technique versus three equations with mesh analysis.

    ATTEMPT a problem solution.

    Solve for initial conditions using the circuit below.

    5 A 15 iL(0) +

    vC(0)

    15 15 H 1/30 F

    iL

    vL

    5 A

    5 A 15 iL(0) +

    vC(0)

  • At 0t = ,75)15)(5()0(v)0(v CC === + volts

    0)0(i)0(i LL == +75)0(v)0(v CL == ++ volts

    Thus,

    51575

    L)0(v

    dt)0(di LL

    ===

    ++

    amp/sec

    Solving for final values, the forced response,

    5i)(ifLL== amps

    Now, solve for the natural response.

    Writing the node equation,

    0dt)0v(d

    301

    i15

    0v LL

    L=

    ++

    where dtdi

    15v LL =

    So,

    0dtid

    3015

    idtdi

    1515

    2L

    2

    LL

    =++

    Simplifying and rearranging the terms,

    0i2dtdi

    2dtid

    LL

    2L

    2

    =++

    15 15 H 1/30 F

    iL

    vL

    5 A 15 iL()+

    vC()

  • Substituting the solution stL Aei = yields0Ae2sAe2Aes ststst2 =++

    0Ae)2s2s( st2 =++

    Thus,0)j1s)(j1s()2s2s( 2 =+++=++

    which gives complex roots at j1-s 2,1 #= .

    Hence,( ) ( ) tj1-tj1-

    L BeAei n+ +=

    This means that the complete response is( ) ( ) tj1-tj1-

    LLL BeAe5iii nf+ ++=+=

    At 0t = ,0BA5BeAe5)0(i 00L =++=++=

    -5BA =+ or 5-AB =

    Also,

    5B)j1(A)j1(0dt)0(diL

    =+=

    5jBBAjA- =+55)(-Aj5)(-AjAA- =+

    5j552jA- =+

    25-

    2j-5j55

    A =+

    =

    Then,

    25-

    525

    5-AB ===

    Therefore,( ) ( ) tj1-tj1-

    L e)25-(e)25-(5i + ++=

    +

    =

    2ee

    e55)t(ijt-jt

    t-L

    [ ] 0tamps)tcos(e55)t(i -tL >= EVALUATE the solution and check for accuracy.

    Check to see if the answer satisfies the initial conditions.

    055)0cos(e55)0(i -0L === amps

  • This matches the initial condition that0)0(i)0(i LL == + amps

    )tcos(e5)tsin(e50dt)t(di

    t-t-L ++=

    550)0cos(e5)0sin(e5dt)0(di 0-0-L

    =+=+= amps/sec

    This matches the initial condition that

    51575

    L)0(v

    dt)0(di LL

    ===

    ++

    amp/sec

    Has the problem been solved SATISFACTORILY? If so, present the solution; if not,then return to ALTERNATIVE solutions and continue through the process again.This problem has been solved satisfactorily.

    =)t(iL (((( ))))[[[[ ]]]] 0tampstcose55 -t >>>>

    SECOND-ORDER OP AMP CIRCUITS

    Problem 8.16 Given the circuit shown in Figure 8.1, find ov in terms of iv .

    Figure 8.1

    Clearly,

    1

    i1 R

    vi =

    +vi

    +

    10 F

    100 k

    i1 i2

    +

    10 F

    +vo

    100 k

    v2

  • and

    == dtvCR 1-dtRvC1-v i111i12where 1)10)(10(CR -5511 == .Hence, = dtv-v i2Also,

    2

    22 R

    vi =

    and

    == dtvCR 1-dtRvC1-v 222222owhere 1)10)(10(CR -5522 == .

    So, = dtv-v 2oTherefore,

    =ov 2i dtvCircuits like these have many applications, especially in analog computers.

    SearchHelpEWB Help PageWe want your feedbacke-Text Main MenuTextbook Table of ContentsProblem Solving WorkbookWeb LinksTextbook WebsiteOLC Student Center WebsiteMcGraw-Hill Website

    PrefaceChapter 1 Basic ConceptsChapter 2 Basic LawsChapter 3 Methods of AnalysisChapter 4 Circuit TheoremsChapter 5 Operational AmplifierChapter 6 Capacitors and InductorsChapter 7 First-Order CircuitsChapter 8 Second-Order CircuitsFinding Initial ValuesThe Source-Free Series RLC CircuitThe Source-Free Parallel RLC CircuitStep Response of a Series RLC CircuitStep Response of a Parallel RLC CircuitGeneral Second-Order CircuitsSecond-Order Op Amp Circuits

    Chapter 9 Sinusoids and PhasorsChapter 10 Sinusoidal Steady-State AnalysisChapter 11 AC Power AnalysisChapter 12 Three-Phase CircuitsChapter 13 Magnetically Coupled CircuitsChapter 14 Frequency ResponseChapter 15 Laplace TransformChapter 16 Fourier SeriesChapter 17 Fourier TransformChapter 18 Two-Port Networks

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