Chapter 14, Problem 1.
Find the transfer function V o /V i of the RC circuit in Fig. 14.68. Express it using ω o =
1/RC.
Figure 14.68
For Prob. 14.1.
Chapter 14, Solution 1.
Vo
R
jωRC
=
=
H (ω) =
Vi R + 1 jωC 1 + jωRC
jω ω0
1
H (ω) =
,
where ω0 =
1 + jω ω0
RC
H = H (ω) =
ω ω0
1 + (ω ω0 ) 2
φ = ∠H (ω) =
⎛ω⎞
π
− tan -1 ⎜ ⎟
2
⎝ ω0 ⎠
This is a highpass filter. The frequency response is the same as that for P.P.14.1 except
that ω0 = 1 RC . Thus, the sketches of H and φ are shown below.
H
1
0.7071
0
ω0 = 1/RC
ω
φ
90°
45°
0
ω0 = 1/RC
ω
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Chapter 14, Problem 2.
Obtain the transfer function V o (s)/V i of the circuit in Fig. 14.69.
Figure 14.69
For Prob. 14.2.
Chapter 14, Solution 2.
V
H(s) = o =
Vi
2+
1
s/8
10 + 20 +
1
s/8
=
2 + 8/s 1 s + 4
=
12 + 8 / s 6 s + 0.6667
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Chapter 14, Problem 3.
For the circuit shown in Fig. 14.70, find H(s) = V o /V i (s).
Figure 14.70
For Prob. 14.3.
Chapter 14, Solution 3.
1
1
5
=
=
jωC s (0.2) s
1
10
0.1F
⎯⎯
→
=
s (0.1) s
The circuit becomes that shown below.
0.2 F
⎯⎯
→
2
V1
5
s
+
Vi
+
_
10
s
5
Vo
_
10
5
10 1 + s
(5 + )
5(
)
10
5
10( s + 1)
s
s
s
s
=
=
Let Z = //(5 + ) =
15
5
s
s
s ( s + 3)
5+
(3 + s )
s
s
Z
V1 =
Vi
Z +2
5
s
s
Z
Vo =
V1 =
V1 =
•
Vi
5+5/ s
s +1
s +1 Z + 2
10( s + 1)
10s
5s
s
s ( s + 3)
V
•
=
= 2
H (s) = o =
Vi s + 1
10( s + 1) 2s ( s + 3) + 10( s + 1) s + 8s + 5
2+
s ( s + 3)
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Chapter 14, Problem 4.
Find the transfer function H( ω ) = V O /V i of the circuits shown in Fig. 14.71.
Figure 14.71
For Prob. 14.4.
Chapter 14, Solution 4.
(a)
R ||
1
R
=
jωC 1 + jωRC
R
Vo
R
1 + jωRC
=
=
H (ω) =
R
Vi
R + jωL (1 + jωRC)
jωL +
1 + jωRC
(b)
H (ω) =
R
- ω RLC + R + jωL
H (ω) =
jωC (R + jωL)
R + jωL
=
R + jωL + 1 jωC 1 + jωC (R + jωL)
2
- ω 2 LC + jωRC
H (ω) =
1 − ω 2 LC + jωRC
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Chapter 14, Problem 5.
For each of the circuits shown in Fig. 14.72, find H(s) = V o (s)/V s (s).
Figure 14.72
For Prob. 14.5.
Chapter 14, Solution 5.
(a) Let Z = R // sL =
Vo =
sRL
R + sL
Z
Vs
Z + Rs
sRL
Vo
Z
sRL
H (s) = =
= R + sL =
Vs Z + Rs R + sRL
RRs + s ( R + Rs ) L
s
R + sL
1
Rx
1
sC = R
(b) Let Z = R //
=
sC R + 1 1 + sRC
sC
Z
Vo =
Vs
Z + sL
V
Z
H(s) = o =
=
Vi Z + sL
R
R
1 + sRC =
R
s 2 LRC + sL + R
sL +
1 + sRC
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Chapter 14, Problem 6.
For the circuit shown in Fig. 14.73, find H(s) = I o (s)/I s (s).
Figure 14.73
For Prob. 14.6.
Chapter 14, Solution 6.
1H
⎯⎯
→
Let Z = s //1 =
jω L = sL = s
s
s +1
We convert the current source to a voltage source as shown below.
1
Is ⋅ 1
S
+
+
_
Vo
Z
_
s
s +1
sI s
sI
Z
Is =
= 2 s
( I s x1) =
2
s
( s + 1) + s s + 3s + 1
Z + s +1
s +1+
s +1
Vo
sI s
Io = = 2
1 s + 3s + 1
I
s
H (s) = o = 2
I s s + 3s + 1
Vo =
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Chapter 14, Problem 7.
Calculate H (ω ) if H dB equals
(a) 0.05dB
(b) -6.2 dB
(c) 104.7 dB
Chapter 14, Solution 7.
(a)
0.05 = 20 log10 H
2.5 × 10 -3 = log10 H
H = 10 2.5×10 = 1.005773
-3
(b)
- 6.2 = 20 log10 H
- 0.31 = log10 H
H = 10 -0.31 = 0.4898
(c)
104.7 = 20 log10 H
5.235 = log10 H
H = 10 5.235 = 1.718 × 10 5
Chapter 14, Problem 8.
Determine the magnitude (in dB) and the phase (in degrees) of H( ω ) = at ω = 1 if
H (ω ) equals
(a) 0.05 dB
(b) 125
(c)
Chapter 14, Solution 8.
(a)
H = 0.05
H dB = 20 log10 0.05 = - 26.02 ,
(b)
(c)
H = 125
H dB = 20 log10 125 = 41.94 ,
(d)
3
6
+
2 + jω
1 + jω
φ = 0°
φ = 0°
H(1) =
H dB
(d)
j10
= 4.472∠63.43°
2+ j
= 20 log10 4.472 = 13.01 ,
10 jω
2 + jω
φ = 63.43°
3
6
+
= 3.9 − j2.7 = 4.743∠ - 34.7°
1+ j 2 + j
H dB = 20 log10 4.743 = 13.521,
φ = –34.7˚
H(1) =
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Chapter 14, Problem 9.
A ladder network has a voltage gain of
H( ω ) =
10
(1 + jω )(10 + jω )
Sketch the Bode plots for the gain.
Chapter 14, Solution 9.
H (ω) =
1
(1 + jω)(1 + jω 10)
H dB = -20 log10 1 + jω − 20 log10 1 + jω / 10
φ = - tan -1 (ω) − tan -1 (ω / 10)
The magnitude and phase plots are shown below.
HdB
0.1
1
10
ω
100
20 log 10
-20
1
1 + jω / 10
20 log10
-40
1
1 + jω
φ
0.1
-45°
1
10
ω
100
arg
1
1 + jω / 10
-90°
-135°
arg
1
1 + jω
-180°
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Chapter 14, Problem 10.
Sketch the Bode magnitude and phase plots of:
H(j ω ) =
50
jω (5 + jω )
Chapter 14, Solution 10.
H( jω) =
50
=
jω(5 + jω)
10
jω ⎞
⎛
1 jω⎜1 + ⎟
5 ⎠
⎝
HdB
40
20 log1
20
10
0.1
-20
1
100
⎛
⎜
1
20 log⎜
⎜
jω
⎜ 1+
5
⎝
⎛ 1 ⎞
⎟
20 log⎜⎜
⎟
⎝ jω ⎠
-40
φ
0.1
-45°
⎞
⎟
⎟
⎟
⎟
⎠
ω
1
10
ω
100
arg
1
1 + jω / 5
-90°
arg
-135°
1
jω
-180°
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Chapter 14, Problem 11.
Sketch the Bode plots for
H( ω ) =
10 + jω
jω ( 2 + jω )
Chapter 14, Solution 11.
5 (1 + jω 10)
H (ω) =
jω (1 + jω 2)
H dB = 20 log10 5 + 20 log10 1 + jω 10 − 20 log10 jω − 20 log10 1 + jω 2
φ = -90° + tan -1 ω 10 − tan -1 ω 2
The magnitude and phase plots are shown below.
HdB
40
34
20
14
0.1
-20
1
10
100
ω
1
10
100
ω
-40
φ
90°
45°
0.1
-45°
-90°
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Chapter 14, Problem 12.
A transfer function is given by
T(s) =
s +1
s ( s + 10)
Sketch the magnitude and phase Bode plots.
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Chapter 14, Solution 12.
T ( w) =
0.1(1 + jω )
,
jω (1 + jω / 10)
20 log 0.1 = −20
The plots are shown below.
|T|
(db)
20
ω
0
0.1
1
10
100
-20
-40
arg T
90o
ω
0
0.1
1
10
100
-90o
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Chapter 14, Problem 13.
Construct the Bode plots for
G(s) =
s +1
,
s ( s + 10)
s=j ω
2
Chapter 14, Solution 13.
G (ω) =
(1 10)(1 + jω)
1 + jω
=
2
( jω) (10 + jω) ( jω) 2 (1 + jω 10)
G dB = -20 + 20 log10 1 + jω − 40 log10 jω − 20 log10 1 + jω 10
φ = -180° + tan -1ω − tan -1 ω 10
The magnitude and phase plots are shown below.
GdB
40
20
0.1
-20
1
10
100
ω
1
10
100
ω
-40
φ
90°
0.1
-90°
-180°
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Chapter 14, Problem 14.
Draw the Bode plots for
H( ω ) =
50( jω + 1)
jω (−ω 2 + 10 jω + 25)
Chapter 14, Solution 14.
50
H (ω) =
25
1 + jω
⎛ jω10 ⎛ jω ⎞ 2 ⎞
+ ⎜ ⎟ ⎟⎟
jω⎜⎜1 +
⎝5⎠ ⎠
25
⎝
H dB = 20 log10 2 + 20 log10 1 + jω − 20 log10 jω
− 20 log10 1 + jω2 5 + ( jω 5) 2
⎛ ω10 25 ⎞
⎟
φ = -90° + tan -1 ω − tan -1 ⎜
⎝ 1 − ω2 5 ⎠
The magnitude and phase plots are shown below.
HdB
40
26
20
6
0.1
-20
1
10
100
ω
1
10
100
ω
-40
φ
90°
0.1
-90°
-180°
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Chapter 14, Problem 15.
Construct the Bode magnitude and phase plots for
H(s) =
40( s + 1)
,
( s + 2)( s + 10)
s=j ω
Chapter 14, Solution 15.
H (ω) =
40 (1 + jω)
2 (1 + jω)
=
(2 + jω)(10 + jω) (1 + jω 2)(1 + jω 10)
H dB = 20 log10 2 + 20 log10 1 + jω − 20 log10 1 + jω 2 − 20 log10 1 + jω 10
φ = tan -1 ω − tan -1 ω 2 − tan -1 ω 10
The magnitude and phase plots are shown below.
HdB
40
20
6
0.1
-20
1
10
100
ω
1
10
100
ω
-40
φ
90°
45°
0.1
-45°
-90°
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of this Manual may be displayed, reproduced or distributed in any form or by any means, without the prior
written permission of the publisher, or used beyond the limited distribution to teachers and educators
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Chapter 14, Problem 16.
Sketch Bode magnitude and phase plots for
H(s) =
10
,
s ( s + s + 16)
s=j ω
2
Chapter 14, Solution 16.
10 /16
0.625
H (ω ) =
=
2
2
⎡
⎡
⎛ jω ⎞ ⎤
⎛ jω ⎞ ⎤
jω ⎢1 + jω + ⎜
⎟ ⎥ jω ⎢1 + jω + ⎜
⎟ ⎥
⎝ 4 ⎠ ⎥⎦
⎝ 4 ⎠ ⎥⎦
⎢⎣
⎢⎣
2
⎛ jω ⎞
H dB = 20 log 0.625 − 20 log | jω | −20 log |1 + jω + ⎜
⎟ |
⎝ 4 ⎠
(20log0,625= –4.082)
The magnitude and phase plots are shown below.
H
20
20 log (jω)
1
10
4
40
100
ω
0.1
–4.082
–20
⎛ jω ⎞
20 log 1 + jω + ⎜
⎟
⎝ 4 ⎠
–40
2
–60
φ
-90
-180
0.4
1
4
10
40
90°
100
-tan-1
ω
ω2
1−
ω
16
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Chapter 14, Problem 17.
Sketch the Bode plots for
G(s) =
s
,
( s + 2) + ( s + 1)
2
s=j ω
Chapter 14, Solution 17.
G (ω) =
(1 4) jω
(1 + jω)(1 + jω 2) 2
G dB = -20log10 4 + 20 log10 jω − 20 log10 1 + jω − 40 log10 1 + jω 2
φ = -90° - tan -1ω − 2 tan -1 ω 2
The magnitude and phase plots are shown below.
GdB
20
0.1
1
10
100
ω
-12
-20
-40
φ
90°
0.1
1
10
100
ω
-90°
-180°
PROPRIETARY MATERIAL. © 2007 The McGraw-Hill Companies, Inc. All rights reserved. No part
of this Manual may be displayed, reproduced or distributed in any form or by any means, without the prior
written permission of the publisher, or used beyond the limited distribution to teachers and educators
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Chapter 14, Problem 18.
A linear network has this transfer function
H(s) =
7s 2 + s + 4
,
( s 3 + 8s 2 + 14 s + 5)
s=j ω
Use MATLAB or equivalent to plot the magnitude and phase (in degrees) of the transfer
function. Take 0.1 < ω < 10 rads/s.
Chapter 14, Solution 18.
The MATLAB code is shown below.
>> w=logspace(-1,1,200);
>> s=i*w;
>> h=(7*s.^2+s+4)./(s.^3+8*s.^2+14*s+5);
>> Phase=unwrap(angle(h))*57.23;
>> semilogx(w,Phase)
>> grid on
60
40
H (jw ) P h a s e
20
0
-2 0
-4 0
-6 0
-1
10
10
w
0
10
1
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Now for the magnitude, we need to add the following to the above,
>> H=abs(h);
>> HdB=20*log10(H);
>> semilogx(w,HdB);
>> grid on
0
-5
HdB
-1 0
-1 5
-2 0
-2 5
-1
10
10
w
0
10
1
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Chapter 14, Problem 19.
Sketch the asymptotic Bode plots of the magnitude and phase for
H(s) =
100s
,
( s + 10)( s + 20)( s + 40)
s=j ω
Chapter 14, Solution 19.
H (ω ) =
100 jω
jω / 80
=
j
jω
jω
ω
( jω + 10)( jω + 20)( jω + 40) (1 +
)(1 +
)(1 +
)
10
20
40
H dB = 20 log(1/ 80) + 20 log | jω /1| −20 log |1 +
jω
jω
jω
| −20 log |1 +
| −20 log |1 +
|
10
20
40
(20log(1/80) = -38.06)
The magnitude and phase plots are shown below.
H
20 log jω
ω
0.1
1
20
20 log
10
20
40
1
80
100
20 log 1 +
40
60
φ
90
0.1
ω
1
2
4
10
20
40
100 200 400
-90
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of this Manual may be displayed, reproduced or distributed in any form or by any means, without the prior
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jω
10
Chapter 14, Problem 20.
Sketch the magnitude Bode plot for the transfer function
H( ω ) =
10 jω
( jω + 1)( jω + 5) 2 ( jω + 40)
Chapter 14, Solution 20.
H (ω ) =
10 jω
jω /100
=
2
(25)(40)(1 + jω )(1 + jω / 5) (1 + jω / 40) (1 + jω )(1 + jω / 5)2 (1 + jω / 40)
20log(1/100) = -40
The magnitude plot is shown below.
20 log jω
40
1
20 log
1+
20
0.1
jω
10
ω
1
5
10
50
100
20 log
-20
20 log
1
100
1
1 + jω
-40
-60
20 log
1
jω ⎞
⎛
⎜1 +
⎟
5 ⎠
⎝
PROPRIETARY MATERIAL. © 2007 The McGraw-Hill Companies, Inc. All rights reserved. No part
of this Manual may be displayed, reproduced or distributed in any form or by any means, without the prior
written permission of the publisher, or used beyond the limited distribution to teachers and educators
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2
Chapter 14, Problem 21.
Sketch the magnitude Bode plot for
H(s) =
s ( s + 20)
,
( s + 1)( s 2 + 60s ) = (400)
Chapter 14, Solution 21.
jω ( jω + 20)
=
H (ω ) =
( jω + 1)(−ω 2 + 60 jω + 400)
H(ω) =
0.05 jω(1 + jω / 20)
⎛ 6 jω ⎛ jω ⎞ 2 ⎞
(1 + jω)⎜1 +
+⎜ ⎟ ⎟
⎜
40 ⎝ 20 ⎠ ⎟
⎠
⎝
s=j ω
20 jω (1 + jω / 20)
2
⎛ jω ⎞
400( jω + 1)(1 + 60 jω / 400 + ⎜
⎟ )
⎝ 20 ⎠
H dB = 20 log(0.05) + 20 log jω + 20 log 1 +
jω
j6ω ⎛ jω ⎞
− 20 log 1 + jω − 20 log 1 +
+⎜ ⎟
20
40 ⎝ 20 ⎠
2
The magnitude plot is as sketched below.
H&B
40
20log|jω|
20 log |1+jω/20|
20
0.1
–20
–40
–60
ω
1
10
20
100
20 log 0.05
–20 log 1 + jω
j 6ω ⎛ jω ⎞
+⎜
–20 log 1 +
⎟
40 ⎝ 20 ⎠
–80
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2
Chapter 14, Problem 22.
Find the transfer function H( ω ) with the Bode magnitude plot shown in Fig. 14.74.
Figure 14.74
For Prob. 14.22.
Chapter 14, Solution 22.
20 = 20 log10 k
⎯
⎯→ k = 10
A zero of slope + 20 dB / dec at ω = 2 ⎯
⎯→ 1 + jω 2
1
A pole of slope - 20 dB / dec at ω = 20 ⎯
⎯→
1 + jω 20
1
A pole of slope - 20 dB / dec at ω = 100 ⎯
⎯→
1 + jω 100
Hence,
H (ω) =
10 (1 + jω 2)
(1 + jω 20)(1 + jω 100)
10 4 ( 2 + jω)
H (ω) =
( 20 + jω)(100 + jω)
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Chapter 14, Problem 23.
The Bode magnitude plot of H( ω ) is shown in Fig. 14.75. Find H( ω ).
Figure 14.75
For Prob. 14.23.
Chapter 14, Solution 23.
A zero of slope + 20 dB / dec at the origin
⎯
⎯→
jω
1
1 + jω 1
1
A pole of slope - 40 dB / dec at ω = 10 ⎯
⎯→
(1 + jω 10) 2
A pole of slope - 20 dB / dec at ω = 1 ⎯
⎯→
Hence,
H (ω) =
jω
(1 + jω)(1 + jω 10) 2
H (ω) =
100 jω
(1 + jω)(10 + jω) 2
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Chapter 14, Problem 24.
The magnitude plot in Fig. 14.76 represents the transfer function of a preamplifier. Find
H(s).
Figure 14.76
For Prob. 14.24.
Chapter 14, Solution 24.
40 = 20 log10 K
⎯⎯
→ K = 100
There is a pole at ω=50 giving 1/(1+jω/50)
There is a zero at ω=500 giving (1 + jω/500).
There is another pole at ω=2122 giving 1/(1 + jω/2122).
Thus,
1
40 x
( s + 500)
40(1 + jω / 500)
500
H (ω ) =
=
(1 + jω / 50)(1 + jω / 2122) 1 x 1 ( s + 50)( s + 2122)
50 2122
or
H (s) =
8488( s + 500)
( s + 50)( s + 2122)
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Chapter 14, Problem 25.
A series RLC network has R = 2 k Ω , L = 40 mH, and C = 1 µ F. Calculate the
impedance at resonance and at one-fourth, one-half, twice, and four times the resonant
frequency.
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Chapter 14, Solution 25.
1
1
ω0 =
=
= 5 krad / s
-3
LC
(40 × 10 )(1 × 10 -6 )
Z(ω0 ) = R = 2 kΩ
⎛ ω0
4 ⎞
⎟
Z(ω0 4) = R + j ⎜ L −
ω0 C ⎠
⎝ 4
⎛ 5 × 10 3
⎞
4
⎟
Z(ω0 4) = 2000 + j ⎜
⋅ 40 × 10 -3 −
3
-6
(5 × 10 )(1 × 10 ) ⎠
⎝ 4
Z(ω0 4) = 2000 + j (50 − 4000 5)
Z(ω0 4) = 2 − j0.75 kΩ
⎛ ω0
2 ⎞
⎟
Z(ω0 2) = R + j ⎜ L −
ω0 C ⎠
⎝ 2
⎛ (5 × 10 3 )
⎞
2
⎟
(40 × 10 -3 ) −
Z(ω0 2) = 2000 + j ⎜
3
-6
2
(5 × 10 )(1 × 10 ) ⎠
⎝
Z(ω0/2) = 200+j(100-2000/5)
Z(ω0 2) = 2 − j0.3 kΩ
⎛
1 ⎞
⎟
Z(2ω0 ) = R + j ⎜ 2ω0 L −
2ω0 C ⎠
⎝
⎛
⎞
1
⎟
Z(2ω0 ) = 2000 + j ⎜ (2)(5 × 10 3 )(40 × 10 -3 ) −
3
-6
(2)(5 × 10 )(1 × 10 ) ⎠
⎝
Z(2ω0 ) = 2 + j0.3 kΩ
⎛
1 ⎞
⎟
Z(4ω0 ) = R + j ⎜ 4ω0 L −
4ω0 C ⎠
⎝
⎛
⎞
1
⎟
Z(4ω0 ) = 2000 + j ⎜ (4)(5 × 10 3 )(40 × 10 -3 ) −
3
-6
(4)(5 × 10 )(1 × 10 ) ⎠
⎝
Z(4ω0 ) = 2 + j0.75 kΩ
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Chapter 14, Problem 26.
A coil with resistance 3 Ω and inductance 100 mH is connected in series with a capacitor
of 50 pF, a resistor of 6 Ω and a signal generator that gives 110 V rms at all frequencies.
Calculate ω o , Q, and B at resonance of the resultant series RLC circuit.
Chapter 14, Solution 26.
Consider the circuit as shown below. This is a series RLC resonant circuit.
6Ω
50 µF
3Ω
+
_
100 mH
R=6+3=9Ω
1
1
=
= 447.21 krad/s
LC
100 x10−3 x50 x10−12
ωo =
Q=
ωo L
B=
ωo
R
Q
=
=
447.21x103 x100 x103
= 4969
9
447.21x103
= 90 rad/s
4969
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Chapter 14, Problem 27.
Design a series RLC resonant circuit with ωo = 40 rad/s and B = 10 rad/s.
Chapter 14, Solution 27.
1
= 40
LC
ωo =
⎯⎯
→ LC =
1
402
R
= 10
⎯⎯
→ R = 10 L
L
If we select R =1 Ω, then L = R/10 = 0.1 H and
B=
C=
1
1
= 2
= 6.25 mF
2
40 L 40 x0.1
Chapter 14, Problem 28.
Design a series RLC circuit with B = 20 rad/s and ωo = 1,000 rad/s. Find the circuit’s Q.
Let R = 10 Ω .
Chapter 14, Solution 28.
Let R = 10 Ω .
L=
R 10
=
= 0.5 H
B 20
C=
1
1
=
= 2 µF
2
ω0 L (1000) 2 (0.5)
Q=
ω0 1000
=
= 50
B
20
Therefore, if R = 10 Ω then
L = 0.5 H ,
C = 2 µF ,
Q = 50
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Chapter 14, Problem 29.
Let v s = 20 cos(at) V in the circuit of Fig. 14.77. Find ωo , Q, and B, as seen by the
capacitor.
Figure 14.77
For Prob. 14.29.
Chapter 14, Solution 29.
We convert the voltage source to a current source as shown below.
12 k
is
is =
45 k
1 µF
60 mH
20
cos ωt , R = 12//45= 12x45/57 = 9.4737 kΩ
12
1
1
ωo =
=
= 4.082 krad/s
LC
60 x10−3 x1x10−6
1
1
=
= 105.55 rad/s
RC 9.4737 x103 x10−6
ω
4082
Q= o =
= 38.674 = 38.67
B 105.55
B=
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Chapter 14, Problem 30.
A circuit consisting of a coil with inductance 10 mH and resistance 20 Ω is connected in
series with a capacitor and a generator with an rms voltage of 120 V. Find:
(a) the value of the capacitance that will cause the circuit to be in resonance at 15 kHz
(b) the current through the coil at resonance
(c) the Q of the circuit
Chapter 14, Solution 30.
Select R = 10 Ω .
R
10
=
= 0.05 H = 50 mH
ω0 Q (10)(20)
1
1
C= 2 =
= 0.2 F
ω0 L (100)(0.05)
1
1
B=
=
= 0.5 rad / s
RC (10)(0.2)
L=
Therefore, if R = 10 Ω then
L = 50 mH, C = 0.2 F ,
B = 0.5 rad / s
Chapter 14, Problem 31.
Design a parallel resonant RLC circuit with ω o = 10rad/s and Q = 20. Calculate the
bandwidth of the circuit. Let R = 10 Ω .
Chapter 14, Solution 31.
X L = ωL
B=
⎯⎯→
L=
XL
ω
R ωR 2πx10 x10 6 x 5.6 x10 3
=
=
= 8.796 x10 6 rad/s
3
L XL
40 x10
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Chapter 14, Problem 32.
A parallel RLC circuit has the following values:
R = 60 Ω , L = 1 mH, and C = 50 µF .
Find the quality factor, the resonant frequency, and the bandwidth of the RLC circuit.
Chapter 14, Solution 32.
1
1
=
= 4.472 krad/s
LC
10−3 x50 x10−6
1
1
B=
=
= 333.33 rad/s
RC 60 x50 x10−6
ω
4472
Q= o =
= 13.42
B 333.33
ωo =
Chapter 14, Problem 33.
A parallel resonant circuit with quality factor 120 has a resonant frequency of 6 × 10 6
rad/s. Calculate the bandwidth and half-power frequencies.
Chapter 14, Solution 33.
Q = ωo RC
R
Q=
ωo L
⎯⎯→
⎯
⎯→
C=
Q
80
=
= 56.84 pF
2πf o R 2πx5.6x10 6 x 40x10 3
R
40 x10 3
L=
=
= 14.21 µH
2πf o Q 2πx 5.6 x10 6 x80
Chapter 14, Problem 34.
A parallel RLC circuit is resonant at 5.6 MHz, has a Q of 80, and has a resistive branch of
40 k Ω . Determine the values of L and C in the other two branches.
Chapter 14, Solution 34.
1
1
(a)
ωo =
=
= 1.443 krad/s
−3
−6
LC
8x10 x 60x10
1
1
=
= 3.33 rad/s
RC 5x10 3 x 60x10 − 6
(b)
B=
(c)
Q = ωo RC = 1.443x10 3 x 5x10 3 x 60x10 −6 = 432.9
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Chapter 14, Problem 35.
A parallel RLC circuit has R = 5k Ω , L = 8 mH, and C = µF . Determine:
(a) the resonant frequency
(b) the bandwidth
(c) the quality factor
Chapter 14, Solution 35.
At resonance,
1
1
=
= 40 Ω
Y 25 × 10 -3
Q
80
Q = ω0 RC ⎯
⎯→ C =
=
= 10 µF
ω0 R (200 × 10 3 )(40)
1
1
1
ω0 =
⎯
⎯→ L = 2 =
= 2.5 µH
10
ω0 C (4 × 10 )(10 × 10 -6 )
LC
Y=
1
R
⎯
⎯→ R =
ω0 200 × 10 3
=
= 2.5 krad / s
Q
80
B
ω1 = ω0 − = 200 − 1.25 = 198.75 krad/s
2
B
ω 2 = ω0 + = 200 + 1.25 = 201.25 krad/s
2
B=
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Chapter 14, Problem 36.
It is expected that a parallel RLC resonant circuit has a midband admittance of 25 ×
110 −3 S, quality factor of 80, and a resonant frequency of 200 krad/s. Calculate the
values of R, L, and C. Find the bandwidth and the half-power frequencies.
Chapter 14, Solution 36.
ω0 =
1
LC
= 5000 rad / s
Y(ω0 ) =
1
R
⎯
⎯→ Z(ω0 ) = R = 2 kΩ
⎛ω
4 ⎞
1
⎟ = 0.5 − j18.75 mS
+ j ⎜⎜ 0 C −
ω 0 L ⎟⎠
R
⎝ 4
1
Z(ω0 4) =
= 1.4212 + j53.3 Ω
0.0005 − j0.01875
Y ( ω 0 4) =
⎛ω
2 ⎞
1
⎟ = 0.5 − j7.5 mS
+ j ⎜⎜ 0 C −
ω0 L ⎟⎠
R
⎝ 2
1
Z(ω0 2) =
= 8.85 + j132.74 Ω
0.0005 − j0.0075
Y ( ω 0 2) =
⎛
1 ⎞
1
⎟ = 0.5 + j7.5 mS
+ j ⎜⎜ 2ω 0 L −
2ω0 C ⎟⎠
R
⎝
Z(2ω0 ) = 8.85 − j132.74 Ω
Y ( 2ω 0 ) =
⎛
1 ⎞
1
⎟ = 0.5 + j18.75 mS
+ j ⎜⎜ 4ω 0 L −
4ω 0 C ⎟⎠
R
⎝
Z(4ω0 ) = 1.4212 − j53.3 Ω
Y ( 4ω 0 ) =
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Chapter 14, Problem 37.
Rework Prob. 14.25 if the elements are connected in parallel.
Chapter 14, Solution 37.
1 ⎞
⎛L
⎞⎛
)⎟
⎜ + jωLR ⎟⎜ R − j(ωL −
1
ωC ⎠
C
⎝
⎠
⎝
=
)=
Z = jωL //( R +
1 2
1
jω C
+ jω L
R 2 + (ωL −
)
R+
jωC
ωC
jωL(R +
1
)
jω C
L⎛
1 ⎞
⎜ ωL −
⎟
C⎝
ωC ⎠
=0
1 2
2
R + ( ωL −
)
ωC
ωLR 2 −
Im(Z) =
⎯
⎯→
ω 2 ( LC − R 2 C 2 ) = 1
Thus,
ω=
1
LC − R 2 C 2
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Chapter 14, Problem 38.
Find the resonant frequency of the circuit in Fig. 14.78.
Figure 14.78
For Prob. 14.38.
Chapter 14, Solution 38.
Y=
1
R − jω L
+ jωC = jωC +
R + jωL
R 2 + ω 2 L2
At resonance, Im(Y) = 0 , i.e.
ω0 L
ω0 C − 2
=0
R + ω02 L2
L
R 2 + ω02 L2 =
C
ω0 =
1
R2
−
=
LC L2
⎛ 50 ⎞
⎟⎟
− ⎜⎜
(40 × 10 -3 )(1 × 10 -6 ) ⎝ 40 × 10 -3 ⎠
1
2
ω0 = 4841 rad / s
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Chapter 14, Problem 39.
For the “tank” circuit in Fig. 14.79, find the resonant frequency.
Figure 14.79
For Probs. 14.39 and 14.91.
Chapter 14, Solution 39.
(a)
B = ω 2 − ω1 = 2π(f 2 − f1 ) = 2π(90 − 86) x10 3 = 8πkrad / s
1
(ω1 + ω 2 ) = 2π(88) x10 3 = 176πX10 3
2
1
1
1
B=
⎯
⎯→ C =
=
= 19.89nF
RC
BR 8πx10 3 x 2x10 3
ωo =
1
(b)
ωo =
(c )
ωo = 176π = 552.9krad / s
(d)
B = 8π = 25.13krad / s
(e)
Q=
LC
⎯
⎯→
L=
1
ω2 o C
=
1
(176πX10 3 ) 2 x19.89 x10 − 9
= 164.45 µH
ωo 176π
= 22
=
B
8π
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Chapter 14, Problem 40.
A parallel resonance circuit has a resistance of 2 k Ω and half-power frequencies of 86
kHz and 90 kHz. Determine:
(a) the capacitance
(b) the inductance
(c) the resonant frequency
(d) the bandwidth
(e) the quality factor
Chapter 14, Solution 40.
(a)
L = 5 + 10 = 15 mH
ω0 =
1
LC
=
1
15x10 − 3 x 20x10 − 6
= 1.8257 k rad/sec
Q = ω0 RC = 1.8257 x10 3 x 25x10 3 x 20x10 −6 = 912.8
1
1
=
= 2 rad/s
3
RC 25x10 20x10 − 6
B=
(b)
To increase B by 100% means that B’ = 4.
C′ =
Since C′ =
1
1
=
= 10 µF
RB′ 25x10 3 x 4
C1C 2
= 10µF and C1 = 20 µF, we then obtain C2 = 20 µF.
C1 + C 2
Therefore, to increase the bandwidth, we merely add another 20 µF in series
with the first one.
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Chapter 14, Problem 41.
For the circuit shown in Fig. 14.80, next page:
(a) Calculate the resonant frequency ω o , the quality factor Q, and the bandwidth B.
(b) What value of capacitance must be connected in series with the 20- µF capacitor in
order to double the bandwidth?
Figure 14.80
For Prob. 14.41.
Chapter 14, Solution 41.
(a)
This is a series RLC circuit.
R = 2+ 6 = 8Ω,
L =1H,
ω0 =
(b)
1
LC
=
1
0.4
C = 0.4 F
= 1.5811 rad / s
Q=
ω 0 L 1.5811
=
= 0.1976
R
8
B=
R
= 8 rad / s
L
This is a parallel RLC circuit.
(3)(6)
3 µF and 6 µF ⎯
⎯→
= 2 µF
3+ 6
C = 2 µF ,
R = 2 kΩ ,
L = 20 mH
ω0 =
1
LC
=
1
(2 × 10 )(20 × 10 -3 )
-6
= 5 krad / s
R
2 × 10 3
Q=
=
= 20
ω0 L (5 × 10 3 )(20 × 10 -3 )
B=
1
1
=
= 250 rad/s
3
RC (2 × 10 )(2 × 10 -6 )
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Chapter 14, Problem 42.
For the circuits in Fig. 14.81, find the resonant frequency ω o , the quality factor Q, and
the bandwidth B.
Figure 14.81
For Prob. 14.42.
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Chapter 14, Solution 42.
(a)
Z in = (1 jωC) || (R + jωL)
Z in =
R + jωL
jωC
=
R + jωL
1 − ω2 LC + jωRC
1
jωC
(R + jωL)(1 − ω2 LC − jωRC)
Z in =
(1 − ω2 LC) 2 + ω2 R 2 C 2
R + jωL +
At resonance, Im(Z in ) = 0 , i.e.
0 = ω 0 L(1 − ω 02 LC) − ω 0 R 2 C
ω02 L2 C = L − R 2 C
ω0 =
(b)
L − R 2C
L2 C
=
1
R2
−
LC L2
Z in = R || ( jωL + 1 jωC)
Z in =
R ( jωL + 1 jωC)
R (1 − ω2 LC)
=
R + jωL + 1 jωC (1 − ω 2 LC) + jωRC
Z in =
R (1 − ω2 LC)[(1 − ω2 LC) − jωRC]
(1 − ω2 LC) 2 + ω2 R 2 C 2
At resonance, Im(Z in ) = 0 , i.e.
0 = R (1 − ω2 LC) ωRC
1 − ω2 LC = 0
1
ω0 =
LC
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Chapter 14, Problem 43.
Calculate the resonant frequency of each of the circuits in Fig. 14.82.
Figure 14.82
For Prob. 14.43.
Chapter 14, Solution 43.
Consider the circuit below.
1/jωC
Zin
(a)
R1
jωL
R2
Z in = (R 1 || jωL) || (R 2 + 1 jωC)
⎛ R 1 jωL ⎞ ⎛
1 ⎞
⎟ || ⎜ R 2 +
⎟
Z in = ⎜
jωC ⎠
⎝ R 1 + jωL ⎠ ⎝
jωR 1 L ⎛
1 ⎞
⎟
⋅ ⎜R 2 +
R 1 + jωL ⎝
jωC ⎠
Z in =
jR 1ωL
1
R2 +
+
jωC R 1 + jωL
jωR 1 L (1 + jωR 2 C)
Z in =
(R 1 + jωL)(1 + jωR 2 C) − ω2 LCR 1
- ω2 R 1 R 2 LC + jωR 1 L
Z in =
R 1 − ω2 LCR 1 − ω2 LCR 2 + jω (L + R 1 R 2 C)
(-ω2 R 1 R 2 LC + jωR 1 L)[R 1 − ω2 LCR 1 − ω2 LCR 2 − jω (L + R 1 R 2 C)]
Z in =
(R 1 − ω2 LCR 1 − ω2 LCR 2 ) 2 + ω2 (L + R 1 R 2 C) 2
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At resonance, Im(Z in ) = 0 , i.e.
0 = ω3 R 1 R 2 LC (L + R 1 R 2 C) + ωR 1 L (R 1 − ω2 LCR 1 − ω2 LCR 2 )
0 = ω3 R 12 R 22 LC 2 + R 12 ωL − ω3 R 12 L2 C
0 = ω2 R 22 C 2 + 1 − ω2 LC
ω2 (LC − R 22 C 2 ) = 1
ω0 =
1
LC − R 22 C 2
ω0 =
1
(0.02)(9 × 10 -6 ) − (0.1) 2 (9 × 10 -6 ) 2
ω0 = 2.357 krad / s
(b)
At ω = ω0 = 2.357 krad / s ,
jωL = j(2.357 × 10 3 )(20 × 10 -3 ) = j47.14
R 1 || jωL =
R2 +
j47.14
= 0.9996 + j0.0212
1 + j47.14
1
1
= 0.1 +
= 0.1 − j47.14
jωC
j (2.357 × 10 3 )(9 × 10 -6 )
Z in (ω0 ) = (R 1 || jωL) || (R 2 + 1 jωC)
(0.9996 + j0.0212)(0.1 − j47.14)
(0.9996 + j0.0212) + (0.1 − j47.14)
Z in (ω0 ) = 1 Ω
Z in (ω0 ) =
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Chapter 14, Problem 44.
* For the circuit in Fig. 14.83, find:
(a) the resonant frequency ω o
(b) Z in ( ω o )
Figure 14.83
For Prob. 14.44.
* An asterisk indicates a challenging problem.
Chapter 14, Solution 44.
We find the input impedance of the circuit shown below.
1
Z
jω(2/3)
1/jω
1/jωC
1
3
1
=
+ jω +
,
Z jω2
1 + 1 jωC
ω=1
1
jC
C 2 + jC
= -j1.5 + j +
= -j0.5 +
Z
1 + jC
1+ C2
v( t ) and i( t ) are in phase when Z is purely real, i.e.
C
0 = -0.5 +
⎯
⎯→ (C − 1) 2 = 1
or
1 + C2
1
C2
1
=
2 =
Z 1+ C
2
C = 1F
⎯
⎯→ Z = 2 Ω
V = Z I = (2)(10) = 20
v( t ) = 20 sin( t ) V , i.e.
Vo = 20 V
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Chapter 14, Problem 45.
For the circuit shown in Fig. 14.84, find ω o , B, and Q, as seen by the voltage across the
inductor.
Figure 14.84
For Prob. 14.45.
Chapter 14, Solution 45.
Convert the voltage source to a current source as shown below.
Is
30 kΩ
50 µF
10 mH
50 kΩ
R = 30//50 = 30x50/80 = 18.75 kΩ
This is a parallel resonant circuit.
1
1
ωo =
=
= 447.21 rad/s
−3
LC
10 x10 x50 x10−6
1
1
B=
=
= 1.067 rad/s
RC 18.75 x103 x50 x10−6
ω 447.21
Q= o =
= 419.13
B
1.067
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Chapter 14, Problem 46.
For the network illustrated in Fig. 14.85, find
(a) the transfer function H( ω ) = V o ( ω )/I( ω ),
(b) the magnitude of H at ω o = 1 rad/s.
Figure 14.85
For Probs. 14.46, 14.78, and 14.92.
Chapter 14, Solution 46.
(a) This is an RLC series circuit.
ωo =
1
⎯⎯→
LC
(b)
(c )
C=
1
ω2 o L
=
1
(2πx15x10 3 ) 2 x10x10 −3
= 11.26nF
Z = R, I = V/Z = 120/20 = 6 A
Q=
ωo L 2πx15x10 3 x10 x10 −3
=
= 15π = 47.12
R
20
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Chapter 14, Problem 47.
Show that a series LR circuit is a lowpass filter if the output is taken across the resistor.
Calculate the corner frequency f c if L = 2 mH and R = 10 k Ω .
Chapter 14, Solution 47.
H (ω) =
Vo
R
1
=
=
Vi R + jωL 1 + jωL R
H(0) = 1 and H(∞) = 0 showing that this circuit is a lowpass filter.
1
At the corner frequency, H(ωc ) =
, i.e.
2
ωc L
1
1
R
=
⎯
⎯→
=
or
ωc =
1
2
R
L
2
⎛ωc L ⎞
⎟
1+ ⎜
⎝ R ⎠
Hence,
ωc =
R
= 2πf c
L
fc =
1 R
1 10 × 10 3
⋅ =
⋅
= 796 kHz
2π L 2π 2 × 10 -3
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Chapter 14, Problem 48.
Find the transfer function V o /V s of the circuit in Fig. 14.86. Show that the circuit is a
lowpass filter.
Figure 14.86
For Prob. 14.48.
Chapter 14, Solution 48.
R ||
H (ω) =
1
jωC
1
jωC
R jωC
R + 1 jωC
H (ω) =
R jωC
jωL +
R + 1 jωC
R
H (ω) =
R + jωL − ω 2 RLC
jωL + R ||
H(0) = 1 and H(∞) = 0 showing that this circuit is a lowpass filter.
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Chapter 14, Problem 49.
Determine the cutoff frequency of the lowpass filter described by
H( ω ) =
4
2 + jω10
Find the gain in dB and phase of H( ω ) at ω = 2 rad/s.
Chapter 14, Solution 49.
At dc, H(0) =
Hence,
H(ω) =
2
2
=
1
2
4
= 2.
2
H(0) =
2
2
4
4 + 100ωc2
4 + 100ωc2 = 8 ⎯
⎯→ ωc = 0.2
H(2) =
4
2
=
2 + j20 1 + j10
H(2) =
2
101
= 0.199
In dB, 20 log10 H(2) = - 14.023
arg H(2) = -tan -110 = - 84.3°
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Chapter 14, Problem 50.
Determine what type of filter is in Fig. 14.87. Calculate the corner frequency f c .
Figure 14.87
For Prob. 14.50.
Chapter 14, Solution 50.
H (ω) =
Vo
jωL
=
Vi R + jωL
H(0) = 0 and H(∞) = 1 showing that this circuit is a highpass filter.
H (ωc ) =
or
fc =
1
2
ωc =
=
1
⎛ R ⎞
⎟
1+ ⎜
⎝ ωc L ⎠
2
⎯
⎯→ 1 =
R
ωc L
R
= 2πf c
L
1 R
1 200
⋅ =
⋅
= 318.3 Hz
2π L 2π 0.1
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Chapter 14, Problem 51.
Design an RL lowpass filter that uses a 40-mH coil and has a cutoff frequency of 5 kHz.
Chapter 14, Solution 51.
The lowpass RL filter is shown below.
L
+
+
vs
R
vo
-
-
H=
ωc =
R
= 2πf c
L
Vo
R
1
=
=
Vs R + jωL 1 + jωL / R
⎯⎯→
R = 2πf c L = 2πx 5x10 3 x 40x10 −3 = 1.256kΩ
Chapter 14, Problem 52.
In a highpass RL filter with a cutoff frequency of 100 kHz, L = 40 mH. Find R.
Chapter 14, Solution 52.
ωc =
R
= 2πf c
L
R = 2πf c L = (2π)(10 5 )(40 × 10 -3 ) = 25.13 kΩ
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Chapter 14, Problem 53.
Design a series RLC type bandpass filter with cutoff frequencies of 10 kHz and 11 kHz.
Assuming C = 80 pF, find R, L, and Q.
Chapter 14, Solution 53.
ω1 = 2πf 1 = 20π × 10 3
ω2 = 2πf 2 = 22π × 10 3
B = ω2 − ω1 = 2π × 10 3
ω2 + ω1
ω0 =
= 21π × 10 3
2
ω0 21π
Q=
=
= 10.5
B
2π
ω0 =
L=
1
LC
⎯
⎯→ L =
1
ω02 C
1
= 2.872 H
(21π × 10 ) (80 × 10 -12 )
3 2
R
⎯
⎯→ R = BL
L
R = (2π × 10 3 )(2.872) = 18.045 kΩ
B=
Chapter 14, Problem 54.
Design a passive bandstop filter with ω o = 10 rad/s and Q = 20.
Chapter 14, Solution 54.
This is an open-ended problem with several possible solutions. We may choose the
bandstop filter in Fig. 14.38.
1
= 10
⎯⎯
→ LC = 0.01
LC
ωL
⎯⎯
→ L = 2R
Q = o = 10 L = 20
R
R
ωo =
If we select L = 1H, then R=0.5 Ω, and C = 0.01/L = 10 mF.
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Chapter 14, Problem 55.
Determine the range of frequencies that will be passed by a series RLC bandpass filter
with R = 10 Ω , L = 25mH, and C = 0.4 µ F. Find the quality factor.
Chapter 14, Solution 55.
ωo =
1
LC
=
1
(25 × 10 −3 )(0.4 × 10 − 6 )
B=
R
10
=
= 0.4 krad / s
L 25 × 10 -3
Q=
10
= 25
0.4
= 10 krad / s
ω1 = ωo − B 2 = 10 − 0.2 = 9.8 krad / s
or
ω2 = ωo + B 2 = 10 + 0.2 = 10.2 krad / s
or
9.8
= 1.56 kHz
2π
10.2
= 1.62 kHz
f2 =
2π
f1 =
Therefore,
1.56 kHz < f < 1.62 kHz
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Chapter 14, Problem 56.
(a) Show that for a bandpass filter,
H(s) =
sB
, s = jw
s + sB + ω 02
2
where B = bandwidth of the filter and ω o is the center frequency.
(b) Similarly, show that for a bandstop filter,
s 2 + ω 02
H(s) = 2
, s = jw
s + sB + ω 02
Chapter 14, Solution 56.
(a)
From Eq 14.54,
R
R
sRC
L
H (s) =
=
=
R
1
1 1 + sRC + s 2 LC
2
s +s +
R + sL +
L LC
sC
s
R
1
,
and ω0 =
L
LC
sB
H (s) = 2
s + sB + ω02
Since B =
(b)
From Eq. 14.56,
H (s) =
sL +
1
sC
R + sL +
H (s) =
1
sC
=
s2 +
s2 + s
1
LC
R
1
+
L LC
s 2 + ω02
s 2 + sB + ω02
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Chapter 14, Problem 57.
Determine the center frequency and bandwidth of the bandpass filters in Fig. 14.88.
Figure 14.88
For Prob. 14.57.
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Chapter 14, Solution 57.
(a)
Consider the circuit below.
I
Vs
+
−
R
I1
1/sC
+
1/sC
R
Vo
−
1 ⎛
1⎞
⎜R + ⎟
1⎞
1 ⎛
sC ⎝
sC ⎠
|| ⎜ R + ⎟ = R +
Z(s) = R +
2
sC ⎝
sC ⎠
R+
sC
1 + sRC
Z(s) = R +
sC (2 + sRC)
Z(s) =
I=
1 + 3sRC + s 2 R 2 C 2
sC (2 + sRC)
Vs
Z
I1 =
Vs
1 sC
I=
2 sC + R
Z (2 + sRC)
Vo = I 1 R =
H (s) =
R Vs
sC (2 + sRC)
⋅
2 + sRC 1 + 3sRC + s 2 R 2 C 2
Vo
sRC
=
Vs 1 + 3sRC + s 2 R 2 C 2
⎡
3
s
⎢
1
RC
H (s) = ⎢
3
1
3 2
⎢s +
s+ 2 2
⎣
RC
R C
1
Thus, ω02 = 2 2 or
R C
B=
⎤
⎥
⎥
⎥
⎦
ω0 =
1
= 1 rad / s
RC
3
= 3 rad / s
RC
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(b)
Similarly,
Z(s) = sL + R || (R + sL) = sL +
Z(s) =
I=
R (R + sL)
2R + sL
R 2 + 3sRL + s 2 L2
2R + sL
Vs
,
Z
I1 =
Vo = I 1 ⋅ sL =
R Vs
R
I=
2R + sL
Z (2R + sL)
sLR Vs
2R + sL
⋅ 2
2R + sL R + 3sRL + s 2 L2
1 ⎛ 3R ⎞
⎜
s⎟
Vo
sRL
3⎝ L ⎠
H (s) =
=
=
3R
R2
Vs R 2 + 3sRL + s 2 L2
s2 +
s+ 2
L
L
Thus, ω0 =
B=
R
= 1 rad / s
L
3R
= 3 rad / s
L
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Chapter 14, Problem 58.
The circuit parameters for a series RLC bandstop filter are R = 2 k Ω , L = 0.1 H, C = 40
pF. Calculate:
(a) the center frequency
(b) the half-power frequencies
(c) the quality factor
Chapter 14, Solution 58.
(a)
(b)
ω0 =
1
LC
=
1
(0.1)(40 × 10 )
-12
= 0.5 × 10 6 rad / s
R 2 × 10 3
B= =
= 2 × 10 4
L
0 .1
ω
0.5 × 10 6
Q= 0 =
= 25
B
2 × 10 4
As a high Q circuit,
B
ω1 = ω0 − = 10 4 (50 − 1) = 490 krad / s
2
B
ω2 = ω0 + = 10 4 (50 + 1) = 510 krad / s
2
(c)
As seen in part (b),
Q = 25
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Chapter 14, Problem 59.
Find the bandwidth and center frequency of the bandstop filter of Fig. 14.89.
Figure 14.89
For Prob. 14.59.
Chapter 14, Solution 59.
Consider the circuit below.
Ro
+
1/sC
Vi
+
−
R
Vo
sL
−
⎛
1 ⎞ R (sL + 1 sC)
Z(s) = R || ⎜sL + ⎟ =
⎝
sC ⎠ R + sL + 1 sC
R (1 + s 2 LC)
Z(s) =
1 + sRC + s 2 LC
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Vo
R (1 + s 2 LC)
Z
=
=
H=
Vi Z + R o R o + sRR o C + s 2 LCR o + R + s 2 LCR
Z in = R o + Z = R o +
R (1 + s 2 LC)
1 + sRC + s 2 LC
R o + sRR o C + s 2 LCR o + R + s 2 LCR
Z in =
1 + sRC + s 2 LC
s = jω
R o + jωRR o C − ω2 LCR o + R − ω2 LCR
Z in =
1 − ω2 LC + jωRC
Z in =
(R o + R − ω2 LCR o − ω2 LCR + jωRR o C)(1 − ω2 LC − jωRC)
(1 − ω2 LC) 2 + (ωRC) 2
Im(Z in ) = 0 implies that
- ωRC [R o + R − ω2 LCR o − ω2 LCR ] + ωRR o C (1 − ω2 LC) = 0
R o + R − ω2 LCR o − ω2 LCR − R o + ω2 LCR o = 0
ω2 LCR = R
1
ω0 =
=
LC
H=
1
(1 × 10 )(4 × 10 -6 )
-3
R (1 − ω2 LC)
R o + jωRR o C + R − ω2 LCR o − ω2 LCR
H max = H(0) =
or
= 15.811 krad / s
H max
R
Ro + R
⎛ 1
⎞
R ⎜ 2 − LC ⎟
R
⎝ω
⎠
=
= H(∞) = lim
ω→ ∞ R o + R
RR o C
+j
− LC (R + R o ) R + R o
2
ω
ω
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At ω1 and ω2 , H =
R
2 (R o + R )
1
2
1
2
0=
=
=
=
1
2
H mzx
R (1 − ω2 LC)
R o + R − ω2 LC (R o + R ) + jωRR o C
(R o + R )(1 − ω2 LC)
(ωRR o C) 2 + (R o + R − ω2 LC(R o + R )) 2
10 (1 − ω2 ⋅ 4 × 10 -9 )
(96 × 10 -6 ω) 2 + (10 − ω2 ⋅ 4 × 10 -8 ) 2
10 (1 − ω2 ⋅ 4 × 10 -9 )
(96 × 10 ω) + (10 − ω ⋅ 4 × 10 )
-6
2
2
-8 2
−
1
2
(10 − ω2 ⋅ 4 × 10 -8 )( 2 ) − (96 × 10 -6 ω) 2 + (10 − ω2 ⋅ 4 × 10 -8 ) 2 = 0
(2)(10 − ω2 ⋅ 4 × 10 -8 ) 2 = (96 × 10 -6 ω) 2 + (10 − ω2 ⋅ 4 × 10 -8 ) 2
(96 × 10 -6 ω) 2 − (10 − ω2 ⋅ 4 × 10 -8 ) 2 = 0
1.6 × 10 -15 ω4 − 8.092 × 10 -7 ω2 + 100 = 0
ω4 − 5.058 × 10 8 + 6.25 × 1016 = 0
⎧ 2.9109 × 10 8
2
ω =⎨
⎩ 2.1471 × 10 8
Hence,
ω1 = 14.653 krad / s
ω2 = 17.061 krad / s
B = ω2 − ω1 = 17.061 − 14.653 = 2.408 krad / s
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Chapter 14, Problem 60.
Obtain the transfer function of a highpass filter with a passband gain of 10 and a cutoff
frequency of 50 rad/s.
Chapter 14, Solution 60.
H ′(ω) =
jωRC
jω
=
1 + jωRC jω + 1 RC
(from Eq. 14.52)
This has a unity passband gain, i.e. H(∞) = 1 .
1
= ωc = 50
RC
j10ω
H ^ (ω) = 10 H ′(ω) =
50 + jω
j10ω
H (ω) =
50 + jω
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Chapter 14, Problem 61.
Find the transfer function for each of the active filters in Fig. 14.90.
Figure 14.90
For Probs. 14.61 and 14.62.
Chapter 14, Solution 61.
1 jωC
(a)
V+ =
V,
R + 1 jωC i
V− = Vo
Since V+ = V− ,
1
V = Vo
1 + jωRC i
H (ω) =
(b)
V+ =
Vo
1
=
Vi 1 + jωRC
R
V,
R + 1 jωC i
V− = Vo
Since V+ = V− ,
jωRC
V = Vo
1 + jωRC i
H (ω) =
Vo
jωRC
=
Vi 1 + jωRC
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Chapter 14, Problem 62.
The filter in Fig. 14.90(b) has a 3-dB cutoff frequency at 1 kHz. If its input is connected
to a 120-mV variable frequency signal, find the output voltage at:
(a) 200 Hz
(b) 2 kHz
(c) 10 kHz
Chapter 14, Solution 62.
This is a highpass filter.
jωRC
1
=
1 + jωRC 1 − j ωRC
1
1
H (ω) =
,
ωc =
= 2π (1000)
1 − j ωc ω
RC
1
1
H (ω) =
=
1 − j f c f 1 − j1000 f
H (ω) =
(a)
H (f = 200 Hz) =
Vo =
(b)
1 − j5
H (f = 2 kHz) =
Vo =
(c)
120 mV
= 23.53 mV
Vo
1
=
1 − j0.5 Vi
120 mV
1 − j0.5
H (f = 10 kHz) =
Vo =
120 mV
1 − j0.1
Vo
1
=
1 − j5 Vi
= 107.3 mV
Vo
1
=
1 − j0.1 Vi
= 119.4 mV
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Chapter 14, Problem 63.
Design an active first-order highpass filter with
H(s) = -
100s
,
s + 10
s = jω
Use a 1- µF capacitor.
Chapter 14, Solution 63.
For an active highpass filter,
H(s) = −
sC i R f
1 + sC i R i
(1)
H(s) = −
10s
1 + s / 10
(2)
But
Comparing (1) and (2) leads to:
C i R f = 10
C i R i = 0.1
⎯⎯→
Ri =
⎯
⎯→
Rf =
10
= 10MΩ
Ci
0.1
= 100kΩ
Ci
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Chapter 14, Problem 64.
Obtain the transfer function of the active filter in Fig. 14.91 on the next page. What kind
of filter is it?
Figure 14.91
For Prob. 14.64.
Chapter 14, Solution 64.
Z f = R f ||
Rf
1
=
jωC f 1 + jωR f C f
Zi = R i +
1 + jωR i C i
1
=
jωC i
jωC i
Hence,
H (ω) =
Vo - Z f
- jωR f C i
=
=
Vi
Zi
(1 + jωR f C f )(1 + jωR i C i )
This is a bandpass filter. H (ω) is similar to the product of the transfer function
of a lowpass filter and a highpass filter.
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Chapter 14, Problem 65.
A highpass filter is shown in Fig. 14.92. Show that the transfer function is
⎛ Rf
H( ω ) = ⎜⎜1 +
⎝ Ri
⎞
jωRC
⎟⎟
⎠ 1 + jωRC
Figure 14.92
For Prob. 14.65.
Chapter 14, Solution 65.
V+ =
R
jωRC
Vi =
V
R + 1 jωC
1 + jωRC i
V− =
Ri
V
Ri + Rf o
Since V+ = V− ,
Ri
jωRC
Vo =
V
Ri + Rf
1 + jωRC i
H (ω) =
Vo ⎛
R f ⎞ ⎛ jωRC ⎞
⎟⎜
⎟
= ⎜1 +
Vi ⎝
R i ⎠ ⎝ 1 + jωRC ⎠
It is evident that as ω → ∞ , the gain is 1 +
Rf
1
and that the corner frequency is
.
Ri
RC
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Chapter 14, Problem 66.
A “general” first-order filter is shown in Fig. 14.93.
(a) Show that the transfer function is
H(s) =
s + (1 / R1C )[ R1 / R2 − R3 / R4 ]
R4
×
R3 + R4
s + 1 / R2 C
s=j ω
(b) What condition must be satisfied for the circuit to operate as a highpass filter?
(c) What condition must be satisfied for the circuit to operate as a lowpass filter?
Figure 14.93
For Prob. 14.66.
Chapter 14, Solution 66.
(a)
Proof
(b)
When R 1 R 4 = R 2 R 3 ,
H (s) =
(c)
H (s) =
R4
s
⋅
R 3 + R 4 s + 1 R 2C
When R 3 → ∞ ,
- 1 R 1C
s + 1 R 2C
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Chapter 14, Problem 67.
Design an active lowpass filter with dc gain of 0.25 and a corner frequency of 500 Hz.
Chapter 14, Solution 67.
DC gain =
Rf 1
=
Ri 4
⎯
⎯→ R i = 4R f
Corner frequency = ωc =
1
= 2π (500) rad / s
R f Cf
If we select R f = 20 kΩ , then R i = 80 kΩ and
1
C=
= 15.915 nF
(2π)(500)(20 × 10 3 )
Therefore, if R f = 20 kΩ , then R i = 80 kΩ and C = 15.915 nF
Chapter 14, Problem 68.
Design an active highpass filter with a high-frequency gain of 5 and a corner frequency
of 200 Hz.
Chapter 14, Solution 68.
Rf
⎯
⎯→ R f = 5R i
Ri
1
Corner frequency = ωc =
= 2π (200) rad / s
R i Ci
High frequency gain = 5 =
If we select R i = 20 kΩ , then R f = 100 kΩ and
1
C=
= 39.8 nF
(2π)(200)(20 × 10 3 )
Therefore, if R i = 20 kΩ , then R f = 100 kΩ and C = 39.8 nF
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Chapter 14, Problem 69.
Design the filter in Fig. 14.94 to meet the following requirements:
(a) It must attenuate a signal at 2 kHz by 3 dB compared with its value at 10 MHz.
(b) It must provide a steady-state output of v o (t) = 10 sin(2 π × 10 8 t + 180 o ) V for an
input v s (t) = 4sin(2 π × 10 8 t) V.
Figure 14.94
For Prob. 14.69.
Chapter 14, Solution 69.
This is a highpass filter with f c = 2 kHz.
1
ωc = 2πf c =
RC
1
1
RC =
=
2πf c 4π × 103
10 8 Hz may be regarded as high frequency. Hence the high-frequency gain is
− R f − 10
R f = 2 .5 R
=
or
R
4
If we let R = 10 kΩ , then R f = 25 kΩ , and C =
1
= 7.96 nF .
4000π × 10 4
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Chapter 14, Problem 70.
* A second-order active filter known as a Butterworth filter is shown in Fig. 14.95.
(a) Find the transfer function V o /V i .
(b) Show that it is a lowpass filter.
Figure 14.95
For Prob. 14.70.
* an asterisk indicates a challenging problem.
Chapter 14, Solution 70.
(a)
Vo (s)
Y1 Y2
=
Vi (s) Y1 Y2 + Y4 (Y1 + Y2 + Y3 )
1
1
= G 1 , Y2 =
= G 2 , Y3 = sC1 , Y4 = sC 2 .
where Y1 =
R1
R2
H (s) =
H (s) =
(b)
G 1G 2
G 1 G 2 + sC 2 (G 1 + G 2 + sC 1 )
G 1G 2
H(∞) = 0
= 1,
G 1G 2
showing that this circuit is a lowpass filter.
H ( 0) =
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Chapter 14, Problem 71.
Use magnitude and frequency scaling on the circuit of Fig. 14.76 to obtain an equivalent
circuit in which the inductor and capacitor have magnitude 1 H and 1 F respectively.
Chapter 14, Solution 71.
R = 50 Ω , L = 40 mH , C = 1 µF
L′ =
Km
Km
L ⎯
⎯→ 1 =
⋅ (40 × 10 -3 )
Kf
Kf
25K f = K m
C
C′ =
KmKf
10 6 K f =
(1)
10 -6
⎯
⎯→ 1 =
KmKf
1
Km
(2)
Substituting (1) into (2),
1
10 6 K f =
25K f
K f = 0.2 × 10 -3
K m = 25K f = 5 × 10 -3
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Chapter 14, Problem 72.
What values of K m and K f will scale a 4-mH inductor and a 20- µ F capacitor to 1 H and
2 F respectively?
Chapter 14, Solution 72.
L′C′ =
LC
K f2
⎯
⎯→ K f2 =
LC
L ′C′
(4 × 10 -3 )(20 × 10 -6 )
K =
= 4 × 10 -8
(1)(2)
2
f
K f = 2 × 10 -4
L′ L 2
= K
C′ C m
K 2m =
⎯
⎯→ K 2m =
L′ C
⋅
C′ L
(1)(20 × 10 -6 )
= 2.5 × 10 -3
(2)(4 × 10 -3 )
K m = 5 × 10 -2
Chapter 14, Problem 73.
Calculate the values of R, L, and C that will result in R = 12k Ω , L = 40 µ H and C = 300
nF respectively when magnitude-scaled by 800 and frequency-scaled by 1000.
Chapter 14, Solution 73.
R ′ = K m R = (12)(800 × 10 3 ) = 9.6 MΩ
L′ =
Km
800
L=
(40 × 10 -6 ) = 32 µF
Kf
1000
C
300 × 10 -9
=
= 0.375 pF
C′ =
K m K f (800)(1000)
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Chapter 14, Problem 74.
A circuit has R 1 = 3 Ω , R 2 = 10 Ω , L = 2H and C = 1/10F. After the circuit is
magnitude-scaled by 100 and frequency-scaled by 10 6 , find the new values of the circuit
elements.
Chapter 14, Solution 74.
R '1 = K m R 1 = 3x100 = 300Ω
R ' 2 = K m R 2 = 10 x100 = 1 kΩ
L' =
Km
10 2
L=
(2) = 200 µH
Kf
10 6
1
C
C' =
= 10 = 1 nF
K m K f 108
Chapter 14, Problem 75.
In an RLC circuit, R = 20 Ω , L = 4 H and C = 1 F. The circuit is magnitude-scaled by 10
and frequency-scaled by 10 5 . Calculate the new values of the elements.
Chapter 14, Solution 75.
R ' = K m R = 20 x10 = 200 Ω
L' =
C' =
Km
10
L=
(4) = 400 µH
Kf
10 5
C
1
=
= 1 µF
K m K f 10x10 5
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Chapter 14, Problem 76.
Given a parallel RLC circuit with R = 5 k Ω , L = 10 mH, and C = 20 µ F, if the circuit is
magnitude-scaled by K m = 500 and frequency-scaled by K f = 10 5 , find the resulting
values of R, L, and C.
Chapter 14, Solution 76.
R ' = K m R = 500 x5 x103 = 25 MΩ
L' =
Km
500
L = 5 (10mH ) = 50 µ H
10
Kf
C'=
C
20 x10−6
=
= 0.4 pF
K m K f 500 x105
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Chapter 14, Problem 77.
A series RLC circuit has R = 10 Ω , ω
the circuit is scaled:
0
= 40 rad/s, and B = 5 rad/s. Find L and C when
(a) in magnitude by a factor of 600,
(b) in frequency by a factor of 1,000,
(c) in magnitude by a factor of 400 and in frequency by a factor of 10 5 .
Chapter 14, Solution 77.
L and C are needed before scaling.
B=
R
L
ω0 =
(a)
⎯
⎯→ L =
1
LC
R 10
=
=2H
B 5
⎯
⎯→ C =
1
1
=
= 312.5 µF
2
ω0 L (1600)(2)
L′ = K m L = (600)(2) = 1200 H
C
3.125 × 10 -4
=
= 0.5208 µF
C′ =
Km
600
(b)
L′ =
L
2
= 3 = 2 mH
K f 10
C
3.125 × 10 -4
=
= 312.5 nF
C′ =
Kf
10 3
(c)
L′ =
Km
(400)(2)
= 8 mH
L=
Kf
10 5
C′ =
C
3.125 × 10 -4
=
= 7.81 pF
KmKf
(400)(10 5 )
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Chapter 14, Problem 78.
Redesign the circuit in Fig. 14.85 so that all resistive elements are scaled by a factor of
1,000 and all frequency-sensitive elements are frequency-scaled by a factor of 10 4 .
Chapter 14, Solution 78.
R ′ = K m R = (1000)(1) = 1 kΩ
Km
10 3
L′ =
L = 4 (1) = 0.1 H
Kf
10
C
1
C′ =
=
= 0.1 µF
3
K m K f (10 )(10 4 )
The new circuit is shown below.
1 kΩ
+
I
1 kΩ
0.1 H
0.1 µF
1 kΩ
Vx
−
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Chapter 14, Problem 79.
* Refer to the network in Fig. 14.96.
(a) Find Z in (s).
(b) Scale the elements by K m = 10 and K f = 100. Find Z in (s) and ω 0 .
Figure 14.96
For Prob. 14.79.
* An asterisk indicates a challenging problem.
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Chapter 14, Solution 79.
(a)
Insert a 1-V source at the input terminals.
Ro
Io
R
V1
V2
+ −
+
−
1V
1/sC
3Vo
+
sL
Vo
−
There is a supernode.
V2
1 − V1
=
R
sL + 1 sC
V1 = V2 + 3Vo
But
⎯
⎯→ V2 = V1 − 3Vo
Vo
V2
sL
⎯→
=
V2 ⎯
sL + 1 sC
sL sL + 1 sC
Combining (2) and (3)
sL + 1 sC
V2 = V1 − 3Vo =
Vo
sL
s 2 LC
Vo =
V
1 + 4s 2 LC 1
Substituting (3) and (4) into (1) gives
1 − V1 Vo
sC
=
=
V
R
sL 1 + 4s 2 LC 1
sRC
1 + 4s 2 LC + sRC
=
V
V1
1 = V1 +
1 + 4s 2 LC 1
1 + 4s 2 LC
1 + 4s 2 LC
V1 =
1 + 4s 2 LC + sRC
Also,
Io =
Vo =
(1)
(2)
(3)
(4)
1 − V1
sRC
=
R
R (1 + 4s 2 LC + sRC)
1 1 + sRC + 4s 2 LC
=
Z in =
Io
sC
1
Z in = 4sL + R +
sC
(5)
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When R = 5 , L = 2 , C = 0.1 ,
10
Z in (s) = 8s + 5 +
s
At resonance,
Im(Z in ) = 0 = 4ωL −
or
(b)
ω0 =
1
2 LC
=
1
ωC
1
2 (0.1)(2)
= 1.118 rad / s
After scaling,
⎯→ K m R
R′ ⎯
4Ω ⎯
⎯→ 40 Ω
5Ω ⎯
⎯→ 50 Ω
L′ =
Km
10
L=
( 2 ) = 0 .2 H
Kf
100
C′ =
C
0.1
=
= 10 -4
K m K f (10)(100)
From (5),
Z in (s) = 0.8s + 50 +
ω0 =
1
2 LC
=
10 4
s
1
2 (0.2)(10 -4 )
= 111.8 rad / s
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Chapter 14, Problem 80.
(a) For the circuit in Fig. 14.97, draw the new circuit after it has been scaled by K m =
200 and K f = 10 4 .
(b) Obtain the Thevenin equivalent impedance at terminals a-b of the scaled circuit at ω
= 10 4 rad/s.
Figure 14.97
For Prob. 14.80.
Chapter 14, Solution 80.
(a)
R ′ = K m R = (200)(2) = 400 Ω
K m L (200)(1)
=
= 20 mH
Kf
10 4
C
0.5
C′ =
=
= 0.25 µF
K m K f (200)(10 4 )
L′ =
The new circuit is shown below.
20 mH
a
Ix
0.25 µF
400 Ω
0.5 Ix
b
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(b)
Insert a 1-A source at the terminals a-b.
a
sL
V1
V2
Ix
1A
1/(sC)
R
0.5 Ix
b
At node 1,
1 = sCV1 +
V1 − V2
sL
(1)
At node 2,
V1 − V2
V2
+ 0 .5 I x =
sL
R
But, I x = sC V1 .
V1 − V2
V2
+ 0.5sC V1 =
sL
R
(2)
Solving (1) and (2),
sL + R
V1 = 2
s LC + 0.5sCR + 1
Z Th =
V1
sL + R
= 2
1 s LC + 0.5sCR + 1
At ω = 10 4 ,
Z Th
( j10 4 )(20 × 10 -3 ) + 400
=
( j10 4 ) 2 (20 × 10 -3 )(0.25 × 10 -6 ) + 0.5( j10 4 )(0.25 × 10 -6 )(400) + 1
Z Th =
400 + j200
= 600 − j200
0.5 + j0.5
Z Th = 632.5∠ - 18.435° ohms
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Chapter 14, Problem 81.
The circuit shown in Fig. 14.98 has the impedance
Z(s) =
1,000( s + 1)
,
( s + 1 + j 50)( s + 1 − j 50)
s=j ω
Find:
(a) the values of R, L, C, and G
(b) the element values that will raise the resonant frequency by a factor of 10 3 by
frequency scaling
Figure 14.98
For Prob. 14.81.
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Chapter 14, Solution 81.
(a)
1
(G + jωC)(R + jωL) + 1
1
= G + jωC +
=
R + jω L
R + jω L
Z
which leads to
Z=
jωL + R
2
− ω LC + jω(RC + LG) + GR + 1
ω R
+
C
LC
Z(ω) =
R
G ⎞ GR + 1
⎛
− ω2 + jω⎜ + ⎟ +
LC
⎝L C⎠
j
(1)
We compare this with the given impedance:
Z(ω) =
1000( jω + 1)
(2)
− ω 2 + 2 jω + 1 + 2500
Comparing (1) and (2) shows that
1
= 1000
C
R G
+ =2
L C
⎯
⎯→
⎯
⎯→
C = 1 mF,
R/L = 1
⎯
⎯→
R=L
G = C = 1 mS
GR + 1 10 −3 R + 1
=
2501 =
LC
10 −3 R
⎯⎯→
R = 0 .4 = L
Thus,
R = 0.4Ω, L = 0.4 H, C = 1 mF, G = 1 mS
(b) By frequency-scaling, Kf =1000.
R’ = 0.4 Ω, G’ = 1 mS
L' =
L
0.4
=
= 0.4mH ,
K f 10 3
C' =
C
10 −3
=
= 1µF
K f 10 − 3
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Chapter 14, Problem 82.
Scale the lowpass active filter in Fig. 14.99 so that its corner frequency increases from 1
rad/s to 200 rad/s. Use a 1- µ F capacitor.
Figure 14.99
For Prob. 14.82.
Chapter 14, Solution 82.
C′ =
C
KmKf
Kf =
ω′c 200
=
= 200
ω
1
Km =
C 1
1
1
⋅
= -6 ⋅
= 5000
C′ K f 10 200
R ′ = K m R = 5 kΩ,
thus,
R ′f = 2R i = 10 kΩ
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Chapter 14, Problem 83.
The op amp circuit in Fig. 14.100 is to be magnitude-scaled by 100 and frequency-scaled
by 10 5 . Find the resulting element values.
Figure 14.100
For Prob. 14.83.
Chapter 14, Solution 83.
1
10 −6
C=
= 0.1 pF
K mKf
100 x10 5
1µF
⎯
⎯→
C' =
5µF
⎯
⎯→
C' = 0.5 pF
10 kΩ
⎯⎯→
R ' = K m R = 100x10 kΩ = 1 MΩ
20 kΩ
⎯⎯→
R ' = 2 MΩ
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Chapter 14, Problem 84.
Using PSpice, obtain the frequency response of the circuit in Fig. 14.101 on the next
page.
Figure 14.101
For Prob. 14.84.
Chapter 14, Solution 84.
The schematic is shown below. A voltage marker is inserted to measure vo. In
the AC sweep box, we select Total Points = 50, Start Frequency = 1, and End
Frequency = 1000. After saving and simulation, we obtain the magnitude and
phase plots in the probe menu as shown below.
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Chapter 14, Problem 85.
Use PSpice to obtain the magnitude and phase plots of V 0 /I s of the circuit in Fig.
14.102.
Figure 14.102
For Prob. 14.85.
Chapter 14, Solution 85.
We let I s = 1∠0 o A so that Vo / I s = Vo . The schematic is shown below. The circuit
is simulated for 100 < f < 10 kHz.
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Chapter 14, Problem 86.
Use PSpice to provide the frequency response (magnitude and phase of i) of the circuit in
Fig. 14.103. Use linear frequency sweep from 1 to 10,000 Hz.
Figure 14.103
For Prob. 14.86.
Chapter 14, Solution 86.
The schematic is shown below. A current marker is inserted to measure I. We set
Total Points = 101, start Frequency = 1, and End Frequency = 10 kHz in the
AC sweep box. After simulation, the magnitude and phase plots are obtained in
the Probe menu as shown below.
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Chapter 14, Problem 87.
In the interval 0.1 < f < 100 Hz, plot the response of the network in Fig. 14.104. Classify
this filter and obtain ω 0 .
Figure 14.104
For Prob. 14.87.
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Chapter 14, Solution 87.
The schematic is shown below. In the AC Sweep box, we set Total Points = 50,
Start Frequency = 1, and End Frequency = 100. After simulation, we obtain the
magnitude response as shown below. It is evident from the response that the
circuit represents a high-pass filter.
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Chapter 14, Problem 88.
Use PSpice to generate the magnitude and phase Bode plots of V 0 in the circuit of Fig.
14.105.
Figure 14.105
For Prob. 14.88.
Chapter 14, Solution 88.
The schematic is shown below. We insert a voltage marker to measure Vo. In the
AC Sweep box, we set Total Points = 101, Start Frequency = 1, and End
Frequency = 100. After simulation, we obtain the magnitude and phase plots of
Vo as shown below.
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Chapter 14, Problem 89.
Obtain the magnitude plot of the response V 0 in the network of Fig. 14.106 for the
frequency interval 100 < f < 1,000 Hz..
Figure 14.106
For Prob. 14.89.
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Chapter 14, Solution 89.
The schematic is shown below. In the AC Sweep box, we type Total Points =
101, Start Frequency = 100, and End Frequency = 1 k. After simulation, the
magnitude plot of the response Vo is obtained as shown below.
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Chapter 14, Problem 90.
Obtain the frequency response of the circuit in Fig. 14.40 (see Practice Problem 14.10).
Take R 1 = R 2 = 100 Ω , L =2 mH. Use 1 < f < 100,000 Hz.
Chapter 14, Solution 90.
The schematic is shown below. In the AC Sweep box, we set
Total Points = 1001, Start Frequency = 1, and End Frequency = 100k. After
simulation, we obtain the magnitude plot of the response as shown below. The
response shows that the circuit is a high-pass filter.
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Chapter 14, Problem 91.
For the “tank” circuit of Fig. 14.79, obtain the frequency response (voltage across the
capacitor) using PSpice. Determine the resonant frequency of the circuit.
Chapter 14, Solution 91.
The schematic is shown below. In the AC Sweep box, we select Total Points = 101,
Start Frequency = 10, and End Frequency = 10 k. After simulation, the magnitude plot
of the frequency response is obtained. From the plot, we obtain the resonant frequency fo
is approximately equal to 800 Hz so that
ωo = 2πfo = 5026 rad/s.
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Chapter 14, Problem 92.
Using PSpice, plot the magnitude of the frequency response of the circuit in Fig. 14.85.
Chapter 14, Solution 92.
The schematic is shown below. We type Total Points = 101, Start Frequency =
1, and End Frequency = 100 in the AC Sweep box. After simulating the circuit,
the magnitude plot of the frequency response is shown below.
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Chapter 14, Problem 93.
For the phase shifter circuit shown in Fig. 14.107, find H = V o /V s .
Figure 14.107
For Prob. 14.93.
Chapter 14, Solution 93.
Consider the circuit as shown below.
R
Vs
+
+
_
V1
c
C
V1 =
1
sC
Vs =
Vo
C
_
c
V2
R
V
1 + sRC
1
sC
R
sRC
V2 =
Vs =
Vs
R + sC
1 + sRC
R+
Vo = V1 − V2 =
Hence,
H (s) =
1 − sRC
Vs
1 + sRC
Vo 1 − sRC
=
V s 1 + sRC
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Chapter 14, Problem 94.
For an emergency situation, an engineer needs to make an RC highpass filter. He has one
10-pF capacitor, one 30-pF capacitor, one 1.8-k Ω resistor, and one 3.3-k Ω resistor
available. Find the greatest cutoff frequency possible using these elements.
Chapter 14, Solution 94.
ωc =
1
RC
We make R and C as small as possible. To achieve this, we connect 1.8 k Ω and 3.3 k Ω
in parallel so that
R=
1.8x 3.3
= 1.164 kΩ
1.8 + 3.3
We place the 10-pF and 30-pF capacitors in series so that
C = (10x30)/40 = 7.5 pF
Hence,
ωc =
1
1
=
= 114.55x10 6 rad/s
−
3
12
RC 1.164x10 x 7.5x10
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Chapter 14, Problem 95.
A series-tuned antenna circuit consists of a variable capacitor (40 pF to 360 pF) and a
240- µ H antenna coil that has a dc resistance of 12 Ω .
(a) Find the frequency range of radio signals to which the radio is tunable.
(b) Determine the value of Q at each end of the frequency range.
Chapter 14, Solution 95.
(a)
f0 =
1
2π LC
When C = 360 pF ,
f0 =
1
2π (240 × 10 -6 )(360 × 10 -12 )
= 0.541 MHz
When C = 40 pF ,
f0 =
1
2π (240 × 10 -6 )(40 × 10 -12 )
= 1.624 MHz
Therefore, the frequency range is
0.541 MHz < f 0 < 1.624 MHz
(b)
Q=
2πfL
R
At f 0 = 0.541 MHz ,
Q=
(2π )(0.541 × 10 6 )(240 × 10 -6 )
= 67.98
12
At f 0 = 1.624 MHz ,
Q=
(2π )(1.624 × 10 6 )(240 × 10 -6 )
= 204.1
12
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Chapter 14, Problem 96.
The crossover circuit in Fig. 14.108 is a lowpass filter that is connected to a woofer. Find
the transfer function H( ω ) = V o ( ω )/V i ( ω )
Figure 14.108
For Prob. 14.96.
Chapter 14, Solution 96.
Ri
Vi
L
V1
Vo
+
+
−
C1
C2
RL
Vo
−
Z2
Z1
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Z1 = R L ||
Z2 =
RL
1
=
sC 2 1 + sR 2 C 2
1
1 ⎛ sL + R L + s 2 R L C 2 L ⎞
⎟
|| (sL + Z1 ) =
|| ⎜
sC1
sC1 ⎝
1 + sR L C 2
⎠
1 sL + R L + s 2 R L C 2 L
⋅
sC1
1 + sR L C 2
Z2 =
sL + R L + s 2 R L C 2 L
1
+
sC1
1 + sR L C 2
sL + R L + s 2 R L LC 2
Z2 =
1 + sR L C 2 + s 2 LC1 + sR L C1 + s 3 R L LC1C 2
V1 =
Z2
V
Z2 + R i i
Vo =
Z1
Z2
Z1
⋅
V1 =
V
Z1 + sL
Z 2 + R 2 Z1 + sL i
Vo
Z2
Z1
=
⋅
Vi Z 2 + R 2 Z1 + sL
where
Z2
=
Z2 + R 2
sL + R L + s 2 R L LC 2
sL + R L + s 2 R L LC 2 + R i + sR i R L C 2 + s 2 R i LC1 + sR i R L C1 + s 3 R i R L LC1C 2
RL
Z1
and
=
Z1 + sL R L + sL + s 2 R L LC 2
Therefore,
Vo
R L (sL + R L + s 2 R L LC 2 )
=
Vi (sL + R L + s 2 R L LC 2 + R i + sR i R L C 2 + s 2 R i LC 1 + sR i R L C 1
+ s 3 R i R L LC 1 C 2 )( R L + sL + s 2 R L LC 2 )
where s = jω .
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Chapter 14, Problem 97.
The crossover circuit in Fig. 14.109 is a highpass filter that is connected to a tweeter.
Determine the transfer function H( ω ) = V o ( ω )/V i ( ω ).
Figure 14.109
For Prob. 14.97.
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Chapter 14, Solution 97.
Ri
Vi
L
V1
Vo
+
+
−
C1
C2
RL
Vo
−
Z2
Z1
⎛
1 ⎞ sL (R L + 1 sC 2 )
⎟=
Z = sL || ⎜ R L +
,
sC 2 ⎠ R L + sL + 1 sC 2
⎝
s = jω
V1 =
Z
V
Z + R i + 1 sC1 i
Vo =
RL
RL
Z
⋅
V1 =
V
R L + 1 sC 2
R L + 1 sC 2 Z + R i + 1 sC1 i
H (ω) =
Vo
RL
sL (R L + 1 sC 2 )
=
⋅
Vi R L + 1 sC 2 sL (R L + 1 sC 2 ) + (R i + 1 sC1 )(R L + sL + 1 sC 2 )
s 3 LR L C 1C 2
H (ω) =
(sR i C 1 + 1)(s 2 LC 2 + sR L C 2 + 1) + s 2 LC 1 (sR L C 2 + 1)
where s = jω .
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Chapter 14, Problem 98.
A certain electronic test circuit produced a resonant curve with half-power points at 432
Hz and 454 Hz. If Q = 20, what is the resonant frequency of the circuit?
Chapter 14, Solution 98.
B = ω2 − ω1 = 2π (f 2 − f 1 ) = 2π (454 − 432) = 44π
ω0 = 2πf 0 = QB = (20)(44π )
f0 =
(20)(44π)
= (20)(22) = 440 Hz
2π
Chapter 14, Problem 99.
In an electronic device, a series circuit is employed that has a resistance of 100 Ω , a
capacitive reactance of 5 k Ω , and an inductive reactance of 300 Ω when used at 2 MHz.
Find the resonant frequency and bandwidth of the circuit.
Chapter 14, Solution 99.
Xc =
1
1
=
ωC 2πf C
1
1
10 -9
C=
=
=
2πf X c (2π )(2 × 10 6 )(5 × 10 3 ) 20π
X L = ωL = 2πf L
L=
f0 =
B=
XL
300
3 × 10 -4
=
=
2πf (2π )(2 × 10 6 )
4π
1
2π LC
=
1
3 × 10 -4 10 -9
2π
⋅
4π
20π
= 8.165 MHz
⎛ 4π ⎞
R
6
⎟
= (100) ⎜
- 4 = 4.188 × 10 rad / s
⎝ 3 × 10 ⎠
L
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Chapter 14, Problem 100.
In a certain application, a simple RC lowpass filter is designed to reduce high frequency
noise. If the desired corner frequency is 20 kHz and C = 0.5 µ F find the value of R.
Chapter 14, Solution 100.
ωc = 2πf c =
R=
1
RC
1
1
=
= 15.91 Ω
2πf c C (2π )(20 × 10 3 )(0.5 × 10 -6 )
Chapter 14, Problem 101.
In an amplifier circuit, a simple RC highpass filter is needed to block the dc component
while passing the time-varying component. If the desired rolloff frequency is 15 Hz and
C = 10 µ F find the value of R.
Chapter 14, Solution 101.
ωc = 2πf c =
R=
1
RC
1
1
=
= 1.061 kΩ
2πf c C (2π )(15)(10 × 10 -6 )
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Chapter 14, Problem 102.
Practical RC filter design should allow for source and load resistances as shown in Fig.
14.110. Let R = 4k Ω and C = 40-nF. Obtain the cutoff frequency when:
(a) R s = 0, R L = ∞ ,
(b) R s = 1k Ω , R L = 5k Ω .
Figure 14.110
For Prob. 14.102.
Chapter 14, Solution 102.
(a)
When R s = 0 and R L = ∞ , we have a low-pass filter.
ωc = 2πf c =
fc =
(b)
1
RC
1
1
=
= 994.7 Hz
2πRC (2π)(4 × 10 3 )(40 × 10 -9 )
We obtain R Th across the capacitor.
R Th = R L || (R + R s )
R Th = 5 || (4 + 1) = 2.5 kΩ
fc =
1
1
=
2πR Th C (2π )(2.5 × 10 3 )(40 × 10 -9 )
f c = 1.59 kHz
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Chapter 14, Problem 103.
The RC circuit in Fig. 14.111 is used for a lead compensator in a system design. Obtain
the transfer function of the circuit.
Figure 14.111
For Prob. 14.103.
Chapter 14, Solution 103.
H (ω) =
H (s) =
H(s) =
Vo
R2
=
,
Vi R 2 + R 1 || 1 jωC
s = jω
R 2 (R 1 + 1 sC)
R2
=
R (1 sC) R 1R 2 + (R 1 + R 2 )(1 sC)
R2 + 1
R 1 + 1 sC
R 2 (1 + sCR 1 )
R 1 + R 2 + sCR 1R 2
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Chapter 14, Problem 104.
A low-quality-factor, double-tuned bandpass filter is shown in Fig. 14.112. Use PSpice to
generate the magnitude plot of V o ( ω ).
Figure 14.112
For Prob. 14.104.
PROPRIETARY MATERIAL. © 2007 The McGraw-Hill Companies, Inc. All rights reserved. No part
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Chapter 14, Solution 104.
The schematic is shown below. We click Analysis/Setup/AC Sweep and enter
Total Points = 1001, Start Frequency = 100, and End Frequency = 100 k.
After simulation, we obtain the magnitude plot of the response as shown.
PROPRIETARY MATERIAL. © 2007 The McGraw-Hill Companies, Inc. All rights reserved. No part
of this Manual may be displayed, reproduced or distributed in any form or by any means, without the prior
written permission of the publisher, or used beyond the limited distribution to teachers and educators
permitted by McGraw-Hill for their individual course preparation. If you are a student using this Manual,
you are using it without permission.