CHAPTER
7
CHAPTER CONTENTS
7.1 The Natural Response of an RL Circuit p. 214
7.2 The Natural Response of an RC Circuit p. 220
7.3 The Step Response of RL and
RC Circuits p. 224
7.4 A General Solution for Step and Natural
Responses p. 231
7.5 Sequential Switching p. 236
7.6 Unbounded Response p. 240
7.7 The Integrating Amplifier p. 241
CHAPTER OBJECTIVES
1 Be able to determine the natural response of
both RL and RC circuits.
2 Be able to determine the step response of both
RL and RC circuits.
3 Know how to analyze circuits with sequential
switching.
4 Be able to analyze op amp circuits containing
resistors and a single capacitor.
212
Response of First-Order
RL and RC Circuits
In Chapter 6, we noted that an important attribute of inductors
and capacitors is their ability to store energy. We are now in a
position to determine the currents and voltages that arise when
energy is either released or acquired by an inductor or capacitor
in response to an abrupt change in a dc voltage or current source.
In this chapter, we will focus on circuits that consist only of
sources, resistors, and either (but not both) inductors or capacitors. For brevity, such configurations are called RL (resistorinductor) and RC (resistor-capacitor) circuits.
Our analysis of RL and RC circuits will be divided into three
phases. In the first phase, we consider the currents and voltages
that arise when stored energy in an inductor or capacitor is suddenly released to a resistive network. This happens when the
inductor or capacitor is abruptly disconnected from its dc source.
Thus we can reduce the circuit to one of the two equivalent forms
shown in Fig. 7.1 on page 214. The currents and voltages that arise
in this configuration are referred to as the natural response of the
circuit, to emphasize that the nature of the circuit itself, not external sources of excitation, determines its behavior.
In the second phase of our analysis, we consider the currents
and voltages that arise when energy is being acquired by an inductor or capacitor due to the sudden application of a dc voltage or
current source. This response is referred to as the step response.
The process for finding both the natural and step responses is the
same; thus, in the third phase of our analysis, we develop a general
method that can be used to find the response of RL and RC circuits to any abrupt change in a dc voltage or current source.
Figure 7.2 on page 214 shows the four possibilities for the general configuration of RL and RC circuits. Note that when there
are no independent sources in the circuit, the Thévenin voltage or
Norton current is zero, and the circuit reduces to one of those
shown in Fig. 7.1; that is, we have a natural-response problem.
Practical Perspective
Artificial Pacemaker
The muscle that makes up the heart contracts due to rhythmical
electrical impulses. The frequency of the impulses is controlled
by pacemaker cells. In adults, the pacemaker cells establish a
resting heart rate of about 72 beats per minutes. But sometimes
the pacemaker cells are damaged and may produce a very low
resting heart rate (a condition known as bradycardia) or a very
high resting heart rate (a condition known as tachycardia).
A normal heart rhythm can be restored by implanting an artificial pacemaker that delivers electrical impulses to the heart,
mimicking the pacemaker cells. An example of an artificial
pacemaker both outside and inside the body is shown in the figures below.
Artificial pacemakers are very small and lightweight.
They have a programmable microprocessor that monitors a
few parameters and adjusts the heart rate, an efficient battery with a life of up to 15 years, and circuitry that generates the pulse. The simplest circuit consists of a resistor and
a capacitor. Once we have introduced the first-order RC
circuit, we will look at an RC circuit design for artificial
pacemakers.
Pacemaker
Wires
Electrode
Pacemaker
Electrode
213
214
Leq
Response of First-Order RL and RC Circuits
I0
Req
V0
Ceq
(a)
Req
(b)
Figure 7.1 The two forms of the circuits for natural
response. (a) RL circuit. (b) RC circuit.
RTh
i
VTh
L v
7.1 The Natural Response
of an RL Circuit
(a)
i
VTh
RTh
The natural response of an RL circuit can best be described in terms of the
circuit shown in Fig. 7.3. We assume that the independent current source
generates a constant current of Is A, and that the switch has been in a
closed position for a long time. We define the phrase a long time more
accurately later in this section. For now it means that all currents and voltages have reached a constant value. Thus only constant, or dc, currents can
exist in the circuit just prior to the switch’s being opened, and therefore
the inductor appears as a short circuit (Ldi>dt = 0) prior to the release of
the stored energy.
Because the inductor appears as a short circuit, the voltage across the
inductive branch is zero, and there can be no current in either R0 or R.
Therefore, all the source current Is appears in the inductive branch.
Finding the natural response requires finding the voltage and current at
the terminals of the resistor after the switch has been opened, that is, after
the source has been disconnected and the inductor begins releasing
energy. If we let t = 0 denote the instant when the switch is opened, the
problem becomes one of finding v(t) and i(t) for t Ú 0. For t Ú 0, the circuit shown in Fig. 7.3 reduces to the one shown in Fig. 7.4.
L v
RTh
(b)
RTh
VTh
v
i
C
(c)
v
i
VTh
RTh
RL and RC circuits are also known as first-order circuits, because their
voltages and currents are described by first-order differential equations. No
matter how complex a circuit may appear, if it can be reduced to a Thévenin or
Norton equivalent connected to the terminals of an equivalent inductor or
capacitor, it is a first-order circuit. (Note that if multiple inductors or capacitors
exist in the original circuit, they must be interconnected so that they can be
replaced by a single equivalent element.)
After introducing the techniques for analyzing the natural and step
responses of first-order circuits, we discuss some special cases of interest.
The first is that of sequential switching, involving circuits in which switching
can take place at two or more instants in time. Next is the unbounded
response. Finally, we analyze a useful circuit called the integrating amplifier.
RTh
C
(d)
Deriving the Expression for the Current
Figure 7.2 Four possible first-order circuits.
(a) An inductor connected to a Thévenin equivalent.
(b) An inductor connected to a Norton equivalent.
(c) A capacitor connected to a Thévenin equivalent.
(d) A capacitor connected to a Norton equivalent.
t0
Is
L
R
v
Figure 7.3 An RL circuit.
i
i(0) Is
L
di
+ Ri = 0,
(7.1)
dt
where we use the passive sign convention. Equation 7.1 is known as a firstorder ordinary differential equation, because it contains terms involving
the ordinary derivative of the unknown, that is, di>dt. The highest order
derivative appearing in the equation is 1; hence the term first-order.
We can go one step further in describing this equation. The coefficients in the equation, R and L, are constants; that is, they are not functions of either the dependent variable i or the independent variable t. Thus
the equation can also be described as an ordinary differential equation
with constant coefficients.
To solve Eq. 7.1, we divide by L, transpose the term involving i to the
right-hand side, and then multiply both sides by a differential time dt. The
result is
di
R
dt = - i dt.
(7.2)
dt
L
L
i
R0
To find i(t), we use Kirchhoff’s voltage law to obtain an expression involving i, R, and L. Summing the voltages around the closed loop gives
R v
Figure 7.4 The circuit shown in Fig. 7.3, for t Ú 0.
7.1
The Natural Response of an RL Circuit
215
Next, we recognize the left-hand side of Eq. 7.2 as a differential change in
the current i, that is, di. We now divide through by i, getting
di
R
= - dt.
i
L
(7.3)
We obtain an explicit expression for i as a function of t by integrating both
sides of Eq. 7.3. Using x and y as variables of integration yields
i(t)
t
dx
R
= dy,
L Lt0
Li(t0) x
(7.4)
in which i(t0) is the current corresponding to time t0, and i(t) is the current
corresponding to time t. Here, t0 = 0. Therefore, carrying out the indicated integration gives
ln
i(t)
R
= - t.
i(0)
L
(7.5)
Based on the definition of the natural logarithm,
i(t) = i(0)e-(R>L)t.
(7.6)
Recall from Chapter 6 that an instantaneous change of current cannot
occur in an inductor. Therefore, in the first instant after the switch has
been opened, the current in the inductor remains unchanged. If we use 0to denote the time just prior to switching, and 0+ for the time immediately
following switching, then
Initial inductor current
i(0-) = i(0+) = I0,
where, as in Fig. 7.1, I0 denotes the initial current in the inductor. The initial
current in the inductor is oriented in the same direction as the reference
direction of i. Hence Eq. 7.6 becomes
i(t) = I0e-(R>L)t,
t Ú 0,
(7.7)
which shows that the current starts from an initial value I0 and decreases
exponentially toward zero as t increases. Figure 7.5 shows this response.
We derive the voltage across the resistor in Fig. 7.4 from a direct application of Ohm’s law:
v = iR = I0Re-(R>L)t,
t Ú 0+.
Natural response of an RL circuit
i(t)
I0
(7.8)
Note that in contrast to the expression for the current shown in Eq. 7.7,
the voltage is defined only for t 7 0, not at t = 0. The reason is that a step
change occurs in the voltage at zero. Note that for t 6 0, the derivative of
the current is zero, so the voltage is also zero. (This result follows from
v = Ldi>dt = 0.) Thus
v(0-) = 0,
(7.9)
v(0+) = I0R,
(7.10)
where v(0+) is obtained from Eq. 7.8 with t = 0+.1 With this step change at
an instant in time, the value of the voltage at t = 0 is unknown. Thus we
use t Ú 0+ in defining the region of validity for these solutions.
1 We can define the expressions 0 - and 0+ more formally. The expression x(0 - ) refers to the
limit of the variable x as t : 0 from the left, or from negative time. The expression x(0+)
refers to the limit of the variable x as t : 0 from the right, or from positive time.
0
t
Figure 7.5 The current response for the circuit shown
in Fig. 7.4.
216
Response of First-Order RL and RC Circuits
We derive the power dissipated in the resistor from any of the following expressions:
p = i2R,
p = vi,
p =
or
v2
.
R
(7.11)
Whichever form is used, the resulting expression can be reduced to
p = I20Re-2(R>L)t,
t Ú 0+.
(7.12)
The energy delivered to the resistor during any interval of time after the
switch has been opened is
t
w =
L0
t
pdx =
L0
I20Re-2(R>L)x dx
=
1
I2R(1 - e-2(R>L)t)
2(R>L) 0
=
1 2
LI (1 - e-2(R>L)t),
2 0
t Ú 0.
(7.13)
Note from Eq. 7.13 that as t becomes infinite, the energy dissipated in the
resistor approaches the initial energy stored in the inductor.
The Significance of the Time Constant
The expressions for i(t) (Eq. 7.7) and v(t) (Eq. 7.8) include a term of the
form e-(R>L)t. The coefficient of t—namely, R>L—determines the rate at
which the current or voltage approaches zero. The reciprocal of this ratio
is the time constant of the circuit, denoted
Time constant for RL circuit t = time constant =
L
.
R
(7.14)
Using the time-constant concept, we write the expressions for current,
voltage, power, and energy as
i(t) = I0e-t>t,
v(t) = I0Re
t Ú 0,
,
-t>t
(7.15)
t Ú 0 ,
+
p = I20Re-2t>t,
t Ú 0+,
1
w = LI20(1 - e-2t>t),
t Ú 0.
2
(7.16)
(7.17)
(7.18)
The time constant is an important parameter for first-order circuits, so
mentioning several of its characteristics is worthwhile. First, it is convenient to think of the time elapsed after switching in terms of integral multiples of t. Thus one time constant after the inductor has begun to release
its stored energy to the resistor, the current has been reduced to e-1, or
approximately 0.37 of its initial value.
7.1
Table 7.1 gives the value of e-t>t for integral multiples of t from 1 to
10. Note that when the elapsed time exceeds five time constants, the
current is less than 1% of its initial value. Thus we sometimes say that
five time constants after switching has occurred, the currents and voltages have, for most practical purposes, reached their final values. For
single time-constant circuits (first-order circuits) with 1% accuracy, the
phrase a long time implies that five or more time constants have
elapsed. Thus the existence of current in the RL circuit shown in
Fig. 7.1(a) is a momentary event and is referred to as the transient
response of the circuit. The response that exists a long time after the
switching has taken place is called the steady-state response. The phrase
a long time then also means the time it takes the circuit to reach its
steady-state value.
Any first-order circuit is characterized, in part, by the value of its
time constant. If we have no method for calculating the time constant of
such a circuit (perhaps because we don’t know the values of its components), we can determine its value from a plot of the circuit’s natural
response. That’s because another important characteristic of the time
constant is that it gives the time required for the current to reach its final
value if the current continues to change at its initial rate. To illustrate, we
evaluate di>dt at 0+ and assume that the current continues to change at
this rate:
I0
di +
R
(0 ) = - I0 = - .
t
dt
L
The Natural Response of an RL Circuit
217
TABLE 7.1 Value of e-t>t For t Equal to
Integral Multiples of t
t
e-t>t
t
e-t>t
t
3.6788 * 10-1
6t
2.4788 * 10-3
2t
1.3534 * 10
-1
7t
9.1188 * 10-4
3t
4.9787 * 10-2
8t
3.3546 * 10-4
4t
1.8316 * 10-2
9t
1.2341 * 10-4
5t
6.7379 * 10-3
10t
4.5400 * 10-5
(7.19)
Now, if i starts as I0 and decreases at a constant rate of I0>t amperes per
second, the expression for i becomes
i = I0 -
I0
t.
t
i
(7.20)
I0
i I0et/t
i I0(I0/t)t
Equation 7.20 indicates that i would reach its final value of zero in
t seconds. Figure 7.6 shows how this graphic interpretation is useful in
estimating the time constant of a circuit from a plot of its natural
response. Such a plot could be generated on an oscilloscope measuring
output current. Drawing the tangent to the natural response plot at t = 0
and reading the value at which the tangent intersects the time axis gives
the value of t.
Calculating the natural response of an RL circuit can be summarized
as follows:
1. Find the initial current, I0, through the inductor.
2. Find the time constant of the circuit, t = L>R.
3. Use Eq. 7.15, I0e-t>t, to generate i(t)
from I0 and t.
All other calculations of interest follow from knowing i(t).
Examples 7.1 and 7.2 illustrate the numerical calculations associated with
the natural response of an RL circuit.
0
t
Figure 7.6 A graphic interpretation of the time constant of the RL circuit shown in Fig. 7.4.
Calculating the natural response
of RL circuit
t
218
Response of First-Order RL and RC Circuits
Example 7.1
Determining the Natural Response of an RL Circuit
The switch in the circuit shown in Fig. 7.7 has
been closed for a long time before it is opened at
t = 0. Find
a) iL(t) for t Ú 0,
b) io(t) for t Ú 0 + ,
c) vo(t) for t Ú 0 + ,
d) the percentage of the total energy stored in the
2 H inductor that is dissipated in the 10 Æ resistor.
t0
20 A
0.1 iL
10 vo
2H
io = - iL
10
.
10 + 40
Note that this expression is valid for t Ú 0+
because io = 0 at t = 0-. The inductor behaves as
a short circuit prior to the switch being opened,
producing an instantaneous change in the current
io. Then,
io(t) = - 4e-5t A,
io
2
b) We find the current in the 40 Æ resistor most
easily by using current division; that is,
40 Figure 7.7 The circuit for Example 7.1.
t Ú 0+.
c) We find the voltage vo by direct application of
Ohm’s law:
vo(t) = 40io = - 160e-5t V,
t Ú 0+.
d) The power dissipated in the 10 Æ resistor is
Solution
p10Æ(t) =
a) The switch has been closed for a long time prior
to t = 0, so we know the voltage across the
inductor must be zero at t = 0-. Therefore the
initial current in the inductor is 20 A at t = 0-.
Hence, iL(0+) also is 20 A, because an instantaneous change in the current cannot occur in an
inductor. We replace the resistive circuit connected to the terminals of the inductor with a
single resistor of 10 Æ:
Req = 2 + (40 7 10) = 10 Æ.
The time constant of the circuit is L>Req, or
0.2 s, giving the expression for the inductor
current as
iL(t) = 20e-5t A,
t Ú 0.
v2o
= 2560e-10t W,
10
t Ú 0+.
The total energy dissipated in the 10 Æ resistor is
q
w10Æ(t) =
L0
2560e-10t dt = 256 J.
The initial energy stored in the 2 H inductor is
w(0) =
1 2
1
Li (0) = (2)(400) = 400 J.
2
2
Therefore the percentage of energy dissipated in
the 10 Æ resistor is
256
(100) = 64%.
400
7.1
Example 7.2
The Natural Response of an RL Circuit
Determining the Natural Response of an RL Circuit with Parallel Inductors
In the circuit shown in Fig. 7.8, the initial currents in
inductors L1 and L2 have been established by
sources not shown. The switch is opened at t = 0.
t Ú 0,
= - 1.6 - 2.4e-2t A,
i3 =
a) Find i1, i2, and i3 for t Ú 0.
b) Calculate the initial energy stored in the parallel
inductors.
c) Determine how much energy is stored in the
inductors as t : q .
d) Show that the total energy delivered to the resistive network equals the difference between the
results obtained in (b) and (c).
v(t) 15
= 5.76e-2t A,
10 25
t Ú 0+.
Note that the expressions for the inductor currents
i1 and i2 are valid for t Ú 0, whereas the expression for the resistor current i3 is valid for t Ú 0+.
12 A
i
8
v(t)
4H
Solution
Figure 7.9 A simplification of the circuit shown in Fig. 7.8.
a) The key to finding currents i1, i2, and i3 lies in
knowing the voltage v(t). We can easily find v(t)
if we reduce the circuit shown in Fig. 7.8 to the
equivalent form shown in Fig. 7.9. The parallel
inductors simplify to an equivalent inductance of
4 H, carrying an initial current of 12 A. The resistive network reduces to a single resistance of
8 Æ. Hence the initial value of i(t) is 12 A and
the time constant is 4>8, or 0.5 s. Therefore
b) The initial energy stored in the inductors is
1
1
w = (5)(64) + (20)(16) = 320 J.
2
2
i(t) = 12e-2t A,
c) As t : q , i1 : 1.6 A and i2 : - 1.6 A.
Therefore, a long time after the switch has been
opened, the energy stored in the two inductors is
1
1
w = (5)(1.6)2 + (20)(- 1.6)2 = 32 J.
2
2
t Ú 0.
d) We obtain the total energy delivered to the resistive network by integrating the expression for
the instantaneous power from zero to infinity:
Now v(t) is simply the product 8i, so
v(t) = 96e-2t V,
t Ú 0+ .
q
w =
The circuit shows that v(t) = 0 at t = 0-, so the
expression for v(t) is valid for t Ú 0+. After
obtaining v(t), we can calculate i1, i2, and i3 :
t Ú 0,
1
96e-2x dx - 4
20 L0
4
i1
4A
L1 (5 H)
i2
L2 (20 H)
t0
v(t)
Figure 7.8 The circuit for Example 7.2.
i3
40 L0
1152e-4tdt
e-4t 2
= 288 J.
-4 0
This result is the difference between the initially
stored energy (320 J) and the energy trapped in
the parallel inductors (32 J). The equivalent
inductor for the parallel inductors (which predicts the terminal behavior of the parallel combination) has an initial energy of 288 J; that is,
the energy stored in the equivalent inductor represents the amount of energy that will be delivered to the resistive network at the terminals of
the original inductors.
t
i2 =
pdt =
q
1
96e-2x dx - 8
5 L0
= 1.6 - 9.6e-2t A,
L0
q
= 1152
t
i1 =
8A
219
15 10 220
Response of First-Order RL and RC Circuits
ASSESSMENT PROBLEMS
Objective 1—Be able to determine the natural response of both RL and RC circuits
7.1
The switch in the circuit shown has been closed
for a long time and is opened at t = 0.
Answer:
a) Calculate the initial value of i.
b) Calculate the initial energy stored in the
inductor.
c) What is the time constant of the circuit for
t 7 0?
d) What is the numerical expression for i(t) for
t Ú 0?
e) What percentage of the initial energy stored
has been dissipated in the 2 Æ resistor 5 ms
after the switch has been opened?
7.2
(a) -12.5 A;
(b) 625 mJ;
(c) 4 ms;
(d) -12.5e-250tA, t Ú 0;
(e) 91.8%.
At t = 0, the switch in the circuit shown moves
instantaneously from position a to position b.
a) Calculate vo for t Ú 0+.
b) What percentage of the initial energy stored
in the inductor is eventually dissipated in
the 4 Æ resistor?
6
a
3
120 V
t0
6
i
30 6.4 A
8 mH
vo 10 0.32 H
t0
b
4
2
Answer:
(a) -8e-10t V, t Ú 0;
(b) 80%.
NOTE: Also try Chapter Problems 7.3, 7.8, and 7.9.
7.2 The Natural Response
of an RC Circuit
R1
Vg
a
b
t0
R
C
Figure 7.10 An RC circuit.
C
Vg v i
R
Figure 7.11 The circuit shown in Fig. 7.10, after
switching.
As mentioned in Section 7.1, the natural response of an RC circuit is analogous to that of an RL circuit. Consequently, we don’t treat the RC circuit
in the same detail as we did the RL circuit.
The natural response of an RC circuit is developed from the circuit
shown in Fig. 7.10. We begin by assuming that the switch has been in
position a for a long time, allowing the loop made up of the dc voltage
source Vg, the resistor R1, and the capacitor C to reach a steady-state
condition. Recall from Chapter 6 that a capacitor behaves as an open circuit in the presence of a constant voltage. Thus the voltage source cannot
sustain a current, and so the source voltage appears across the capacitor
terminals. In Section 7.3, we will discuss how the capacitor voltage actually
builds to the steady-state value of the dc voltage source, but for now the
important point is that when the switch is moved from position a to position b (at t = 0), the voltage on the capacitor is Vg. Because there can be
no instantaneous change in the voltage at the terminals of a capacitor, the
problem reduces to solving the circuit shown in Fig. 7.11.
7.2
The Natural Response of an RC Circuit
221
Deriving the Expression for the Voltage
We can easily find the voltage v(t) by thinking in terms of node voltages.
Using the lower junction between R and C as the reference node and summing the currents away from the upper junction between R and C gives
C
dv
v
+
= 0.
dt
R
(7.21)
Comparing Eq. 7.21 with Eq. 7.1 shows that the same mathematical techniques can be used to obtain the solution for v(t). We leave it to you to
show that
v(t) = v(0)e-t>RC,
t Ú 0.
(7.22)
As we have already noted, the initial voltage on the capacitor equals the
voltage source voltage Vg, or
v(0-) = v(0) = v(0+) = Vg = V0,
(7.23)
Initial capacitor voltage
where V0 denotes the initial voltage on the capacitor. The time constant for
the RC circuit equals the product of the resistance and capacitance,
namely,
t = RC.
(7.24)
Time constant for RC circuit
(7.25)
Natural response of an RC circuit
Substituting Eqs. 7.23 and 7.24 into Eq. 7.22 yields
v(t) = V0e-t>t,
t Ú 0,
which indicates that the natural response of an RC circuit is an exponential decay of the initial voltage. The time constant RC governs the rate of
decay. Figure 7.12 shows the plot of Eq. 7.25 and the graphic interpretation of the time constant.
After determining v(t), we can easily derive the expressions for i, p,
and w:
i(t) =
v(t)
V0 -t>t
=
e ,
R
R
p = vi =
V20 -2t>t
R
e
t
w =
=
L0
t Ú 0+ ,
(7.27)
t
p dx =
V0
v(t) = V0et/t
v(t) V0 V0 t
t
0
Figure 7.12 The natural response of an RC circuit.
t Ú 0+ ,
,
(7.26)
v(t)
V20 -2x>t
e
dx
L0 R
1
CV20(1 - e-2t>t), t Ú 0.
2
(7.28)
t
222
Response of First-Order RL and RC Circuits
Calculating the natural response of an RC circuit can be summarized
as follows:
Calculating the natural response of an
RC circuit 1. Find the initial voltage, V0, across the capacitor.
2. Find the time constant of the circuit, t = RC.
3. Use Eq. 7.25, v(t) = V0e-t>t, to generate v(t) from V0 and t.
All other calculations of interest follow from knowing v(t).
Examples 7.3 and 7.4 illustrate the numerical calculations associated with
the natural response of an RC circuit.
Example 7.3
Determining the Natural Response of an RC Circuit
The switch in the circuit shown in Fig. 7.13 has been
in position x for a long time. At t = 0, the switch
moves instantaneously to position y. Find
a) vC(t) for t Ú 0,
b) vo(t) for t Ú 0 + ,
c) io(t) for t Ú 0 + , and
d) the total energy dissipated in the 60 kÆ resistor.
10 k x
t0
100 V
0.5 mF
vC
io
y 32 k
b) The easiest way to find vo(t) is to note that the
resistive circuit forms a voltage divider across
the terminals of the capacitor. Thus
vo(t) =
48
v (t) = 60e-25t V,
80 C
t Ú 0+.
This expression for vo(t) is valid for t Ú 0+
because vo(0-) is zero. Thus we have an instantaneous change in the voltage across the 240 kÆ
resistor.
vo
240 k 60 k
c) We find the current io(t) from Ohm’s law:
Figure 7.13 The circuit for Example 7.3.
io(t) =
vo(t)
60 * 103
= e-25t mA,
t Ú 0+.
d) The power dissipated in the 60 kÆ resistor is
Solution
a) Because the switch has been in position x for a
long time, the 0.5 mF capacitor will charge to
100 V and be positive at the upper terminal. We
can replace the resistive network connected to
the capacitor at t = 0+ with an equivalent resistance of 80 kÆ . Hence the time constant of the
circuit is (0.5 * 10-6)(80 * 103) or 40 ms. Then,
p60kÆ(t) = i2o(t)(60 * 103) = 60e-50t mW,
The total energy dissipated is
q
w60kÆ =
vC(t) = 100e-25t V,
t Ú 0.
t Ú 0+.
L0
i2o(t)(60 * 103) dt = 1.2 mJ.
7.2
Example 7.4
The Natural Response of an RC Circuit
223
Determining the Natural Response of an RC Circuit with Series Capacitors
The initial voltages on capacitors C1 and C2 in the
circuit shown in Fig. 7.14 have been established by
sources not shown. The switch is closed at t = 0.
a) Find v1(t), v2(t), and v(t) for t Ú 0 and i(t) for
t Ú 0+.
b) Calculate the initial energy stored in the capacitors C1 and C2.
c) Determine how much energy is stored in the
capacitors as t : q .
d) Show that the total energy delivered to the
250 kÆ resistor is the difference between the
results obtained in (b) and (c).
C1 (5 mF) v1(t)
4V
v(t) = 20e-t V,
t0 20 V
4 mF v(t)
t
106
v1(t) = 80 * 10-6e-x dx - 4
5 L0
-t
- 20) V,
6
t
t Ú 0,
10
80 * 10-6e-x dx + 24
20 L0
= (4e-t + 20) V,
t Ú 0.
b) The initial energy stored in C1 is
w1 =
1
(5 * 10-6)(16) = 40 mJ.
2
The initial energy stored in C2 is
w2 =
250 k
Figure 7.15 A simplification of the circuit shown in Fig. 7.14.
The total energy stored in the two capacitors is
wo = 40 + 5760 = 5800 mJ.
c) As t : q ,
v1 : -20 V
and v2 : + 20 V.
t Ú 0+.
Knowing i(t), we calculate the expressions for
v1(t) and v2(t):
v2(t) = -
i(t)
Therefore the energy stored in the two capacitors is
v(t)
= 80e-t mA,
250,000
= (16e
Figure 7.14 The circuit for Example 7.4.
t Ú 0.
The current i(t) is
i(t) =
Solution
a) Once we know v(t), we can obtain the current i(t)
from Ohm’s law. After determining i(t), we can
calculate v1(t) and v2(t) because the voltage across
a capacitor is a function of the capacitor current.
To find v(t), we replace the series-connected
capacitors with an equivalent capacitor. It has a
capacitance of 4 mF and is charged to a voltage of
20 V. Therefore, the circuit shown in Fig. 7.14
reduces to the one shown in Fig. 7.15, which
reveals that the initial value of v(t) is 20 V, and that
the time constant of the circuit is (4)(250) * 10-3,
or 1 s. Thus the expression for v(t) is
i(t)
v(t) 250 k
C2 (20 mF) v2(t)
24 V
t0
1
(20 * 10-6)(576) = 5760 mJ.
2
wq =
1
(5 + 20) * 10-6(400) = 5000 mJ.
2
d) The total energy delivered to the 250 kÆ resistor is
q
w =
L0
q
pdt =
400e-2t
dt = 800 mJ.
L0 250,000
Comparing the results obtained in (b) and (c)
shows that
800 mJ = (5800 - 5000) mJ.
The energy stored in the equivalent capacitor in
Fig. 7.15 is 12(4 * 10-6)(400), or 800 mJ. Because
this capacitor predicts the terminal behavior of
the original series-connected capacitors, the
energy stored in the equivalent capacitor is the
energy delivered to the 250 kÆ resistor.
224
Response of First-Order RL and RC Circuits
ASSESSMENT PROBLEMS
Objective 1—Be able to determine the natural response of both RL and RC circuits
7.3
The switch in the circuit shown has been closed
for a long time and is opened at t = 0. Find
a) the initial value of v(t),
b) the time constant for t 7 0,
c) the numerical expression for v(t) after the
switch has been opened,
d) the initial energy stored in the capacitor, and
e) the length of time required to dissipate 75%
of the initially stored energy.
20 k
7.5 mA
80 k
t0
0.4 mF
The switch in the circuit shown has been closed
for a long time before being opened at t = 0.
a) Find vo(t) for t Ú 0.
b) What percentage of the initial energy stored
in the circuit has been dissipated after the
switch has been open for 60 ms?
5 mF
t0
v(t)
20 k
15 k
50 k
15 V
Answer:
7.4
vo(t)
1 mF
40 k
(a) 200 V;
(b) 20 ms;
(c) 200e-50t V, t Ú 0;
(d) 8 mJ;
(e) 13.86 ms.
Answer:
(a) 8e-25t + 4e-10t V, t Ú 0;
(b) 81.05%.
NOTE: Also try Chapter Problems 7.23 and 7.25.
7.3 The Step Response of RL
and RC Circuits
We are now ready to discuss the problem of finding the currents and voltages generated in first-order RL or RC circuits when either dc voltage or
current sources are suddenly applied. The response of a circuit to the sudden application of a constant voltage or current source is referred to as the
step response of the circuit. In presenting the step response, we show how
the circuit responds when energy is being stored in the inductor or capacitor. We begin with the step response of an RL circuit.
The Step Response of an RL Circuit
R
Vs
i
t0
L v(t)
Figure 7.16 A circuit used to illustrate the step
response of a first-order RL circuit.
To begin, we modify the first-order circuit shown in Fig. 7.2(a) by adding a
switch. We use the resulting circuit, shown in Fig. 7.16, in developing the
step response of an RL circuit. Energy stored in the inductor at the time
the switch is closed is given in terms of a nonzero initial current i(0). The
task is to find the expressions for the current in the circuit and for the voltage across the inductor after the switch has been closed. The procedure is
the same as that used in Section 7.1; we use circuit analysis to derive the
7.3
The Step Response of RL and RC Circuits
differential equation that describes the circuit in terms of the variable of
interest, and then we use elementary calculus to solve the equation.
After the switch in Fig. 7.16 has been closed, Kirchhoff’s voltage law
requires that
Vs = Ri + L
di
,
dt
(7.29)
which can be solved for the current by separating the variables i and t and
then integrating. The first step in this approach is to solve Eq. 7.29 for the
derivative di>dt:
-Ri + Vs
Vs
di
-R
=
=
ai - b.
dt
L
L
R
(7.30)
Next, we multiply both sides of Eq. 7.30 by a differential time dt. This step
reduces the left-hand side of the equation to a differential change in the
current. Thus
Vs
di
-R
dt =
ai - b dt,
dt
L
R
(7.31)
or
di =
Vs
-R
ai - b dt.
L
R
We now separate the variables in Eq. 7.31 to get
di
-R
=
dt,
i - (Vs>R)
L
(7.32)
and then integrate both sides of Eq. 7.32. Using x and y as variables for the
integration, we obtain
i(t)
LI0
t
dx
-R
=
dy,
x - (Vs>R)
L L0
(7.33)
where I0 is the current at t = 0 and i(t) is the current at any t 7 0.
Performing the integration called for in Eq. 7.33 generates the expression
ln
from which
i(t) - (Vs>R)
I0 - (Vs>R)
i(t) - (Vs>R)
I0 - (Vs>R)
=
-R
t,
L
(7.34)
= e-(R>L)t,
or
i(t) =
Vs
Vs -(R>L)t
+ a I0 be
.
R
R
(7.35)
When the initial energy in the inductor is zero, I0 is zero. Thus Eq. 7.35
reduces to
i(t) =
Vs
Vs
- e-(R>L)t.
R
R
(7.36)
Equation 7.36 indicates that after the switch has been closed, the current increases exponentially from zero to a final value of Vs>R. The time
constant of the circuit, L>R, determines the rate of increase. One time
Step response of RL circuit
225
226
Response of First-Order RL and RC Circuits
constant after the switch has been closed, the current will have reached
approximately 63% of its final value, or
i(t) =
Vs
Vs -1
Vs
e L 0.6321 .
R
R
R
(7.37)
If the current were to continue to increase at its initial rate, it would reach
its final value at t = t; that is, because
- Vs - 1 -t>t
Vs -t>t
di
=
a
be
=
e ,
t
dt
R
L
(7.38)
the initial rate at which i(t) increases is
Vs
di
(0) =
.
dt
L
(7.39)
If the current were to continue to increase at this rate, the expression for i
would be
Vs
t,
L
(7.40)
Vs L
Vs
=
.
L R
R
(7.41)
i =
from which, at t = t,
i =
Equations 7.36 and 7.40 are plotted in Fig. 7.17. The values given by
Eqs. 7.37 and 7.41 are also shown in this figure.
The voltage across an inductor is Ldi>dt, so from Eq. 7.35, for t Ú 0+,
i(t)
i(t) Vs
t
L
Vs
R
v = La
V
V
i(t) s s et/
R
R
V
0.632 s
R
t
0
2
3
4
5
Figure 7.17 The step response of the RL circuit
shown in Fig. 7.16 when I0 = 0.
v
Vs -(R>L)t
-R
b a I0 be
= (Vs - I0R)e-(R>L)t.
L
R
(7.42)
The voltage across the inductor is zero before the switch is closed.
Equation 7.42 indicates that the inductor voltage jumps to Vs - I0R at
the instant the switch is closed and then decays exponentially to zero.
Does the value of v at t = 0+ make sense? Because the initial current
is I0 and the inductor prevents an instantaneous change in current, the
current is I0 in the instant after the switch has been closed. The voltage
drop across the resistor is I0R, and the voltage impressed across the inductor is the source voltage minus the voltage drop, that is, Vs - I0R.
When the initial inductor current is zero, Eq. 7.42 simplifies to
Vs
v = Vs e-(R>L)t.
R
v Vs Vs t
L
0.368 Vs
0
v Vse(R/L)t
2
3
4
Figure 7.18 Inductor voltage versus time.
5
t
(7.43)
If the initial current is zero, the voltage across the inductor jumps to Vs. We
also expect the inductor voltage to approach zero as t increases, because the
current in the circuit is approaching the constant value of Vs>R. Figure 7.18
shows the plot of Eq. 7.43 and the relationship between the time constant
and the initial rate at which the inductor voltage is decreasing.
7.3
The Step Response of RL and RC Circuits
227
If there is an initial current in the inductor, Eq. 7.35 gives the solution
for it. The algebraic sign of I0 is positive if the initial current is in the same
direction as i; otherwise, I0 carries a negative sign. Example 7.5 illustrates
the application of Eq. 7.35 to a specific circuit.
Example 7.5
Determining the Step Response of an RL Circuit
The switch in the circuit shown in Fig. 7.19 has been
in position a for a long time. At t = 0, the switch
moves from position a to position b. The switch is a
make-before-break type; that is, the connection at
position b is established before the connection at
position a is broken, so there is no interruption of
current through the inductor.
a) Find the expression for i(t) for t Ú 0.
b) What is the initial voltage across the inductor just
after the switch has been moved to position b?
c) How many milliseconds after the switch has been
moved does the inductor voltage equal 24 V?
d) Does this initial voltage make sense in terms of
circuit behavior?
e) Plot both i(t) and v(t) versus t.
t0
2
24 V
b) The voltage across the inductor is
di
v = L
dt
= 0.2(200e-10t)
= 40e-10t V,
t Ú 0+ .
The initial inductor voltage is
v(0+) = 40 V.
c) Yes; in the instant after the switch has been
moved to position b, the inductor sustains a current of 8 A counterclockwise around the newly
formed closed path. This current causes a 16 V
drop across the 2 Æ resistor. This voltage drop
adds to the drop across the source, producing a
40 V drop across the inductor.
d) We find the time at which the inductor voltage
equals 24 V by solving the expression
24 = 40e-10t
b
a
i v
200 mH
for t:
t =
10 8 A
1
40
ln
10 24
= 51.08 * 10-3
= 51.08 ms.
Figure 7.19 The circuit for Example 7.5.
e) Figure 7.20 shows the graphs of i(t) and v(t) versus
t. Note that the instant of time when the current
equals zero corresponds to the instant of time
when the inductor voltage equals the source voltage of 24 V, as predicted by Kirchhoff’s voltage law.
Solution
a) The switch has been in position a for a long time,
so the 200 mH inductor is a short circuit across
the 8 A current source. Therefore, the inductor
carries an initial current of 8 A. This current is
oriented opposite to the reference direction for i;
thus I0 is -8 A. When the switch is in position b,
the final value of i will be 24>2, or 12 A. The time
constant of the circuit is 200>2, or 100 ms.
Substituting these values into Eq. 7.35 gives
v(V) i(A)
40
32
24
16
8
12
8
4
4
v
100
i
200
300
400
500
8
i = 12 + (-8 - 12)e
-t>0.1
= 12 - 20e-10t A,
t Ú 0.
Figure 7.20 The current and voltage waveforms for
Example 7.5.
t (ms)
228
Response of First-Order RL and RC Circuits
ASSESSMENT PROBLEM
Objective 2—Be able to determine the step response of both RL and RC circuits
7.5
Assume that the switch in the circuit shown in
Fig. 7.19 has been in position b for a long time,
and at t = 0 it moves to position a. Find
(a) i(0+); (b) v(0+); (c) t, t 7 0; (d) i(t), t Ú 0;
and (e) v(t), t Ú 0+.
Answer:
(a) 12 A;
(b) -200 V;
(c) 20 ms;
(d) -8 + 20e-50t A, t Ú 0;
(e) -200e-50t V, t Ú 0+.
NOTE: Also try Chapter Problems 7.35–7.37.
We can also describe the voltage v(t) across the inductor in Fig. 7.16
directly, not just in terms of the circuit current. We begin by noting that the
voltage across the resistor is the difference between the source voltage
and the inductor voltage. We write
i(t) =
Vs
v(t)
,
R
R
(7.44)
where Vs is a constant. Differentiating both sides with respect to time yields
di
1 dv
= .
dt
R dt
(7.45)
Then, if we multiply each side of Eq. 7.45 by the inductance L, we get an
expression for the voltage across the inductor on the left-hand side, or
v = -
L dv
.
R dt
(7.46)
Putting Eq. 7.46 into standard form yields
dv
R
+
v = 0.
dt
L
(7.47)
You should verify (in Problem 7.38) that the solution to Eq. 7.47 is identical to that given in Eq. 7.42.
At this point, a general observation about the step response of an
RL circuit is pertinent. (This observation will prove helpful later.) When
we derived the differential equation for the inductor current, we obtained
Eq. 7.29. We now rewrite Eq. 7.29 as
Vs
di
R
+
i = .
dt
L
L
(7.48)
Observe that Eqs. 7.47 and 7.48 have the same form. Specifically, each
equates the sum of the first derivative of the variable and a constant times
the variable to a constant value. In Eq. 7.47, the constant on the right-hand
side happens to be zero; hence this equation takes on the same form as the
natural response equations in Section 7.1. In both Eq. 7.47 and Eq. 7.48,
the constant multiplying the dependent variable is the reciprocal of the
time constant, that is, R>L = 1>t. We encounter a similar situation in the
derivations for the step response of an RC circuit. In Section 7.4, we will
use these observations to develop a general approach to finding the natural and step responses of RL and RC circuits.
7.3
The Step Response of RL and RC Circuits
The Step Response of an RC Circuit
We can find the step response of a first-order RC circuit by analyzing the
circuit shown in Fig. 7.21. For mathematical convenience, we choose the
Norton equivalent of the network connected to the equivalent capacitor.
Summing the currents away from the top node in Fig. 7.21 generates the
differential equation
C
dvC
vC
+
= Is.
dt
R
(7.49)
t0
Is
R
C
i
Figure 7.21 A circuit used to illustrate the step
response of a first-order RC circuit.
Division of Eq. 7.49 by C gives
dvC
vC
Is
+
= .
dt
RC
C
(7.50)
Comparing Eq. 7.50 with Eq. 7.48 reveals that the form of the solution for
vC is the same as that for the current in the inductive circuit, namely,
Eq. 7.35. Therefore, by simply substituting the appropriate variables and
coefficients, we can write the solution for vC directly. The translation
requires that Is replace Vs, C replace L, 1>R replace R, and V0 replace I0.
We get
vC = IsR + (V0 - IsR)e-t>RC,
t Ú 0.
(7.51)
A similar derivation for the current in the capacitor yields the differential
equation
di
1
+
i = 0.
dt
RC
(7.52)
Equation 7.52 has the same form as Eq. 7.47, hence the solution for i is
obtained by using the same translations used for the solution of
Eq. 7.50. Thus
i = a Is -
V0 -t>RC
be
,
R
t Ú 0+ ,
(7.53)
where V0 is the initial value of vC, the voltage across the capacitor.
We obtained Eqs. 7.51 and 7.53 by using a mathematical analogy to
the solution for the step response of the inductive circuit. Let’s see
whether these solutions for the RC circuit make sense in terms of
known circuit behavior. From Eq. 7.51, note that the initial voltage
across the capacitor is V0, the final voltage across the capacitor is IsR,
and the time constant of the circuit is RC. Also note that the solution
for vC is valid for t Ú 0. These observations are consistent with the
behavior of a capacitor in parallel with a resistor when driven by a constant current source.
Equation 7.53 predicts that the current in the capacitor at t = 0+ is
Is - V0>R. This prediction makes sense because the capacitor voltage cannot change instantaneously, and therefore the initial current in the resistor
is V0>R. The capacitor branch current changes instantaneously from zero
at t = 0- to Is - V0>R at t = 0+. The capacitor current is zero at t = q .
Also note that the final value of v = IsR.
Example 7.6 illustrates how to use Eqs. 7.51 and 7.53 to find the step
response of a first-order RC circuit.
vC
Step response of an RC circuit
229
230
Response of First-Order RL and RC Circuits
Example 7.6
Determining the Step Response of an RC Circuit
The switch in the circuit shown in Fig. 7.22 has been
in position 1 for a long time. At t = 0, the switch
moves to position 2. Find
a) vo(t) for t Ú 0 and
b) io(t) for t Ú 0+.
The value of the Norton current source is the
ratio of the open-circuit voltage to the Thévenin
resistance, or - 60>(40 * 103) = - 1.5 mA. The
resulting Norton equivalent circuit is shown in
Fig. 7.23. From Fig. 7.23, IsR = - 60 V and
RC = 10 ms. We have already noted that
vo(0) = 30 V, so the solution for vo is
vo = - 60 + [30 - ( -60)]e-100t
20 k
40 V
1
t0
2 8 k
40 k
io
60 k
0.25 mF
vo
160 k
= - 60 + 90e-100t V, t Ú 0.
75 V
Figure 7.22 The circuit for Example 7.6.
b) We write the solution for io directly from
Eq. 7.53 by noting that Is = - 1.5 mA and
Vo>R = (30>40) * 10-3, or 0.75 mA:
io = - 2.25e-100t mA, t Ú 0+.
We check the consistency of the solutions for vo
and io by noting that
Solution
a) The switch has been in position 1 for a long time,
so the initial value of vo is 40(60>80), or 30 V. To
take advantage of Eqs. 7.51 and 7.53, we find the
Norton equivalent with respect to the terminals
of the capacitor for t Ú 0. To do this, we begin by
computing the open-circuit voltage, which is
given by the -75 V source divided across the
40 kÆ and 160 kÆ resistors:
io = C
dvo
= (0.25 * 10-6)( -9000e-100t)
dt
= - 2.25e-100t mA.
Because dvo(0-)>dt = 0, the expression for io
clearly is valid only for t Ú 0+.
Voc =
160 * 103
( - 75) = - 60 V.
(40 + 160) * 103
Next, we calculate the Thévenin resistance, as
seen to the right of the capacitor, by shorting the
-75 V source and making series and parallel
combinations of the resistors:
RTh = 8000 + 40,000 7 160,000 = 40 kÆ
30 V
0.25 mF
40 k
1.5 mA
Figure 7.23 The equivalent circuit for t 7 0 for the circuit
shown in Fig. 7.22.
7.4
A General Solution for Step and Natural Responses
ASSESSMENT PROBLEM
Objective 2—Be able to determine the step response of both RL and RC circuits
7.6
a) Find the expression for the voltage across
the 160 kÆ resistor in the circuit shown in
Fig. 7.22. Let this voltage be denoted vA, and
assume that the reference polarity for the
voltage is positive at the upper terminal of
the 160 kÆ resistor.
b) Specify the interval of time for which the
expression obtained in (a) is valid.
Answer:
(a) -60 + 72e-100t V;
(b) t Ú 0+.
NOTE: Also try Chapter Problems 7.53 and 7.54.
7.4 A General Solution for Step
and Natural Responses
RTh
The general approach to finding either the natural response or the step
response of the first-order RL and RC circuits shown in Fig. 7.24 is based
on their differential equations having the same form (compare Eq. 7.48
and Eq. 7.50). To generalize the solution of these four possible circuits, we
let x(t) represent the unknown quantity, giving x(t) four possible values. It
can represent the current or voltage at the terminals of an inductor or the
current or voltage at the terminals of a capacitor. From Eqs. 7.47, 7.48,
7.50, and 7.52, we know that the differential equation describing any one
of the four circuits in Fig. 7.24 takes the form
i
VTh
L v
(a)
VTh
RTh
i
L v
RTh
dx
x
+
= K,
t
dt
(b)
(7.54)
RTh
xf = Kt,
(7.55)
where xf represents the final value of the variable.
We solve Eq. 7.54 by separating the variables, beginning by solving for
the first derivative:
-(x - xf)
- (x - Kt)
dx
-x
=
+ K =
=
.
t
t
t
dt
(7.56)
i
VTh C
where the value of the constant K can be zero. Because the sources in the
circuit are constant voltages and/or currents, the final value of x will be
constant; that is, the final value must satisfy Eq. 7.54, and, when x reaches
its final value, the derivative dx>dt must be zero. Hence
v
(c)
VTh
RTh
i
RTh
C
v
(d)
Figure 7.24 Four possible first-order circuits.
(a) An inductor connected to a Thévenin equivalent.
(b) An inductor connected to a Norton equivalent.
(c) A capacitor connected to a Thévenin equivalent.
(d) A capacitor connected to a Norton equivalent.
231
232
Response of First-Order RL and RC Circuits
In writing Eq. 7.56, we used Eq. 7.55 to substitute xf for Kt. We now multiply both sides of Eq. 7.56 by dt and divide by x - xf to obtain
dx
-1
=
dt.
t
x - xf
(7.57)
Next, we integrate Eq. 7.57. To obtain as general a solution as possible, we
use time t0 as the lower limit and t as the upper limit. Time t0 corresponds
to the time of the switching or other change. Previously we assumed that
t0 = 0, but this change allows the switching to take place at any time.
Using u and v as symbols of integration, we get
x(t)
t
du
1
= dv.
t Lt0
u
x
f
Lx(t0)
(7.58)
Carrying out the integration called for in Eq. 7.58 gives
General solution for natural and step
responses of RL and RC circuits x(t) = xf + [x(t0) - xf ]e-(t - t0)>t.
(7.59)
The importance of Eq. 7.59 becomes apparent if we write it out in words:
the unknown
the final
variable as a = value of the
function of time
variable
the initial
the final
- [t - (time of switching)]
+ C value of the - value of the S * e (time constant)
variable
variable
(7.60)
In many cases, the time of switching—that is, t0—is zero.
When computing the step and natural responses of circuits, it may
help to follow these steps:
Calculating the natural or step response of
RL or RC circuits 1. Identify the variable of interest for the circuit. For RC circuits, it is
most convenient to choose the capacitive voltage; for RL circuits,
it is best to choose the inductive current.
2. Determine the initial value of the variable, which is its value at t0.
Note that if you choose capacitive voltage or inductive current as
your variable of interest, it is not necessary to distinguish between
t = t0- and t = t0+.2 This is because they both are continuous variables. If you choose another variable, you need to remember that
its initial value is defined at t = t0+.
3. Calculate the final value of the variable, which is its value as t : q .
4. Calculate the time constant for the circuit.
With these quantities, you can use Eq. 7.60 to produce an equation
describing the variable of interest as a function of time. You can then find
equations for other circuit variables using the circuit analysis techniques
introduced in Chapters 3 and 4 or by repeating the preceding steps for the
other variables.
Examples 7.7–7.9 illustrate how to use Eq. 7.60 to find the step
response of an RC or RL circuit.
2 The expressions t- and t + are analogous to 0- and 0+ . Thus x(t -) is the limit of x(t) as t : t
0
0
0
from the left, and x(t0+) is the limit of x(t) as t : t0 from the right.
0
7.4
Example 7.7
A General Solution for Step and Natural Responses
233
Using the General Solution Method to Find an RC Circuit’s Step Response
The switch in the circuit shown in Fig. 7.25 has been
in position a for a long time. At t = 0 the switch is
moved to position b.
a) What is the initial value of vC?
b) What is the final value of vC?
c) What is the time constant of the circuit when the
switch is in position b?
d) What is the expression for vC(t) when t Ú 0?
e) What is the expression for i(t) when t Ú 0 + ?
f) How long after the switch is in position b does
the capacitor voltage equal zero?
g) Plot vC(t) and i(t) versus t.
400 k
90 V
b
20 a
t0
i vC
60 0.5 mF
40 V
Figure 7.25 The circuit for Example 7.7.
capacitor voltage cannot change instantaneously.
The final value of i(t) = 0, so
i(t) = 0 + (300 - 0)e-5t
= 300e-5t mA, t Ú 0+.
Solution
a) The switch has been in position a for a long time,
so the capacitor looks like an open circuit.
Therefore the voltage across the capacitor is the
voltage across the 60 Æ resistor. From the voltagedivider rule, the voltage across the 60 Æ resistor
is 40 * [60>(60 + 20)], or 30 V. As the reference for vC is positive at the upper terminal of
the capacitor, we have vC(0) = -30 V.
b) After the switch has been in position b for a long
time, the capacitor will look like an open circuit
in terms of the 90 V source. Thus the final value
of the capacitor voltage is + 90 V.
c) The time constant is
t = RC
= (400 * 103)(0.5 * 10-6)
= 0.2 s.
We could have obtained this solution by differentiating the solution in (d) and multiplying by
the capacitance. You may want to do so for yourself. Note that this alternative approach to finding
i(t) also predicts the discontinuity at t = 0.
f) To find how long the switch must be in position b
before the capacitor voltage becomes zero, we
solve the equation derived in (d) for the time
when vC(t) = 0:
120e-5t = 90
or e5t =
120
,
90
so
1
4
ln a b
5
3
= 57.54 ms.
t =
Note that when vC = 0, i = 225 mA and the
voltage drop across the 400 kÆ resistor is 90 V.
g) Figure 7.26 shows the graphs of vC(t) and i(t)
versus t.
d) Substituting the appropriate values for vf, v(0),
and t into Eq. 7.60 yields
vC(t) = 90 + ( -30 - 90)e-5t
= 90 - 120e-5t V,
t Ú 0.
e) Here the value for t doesn’t change. Thus we
need to find only the initial and final values for
the current in the capacitor. When obtaining the
initial value, we must get the value of i(0+),
because the current in the capacitor can change
instantaneously. This current is equal to the current in the resistor, which from Ohm’s law is
[90 - ( -30)]>(400 * 103) = 300 mA. Note that
when applying Ohm’s law we recognized that the
i (mA) vC (V)
300 120
250 100
200 80
150 60
100 40
50 20
0
20
vC
i
200
400
600
800
t (ms)
30
Figure 7.26 The current and voltage waveforms for
Example 7.7.
234
Response of First-Order RL and RC Circuits
Example 7.8
Using the General Solution Method with Zero Initial Conditions
The switch in the circuit shown in Fig. 7.27 has been
open for a long time. The initial charge on the
capacitor is zero. At t = 0, the switch is closed. Find
the expression for
Therefore t = (20 + 30)103(0.1) * 10-6 = 5 ms.
Substituting these values into Eq. 7.60 generates
the expression
i(t) = 0 + (3 - 0)e-t>5 * 10
= 3e-200t mA, t Ú 0+.
-3
a) i(t) for t Ú 0 + and
b) v(t) when t Ú 0 + .
t0
0.1 mF
v(t)
7.5 mA
20 k
i(t)
30 k
Figure 7.27 The circuit for Example 7.8.
Solution
a) Because the initial voltage on the capacitor is
zero, at the instant when the switch is closed the
current in the 30 kÆ branch will be
The final value of the capacitor current will be
zero because the capacitor eventually will
appear as an open circuit in terms of dc current.
Thus if = 0. The time constant of the circuit will
equal the product of the Thévenin resistance (as
seen from the capacitor) and the capacitance.
a) v(t) when t Ú 0 + and
b) i(t) when t Ú 0.
t0
i(t)
1
20 V
Hence the expression for the voltage v(t) is
As one check on this expression, note that it predicts the initial value of the voltage across the
20 Æ resistor as 150 - 60, or 90 V. The instant
the switch is closed, the current in the 20 kÆ
resistor is (7.5)(30>50), or 4.5 mA. This current
produces a 90 V drop across the 20 kÆ resistor,
confirming the value predicted by the solution.
Using the General Solution Method to Find an RL Circuit’s Step Response
The switch in the circuit shown in Fig. 7.28 has been
open for a long time. At t = 0 the switch is closed.
Find the expression for
vC(t) = 150 + (0 - 150)e-200t
= (150 - 150e-200t) V, t Ú 0.
v(t) = 150 - 150e-200t + (30)(3)e-200t
= (150 - 60e-200t) V, t Ú 0+.
(7.5)(20)
50
= 3 mA.
i(0+) =
Example 7.9
b) To find v(t), we note from the circuit that it
equals the sum of the voltage across the capacitor and the voltage across the 30 kÆ resistor. To
find the capacitor voltage (which is a drop in the
direction of the current), we note that its initial
value is zero and its final value is (7.5)(20), or
150 V. The time constant is the same as before, or
5 ms. Therefore we use Eq. 7.60 to write
3
v(t)
Figure 7.28 The circuit for Example 7.9.
80 mH
Solution
a) The switch has been open for a long time, so the
initial current in the inductor is 5 A, oriented
from top to bottom. Immediately after the switch
closes, the current still is 5 A, and therefore the
initial voltage across the inductor becomes
20 - 5(1), or 15 V. The final value of the inductor voltage is 0 V. With the switch closed, the time
constant is 80>1, or 80 ms. We use Eq. 7.60 to
write the expression for v(t):
v(t) = 0 + (15 - 0)e-t>80 * 10
= 15e-12.5t V, t Ú 0+.
-3
b) We have already noted that the initial value of
the inductor current is 5 A. After the switch has
7.4
been closed for a long time, the inductor current
reaches 20>1, or 20 A. The circuit time constant is
80 ms, so the expression for i(t) is
A General Solution for Step and Natural Responses
235
We determine that the solutions for v(t) and i(t)
agree by noting that
di
dt
= 80 * 10-3[15(12.5)e-12.5t]
= 15e-12.5t V, t Ú 0+.
v(t) = L
i(t) = 20 + (5 - 20)e
= (20 - 15e-12.5t) A,
-12.5t
t Ú 0.
NOTE: Assess your understanding of the general solution method by trying Chapter Problems 7.51 and 7.53.
Example 7.10 shows that Eq. 7.60 can even be used to find the step
response of some circuits containing magnetically coupled coils.
Example 7.10
Determining Step Response of a Circuit with Magnetically Coupled Coils
There is no energy stored in the circuit in Fig. 7.29
at the time the switch is closed.
7.5 a) Find the solutions for io, vo, i1, and i2.
b) Show that the solutions obtained in (a) make
sense in terms of known circuit behavior.
120 V
t0
6H
vo 3 H
15 H
i1
i2
Figure 7.29 The circuit for Example 7.10.
Solution
a) For the circuit in Fig. 7.29, the magnetically coupled coils can be replaced by a single inductor
having an inductance of
2
Leq =
io
L1L2 - M
45 - 36
=
= 1.5 H.
L1 + L2 - 2M
18 - 12
(See Problem 6.41.) It follows that the circuit in
Fig. 7.29 can be simplified as shown in Fig. 7.30.
By hypothesis the initial value of io is zero.
From Fig. 7.30 we see that the final value of io
will be 120>7.5 or 16 A. The time constant of the
circuit is 1.5>7.5 or 0.2 s. It follows directly from
Eq. 7.60 that
io = 16 - 16e-5t A,
t Ú 0.
The voltage vo follows from Kirchhoff’s
voltage law. Thus,
vo = 120 - 7.5io
= 120e-5t V,
t Ú 0+ .
To find i1 and i2 we first note from
Fig. 7.29 that
di1
di2
di1
di2
3
+ 6
= 6
+ 15
dt
dt
dt
dt
7.5 120 V
vo 1.5 H
Figure 7.30 The circuit in Fig. 7.29 with the magnetically
coupled coils replaced by an equivalent coil.
It also follows from Fig. 7.29 that because
io = i1 + i2,
dio
di1
di2
=
+
.
dt
dt
dt
Therefore
80e-5t = - 2
di2
.
dt
Because i2(0) is zero we have
t
i2 =
L0
-40e-5x dx
= - 8 + 8e-5t A,
di1
di2
= -3
.
dt
dt
t Ú 0.
Using Kirchhoff’s current law we get
i1 = 24 - 24e-5t A,
or
io
t0
t Ú 0.
b) First we observe that io(0), i1(0), and i2(0) are all
zero, which is consistent with the statement that
236
Response of First-Order RL and RC Circuits
no energy is stored in the circuit at the instant
the switch is closed.
Next we observe vo(0+) = 120 V, which is
consistent with the fact that io(0) = 0.
Now we observe the solutions for i1 and
i2 are consistent with the solution for vo by
observing
di1
di2
vo = 3
+ 6
dt
dt
= 360e
-5t
- 240e
= 120e
-5t
V,
-5t
Regardless of which expression we use, we
obtain
l1 = l2 = 24 - 24e-5t Wb-turns.
Note the solution for l1 or l2 is consistent with
the solution for vo.
The final value of the flux linking either
coil 1 or coil 2 is 24 Wb-turns, that is,
l1( q ) = l2( q ) = 24 Wb-turns.
The final value of i1 is
t Ú 0 ,
+
i1( q ) = 24 A
or
vo = 6
di1
di2
+ 15
dt
dt
and the final value of i2 is
i2( q ) = - 8 A.
= 720e-5t - 600e-5t
= 120e-5t V,
t Ú 0+ .
The final values of i1 and i2 can be checked
using flux linkages. The flux linking the 3 H coil
(l1) must be equal to the flux linking the 15 H
coil (l2), because
vo =
=
dl1
dt
dl2
.
dt
Now
l1 = 3i1 + 6i2 Wb-turns
and
l2 = 6i1 + 15i2 Wb-turns.
The consistency between these final values
for i1 and i2 and the final value of the flux linkage can be seen from the expressions:
l1( q ) = 3i1( q ) + 6i2( q )
= 3(24) + 6( -8) = 24 Wb-turns,
l2( q ) = 6i1( q ) + 15i2( q )
= 6(24) + 15( -8) = 24 Wb-turns.
It is worth noting that the final values of i1
and i2 can only be checked via flux linkage
because at t = q the two coils are ideal short
circuits. The division of current between ideal
short circuits cannot be found from Ohm’s law.
NOTE: Assess your understanding of this material by using the general solution method to solve Chapter
Problems 7.68 and 7.71.
7.5 Sequential Switching
Whenever switching occurs more than once in a circuit, we have sequential
switching. For example, a single, two-position switch may be switched back
and forth, or multiple switches may be opened or closed in sequence. The
time reference for all switchings cannot be t = 0. We determine the voltages and currents generated by a switching sequence by using the techniques described previously in this chapter. We derive the expressions for
v(t) and i(t) for a given position of the switch or switches and then use
these solutions to determine the initial conditions for the next position of
the switch or switches.
With sequential switching problems, a premium is placed on obtaining
the initial value x(t0). Recall that anything but inductive currents and
capacitive voltages can change instantaneously at the time of switching.
Thus solving first for inductive currents and capacitive voltages is even
more pertinent in sequential switching problems. Drawing the circuit that
pertains to each time interval in such a problem is often helpful in the
solution process.
7.5
Sequential Switching
237
Examples 7.11 and 7.12 illustrate the analysis techniques for circuits
with sequential switching. The first is a natural response problem with two
switching times, and the second is a step response problem.
Example 7.11
Analyzing an RL Circuit that has Sequential Switching
4
The two switches in the circuit shown in Fig. 7.31
have been closed for a long time. At t = 0, switch 1
is opened. Then, 35 ms later, switch 2 is opened.
a) Find iL(t) for 0 … t … 35 ms.
b) Find iL for t Ú 35 ms.
c) What percentage of the initial energy stored in
the 150 mH inductor is dissipated in the 18 Æ
resistor?
d) Repeat (c) for the 3 Æ resistor.
e) Repeat (c) for the 6 Æ resistor.
60 V
3
12 Figure 7.32 The circuit shown in Fig. 7.31, for t 6 0.
3
1
2
iL
vL 150 mH
6
t0
1
60 V
12 t 35 ms
3
6
iL(0 ) 6A
Figure 7.33 The circuit shown in Fig. 7.31, for 0 … t … 35 ms.
iL 2
vL 150 mH
18 4
iL(0)
6
18 b) When t = 35 ms, the value of the inductor
current is
iL = 6e-1.4 = 1.48 A.
Figure 7.31 The circuit for Example 7.11.
Thus, when switch 2 is opened, the circuit
reduces to the one shown in Fig. 7.34, and the
time constant changes to (150>9) * 10-3, or
16.67 ms. The expression for iL becomes
Solution
iL = 1.48e-60(t - 0.035) A,
a) For t 6 0 both switches are closed, causing the
150 mH inductor to short-circuit the 18 Æ resistor.The equivalent circuit is shown in Fig. 7.32.We
determine the initial current in the inductor by
solving for iL(0-) in the circuit shown in Fig. 7.32.
After making several source transformations, we
find iL(0-) to be 6 A. For 0 … t … 35 ms, switch 1
is open (switch 2 is closed), which disconnects the
60 V voltage source and the 4 Æ and 12 Æ resistors from the circuit. The inductor is no longer
behaving as a short circuit (because the dc source
is no longer in the circuit), so the 18 Æ resistor is
no longer short-circuited. The equivalent circuit is
shown in Fig. 7.33. Note that the equivalent resistance across the terminals of the inductor is the
parallel combination of 9 Æ and 18 Æ, or 6 Æ.
The time constant of the circuit is (150>6) * 10-3,
or 25 ms. Therefore the expression for iL is
iL = 6e-40t A,
0 … t … 35 ms.
t Ú 35 ms.
Note that the exponential function is shifted in
time by 35 ms.
3
iL
vL 150 mH
6
iL(0.035) ⬵ 1.48A
Figure 7.34 The circuit shown in Fig. 7.31, for t Ú 35 ms.
c) The 18 Æ resistor is in the circuit only during the
first 35 ms of the switching sequence. During this
interval, the voltage across the resistor is
vL = 0.15
d
(6e-40t)
dt
= - 36e-40t V,
0 6 t 6 35 ms.
238
Response of First-Order RL and RC Circuits
The power dissipated in the 18 Æ resistor is
p =
v2L
= 72e-80t W,
18
Hence the energy dissipated in the 3 Æ resistor for
t 7 35 ms is
0 6 t 6 35 ms.
q
w3Æ =
Hence the energy dissipated is
q
0.035
w =
=
72e
L0
i23Æ * 3 dt
L0.035
-80t
dt
=
72 -80t 2 0.035
e
-80
0
3(36)e-2.8e-120(t - 0.035) dt
L0.035
= 108e-2.8 *
= 0.9(1 - e-2.8)
=
= 845.27 mJ.
e-120(t - 0.035) 2 q
- 120
0.035
108 -2.8
e
= 54.73 mJ.
120
The initial energy stored in the 150 mH inductor is
The total energy dissipated in the 3 Æ resistor is
1
(0.15)(36) = 2.7 J = 2700 mJ.
2
w3Æ(total) = 563.51 + 54.73
wi =
= 618.24 mJ.
Therefore (845.27>2700) * 100, or 31.31% of
the initial energy stored in the 150 mH inductor
is dissipated in the 18 Æ resistor.
d) For 0 6 t 6 35 ms, the voltage across the 3 Æ
resistor is
v3Æ = ¢
=
vL
≤ (3)
9
1
v
3 L
The percentage of the initial energy stored is
618.24
* 100 = 22.90%.
2700
e) Because the 6 Æ resistor is in series with the 3 Æ
resistor, the energy dissipated and the percentage of the initial energy stored will be twice that
of the 3 Æ resistor:
= - 12e-40t V.
w6Æ(total) = 1236.48 mJ,
Therefore the energy dissipated in the 3 Æ resistor in the first 35 ms is
and the percentage of the initial energy stored is
45.80%. We check these calculations by observing that
0.035
w3Æ =
L0
144e-80t
dt
3
= 0.6(1 - e-2.8)
= 563.51 mJ.
1236.48 + 618.24 + 845.27 = 2699.99 mJ
and
31.31 + 22.90 + 45.80 = 100.01%.
For t 7 35 ms, the current in the 3 Æ resistor is
i3Æ = iL = (6e-1.4)e-60(t - 0.035) A.
The small discrepancies in the summations are
the result of roundoff errors.
7.5
Example 7.12
Sequential Switching
239
Analyzing an RC Circuit that has Sequential Switching
The uncharged capacitor in the circuit shown in
Fig. 7.35 is initially switched to terminal a of the
three-position switch. At t = 0, the switch is moved
to position b, where it remains for 15 ms. After the
15 ms delay, the switch is moved to position c, where
it remains indefinitely.
a) Derive the numerical expression for the voltage
across the capacitor.
b) Plot the capacitor voltage versus time.
c) When will the voltage on the capacitor equal
200 V?
Solution
a) At the instant the switch is moved to position b,
the initial voltage on the capacitor is zero. If the
switch were to remain in position b, the capacitor
would eventually charge to 400 V. The time constant of the circuit when the switch is in position b
is 10 ms. Therefore we can use Eq. 7.59 with
t0 = 0 to write the expression for the capacitor
voltage:
a
100 k b
400 V
c
50 k
v(t)
0.1mF
Figure 7.35 The circuit for Example 7.12.
In writing the expression for v, we recognized
that t0 = 15 ms and that this expression is valid
only for t Ú 15 ms.
b) Figure 7.36 shows the plot of v versus t.
c) The plot in Fig. 7.36 reveals that the capacitor
voltage will equal 200 V at two different times:
once in the interval between 0 and 15 ms and
once after 15 ms. We find the first time by solving
the expression
200 = 400 - 400e-100t1,
v = 400 + (0 - 400)e
-100t
0 … t … 15 ms.
= (400 - 400e-100t) V,
Note that, because the switch remains in position b for only 15 ms, this expression is valid only
for the time interval from 0 to 15 ms. After the
switch has been in this position for 15 ms, the
voltage on the capacitor will be
which yields t1 = 6.93 ms. We find the second
time by solving the expression
200 = 310.75e-200(t2 - 0.015).
In this case, t2 = 17.20 ms.
v(15 ms) = 400 - 400e-1.5 = 310.75 V.
Therefore, when the switch is moved to position c,
the initial voltage on the capacitor is 310.75 V.
With the switch in position c, the final value of
the capacitor voltage is zero, and the time constant is 5 ms. Again, we use Eq. 7.59 to write the
expression for the capacitor voltage:
v = 0 + (310.75 - 0)e
v (V)
300
v 400 400e100t
v 310.75e200(t 0.015)
200
100
-200(t - 0.015)
= 310.75e-200(t - 0.015) V,
0
15 ms … t.
5
10
15
20
25
t (ms)
Figure 7.36 The capacitor voltage for Example 7.12.
240
Response of First-Order RL and RC Circuits
ASSESSMENT PROBLEMS
Objective 3—Know how to analyze circuits with sequential switching
7.7
In the circuit shown, switch 1 has been closed
and switch 2 has been open for a long time. At
t = 0, switch 1 is opened. Then 10 ms later,
switch 2 is closed. Find
a) vc(t) for 0 … t … 0.01 s,
b) vc(t) for t Ú 0.01 s,
c) the total energy dissipated in the 25 kÆ
resistor, and
d) the total energy dissipated in the 100 kÆ
resistor.
t0
1
t1
1mF
b
8A
t1
2
2
vc(t) 100 k
10 V
Answer:
Switch a in the circuit shown has been open for
a long time, and switch b has been closed for a
long time. Switch a is closed at t = 0 and, after
remaining closed for 1 s, is opened again.
Switch b is opened simultaneously, and both
switches remain open indefinitely. Determine
the expression for the inductor current i that is
valid when (a) 0 … t … 1 s and (b) t Ú 1 s.
t 10 ms
60 k
10 mA 40 k 25 k
7.8
(a) 80e-40t V;
(b) 53.63e-50(t - 0.01) V;
(c) 2.91 mJ;
(d) 0.29 mJ.
Answer:
0.8 3
a
t0
2H
9
i
3
6
(a) (3 - 3e-0.5t) A, 0 … t … 1 s;
(b) ( -4.8 + 5.98e-1.25(t - 1)) A, t Ú 1 s.
NOTE: Also try Chapter Problems 7.72 and 7.80.
7.6 Unbounded Response
A circuit response may grow, rather than decay, exponentially with time.
This type of response, called an unbounded response, is possible if the circuit contains dependent sources. In that case, the Thévenin equivalent
resistance with respect to the terminals of either an inductor or a capacitor
may be negative. This negative resistance generates a negative time constant, and the resulting currents and voltages increase without limit. In an
actual circuit, the response eventually reaches a limiting value when a
component breaks down or goes into a saturation state, prohibiting further increases in voltage or current.
When we consider unbounded responses, the concept of a final value
is confusing. Hence, rather than using the step response solution given in
Eq. 7.59, we derive the differential equation that describes the circuit containing the negative resistance and then solve it using the separation of
variables technique. Example 7.13 presents an exponentially growing
response in terms of the voltage across a capacitor.
7.7
Example 7.13
241
Finding the Unbounded Response in an RC Circuit
a) When the switch is closed in the circuit shown in
Fig. 7.37, the voltage on the capacitor is 10 V.
Find the expression for vo for t Ú 0.
b) Assume that the capacitor short-circuits when
its terminal voltage reaches 150 V. How many
milliseconds elapse before the capacitor shortcircuits?
10 V
The Integrating Amplifier
t0
5 mF vo 10 k
7i
i
20 k
Figure 7.37 The circuit for Example 7.13.
Solution
a) To find the Thévenin equivalent resistance with
respect to the capacitor terminals, we use the testsource method described in Chapter 4. Figure 7.38
shows the resulting circuit, where vT is the test
voltage and iT is the test current. For vT expressed
in volts, we obtain
iT =
vT
vT
vT
- 7( ) +
mA.
10
20
20
Solving for the ratio vT>iT yields the Thévenin
resistance:
vT
RTh =
= - 5 kÆ.
iT
With this Thévenin resistance, we can simplify
the circuit shown in Fig. 7.37 to the one shown in
Fig. 7.39.
iT
vT
10 k
i
7i
20 k
Figure 7.38 The test-source method used to find RTh.
10 V
t0
5 mF vo
5 k
Figure 7.39 A simplification of the circuit shown in
Fig. 7.37.
For t Ú 0, the differential equation describing
the circuit shown in Fig. 7.39 is
(5 * 10 - 6)
dvo
vo
* 10 - 3 = 0.
dt
5
Dividing by the coefficient of the first derivative
yields
dvo
- 40vo = 0.
dt
We now use the separation of variables technique
to find vo(t):
vo(t) = 10e40t V,
t Ú 0.
b) vo = 150 V when e40t = 15. Therefore, 40t = ln 15,
and t = 67.70 ms.
NOTE: Assess your understanding of this material by trying Chapter Problems 7.86 and 7.88.
The fact that interconnected circuit elements may lead to everincreasing currents and voltages is important to engineers. If such interconnections are unintended, the resulting circuit may experience
unexpected, and potentially dangerous, component failures.
Cf
7.7 The Integrating Amplifier
Recall from the introduction to Chapter 5 that one reason for our interest in
the operational amplifier is its use as an integrating amplifier. We are now
ready to analyze an integrating-amplifier circuit, which is shown in Fig. 7.40.
The purpose of such a circuit is to generate an output voltage proportional
to the integral of the input voltage. In Fig. 7.40, we added the branch currents if and is, along with the node voltages vn and vp, to aid our analysis.
Rs
vs
is
vp
vn
VCC if
VCC
Figure 7.40 An integrating amplifier.
vo
242
Response of First-Order RL and RC Circuits
We assume that the operational amplifier is ideal. Thus we take
advantage of the constraints
if + is = 0,
vn = vp.
(7.61)
(7.62)
vs
,
Rs
(7.63)
Because vp = 0,
is =
dvo
.
dt
if = Cf
(7.64)
Hence, from Eqs. 7.61, 7.63, and 7.64,
dvo
1
= v.
dt
RsCf s
(7.65)
Multiplying both sides of Eq. 7.65 by a differential time dt and then integrating from t0 to t generates the equation
t
vo(t) = -
1
v dy + vo(t0).
RsCf Lt0 s
(7.66)
In Eq. 7.66, t0 represents the instant in time when we begin the integration.
Thus vo(t0) is the value of the output voltage at that time. Also, because
vn = vp = 0, vo(t0) is identical to the initial voltage on the feedback
capacitor Cf.
Equation 7.66 states that the output voltage of an integrating amplifier equals the initial value of the voltage on the capacitor plus an inverted
(minus sign), scaled (1>RsCf) replica of the integral of the input voltage. If
no energy is stored in the capacitor when integration commences, Eq. 7.66
reduces to
t
1
vo(t) = v dy.
RsCf Lt0 s
vs
If vs is a step change in a dc voltage level, the output voltage will vary linearly with time. For example, assume that the input voltage is the rectangular voltage pulse shown in Fig. 7.41. Assume also that the initial value of
vo(t) is zero at the instant vs steps from 0 to Vm. A direct application of
Eq. 7.66 yields
Vm
0
(7.67)
t1
2t1
t
vo = -
Vm
1
V t + 0,
RsCf m
0 … t … t1.
(7.68)
When t lies between t1 and 2t1,
Figure 7.41 An input voltage signal.
t
vo = vo(t)
=
0
t1
2t1
Vmt1
RsCf
Figure 7.42 The output voltage of an integrating
amplifier.
t
1
1
( - Vm) dy Vt
RsCf Lt1
RsCf m 1
Vm
2Vm
t t,
RsCf
RsCf 1
t1 … t … 2t1.
(7.69)
Figure 7.42 shows a sketch of vo(t) versus t. Clearly, the output voltage is
an inverted, scaled replica of the integral of the input voltage.
The output voltage is proportional to the integral of the input voltage
only if the op amp operates within its linear range, that is, if it doesn’t saturate. Examples 7.14 and 7.15 further illustrate the analysis of the integrating amplifier.
7.7
Example 7.14
The Integrating Amplifier
243
Analyzing an Integrating Amplifier
Assume that the numerical values for the signal
voltage shown in Fig. 7.41 are Vm = 50 mV and
t1 = 1 s. This signal voltage is applied to the
integrating-amplifier circuit shown in Fig. 7.40. The
circuit parameters of the amplifier are Rs = 100 kÆ,
Cf = 0.1 mF, and VCC = 6 V. The initial voltage on
the capacitor is zero.
For 1 … t … 2 s,
vo = (5t - 10) V.
b) Figure 7.43 shows a plot of vo(t) versus t.
a) Calculate vo(t).
b) Plot vo(t) versus t.
vo(t) (V)
Solution
0
1
2 t (s)
a) For 0 … t … 1 s,
vo =
-1
50 * 10-3t + 0
(100 * 10 )(0.1 * 10-6)
5
3
Example 7.15
Figure 7.43 The output voltage for Example 7.14.
0 … t … 1 s.
= - 5t V,
Analyzing an Integrating Amplifier that has Sequential Switching
At the instant the switch makes contact with terminal a in the circuit shown in Fig. 7.44, the voltage on
the 0.1 mF capacitor is 5 V. The switch remains at
terminal a for 9 ms and then moves instantaneously
to terminal b. How many milliseconds after making
contact with terminal b does the operational amplifier saturate?
Thus, 9 ms after the switch makes contact with terminal a, the output voltage is -5 + 9, or 4 V.
The expression for the output voltage after the
switch moves to terminal b is
t
vo = 4 -
5V a
10 V
= 4 - 800(t - 9 * 10-3)
0.1 mF
6V
100 k
b t 9 ms
8V
6 V
1
8 dy
10-2 L9 * 10-3
= (11.2 - 800t) V.
vo
Figure 7.44 The circuit for Example 7.15.
During this time interval, the voltage is decreasing, and the operational amplifier eventually saturates at -6 V. Therefore we set the expression for vo
equal to -6 V to obtain the saturation time ts:
11.2 - 800ts = - 6,
Solution
The expression for the output voltage during the
time the switch is at terminal a is
t
vo = - 5 -
1
( -10) dy
10-2 L0
= (-5 + 1000t) V.
or
ts = 21.5 ms.
Thus the integrating amplifier saturates 21.5 ms
after making contact with terminal b.
244
Response of First-Order RL and RC Circuits
From the examples, we see that the integrating amplifier can perform
the integration function very well, but only within specified limits that
avoid saturating the op amp. The op amp saturates due to the accumulation of charge on the feedback capacitor. We can prevent it from saturating by placing a resistor in parallel with the feedback capacitor. We
examine such a circuit in Chapter 8.
Note that we can convert the integrating amplifier to a differentiating
amplifier by interchanging the input resistance Rs and the feedback capacitor Cf. Then
vo = - RsCf
dvs
.
dt
(7.70)
We leave the derivation of Eq. 7.70 as an exercise for you. The differentiating amplifier is seldom used because in practice it is a source of unwanted
or noisy signals.
Finally, we can design both integrating- and differentiating-amplifier
circuits by using an inductor instead of a capacitor. However, fabricating
capacitors for integrated-circuit devices is much easier, so inductors are
rarely used in integrating amplifiers.
ASSESSMENT PROBLEMS
Objective 4—Be able to analyze op amp circuits containing resistors and a single capacitor
7.9
There is no energy stored in the capacitor at
the time the switch in the circuit makes contact
with terminal a. The switch remains at position
a for 32 ms and then moves instantaneously to
position b. How many milliseconds after making contact with terminal a does the op amp
saturate?
7.10
a) When the switch closes in the circuit
shown, there is no energy stored in the
capacitor. How long does it take to saturate
the op amp?
b) Repeat (a) with an initial voltage on the
capacitor of 1 V, positive at the upper
terminal.
10 k
0.2 mF
40 k
40 k
a
160 k
10 V 90 k
b t 32 ms
10 V
15 V
5V
160 k
vo
t0
2V
Answer:
262 ms.
NOTE: Also try Chapter Problems 7.94 and 7.95.
5 V
0.01 mF
vo
Answer:
5V
(a) 1.11 ms;
(b) 1.76 ms.
6.8 k
Practical Perspective
Practical Perspective
Artificial Pacemaker
We are now ready to analyze a simple RC circuit, shown in Fig. 7.45,
which can generate periodic electrical impulses. This RC circuit can be
used in an artificial pacemaker to establish a normal heart rhythm. The
box labeled “controller” behaves as an open circuit until the voltage drop
across the capacitor reaches a pre-set limit. Once that limit is reached,
the capacitor discharges its stored energy in the form of an electrical
impulse to the heart and starts to recharge and the process repeats.
Before we develop the analytical expressions that describe the behavior of the circuit, let us develop a feel for how that circuit works. First,
when the controller behaves as an open circuit, the dc voltage source will
charge the capacitor via the resistor R, toward a value of Vs volts. But
once the capacitor voltage reaches Vmax, the controller behaves like a
short circuit, enabling the capacitor to discharge. Once the capacitor discharge is complete, the controller once again acts as an open circuit and
the capacitor starts to recharge. This cycle of charging and discharging
the capacitor establishes the desired heart rhythm, as shown in Fig. 7.46.
Vs
R
C
vC
Controller
Figure 7.45 An artificial pacemaker circuit.
vC(t)
Vmax
etc.
t
tc
Figure 7.46 Capacitor voltage versus time for
the circuit in Fig. 7.45.
In drawing Fig. 7.46 we have chosen t = 0 at the instant the capacitor
starts to charge. This figure also assumes that the circuit has reached the
repetitive stage of its operation, and that the time to discharge the capacitor is negligible when compared to the recharge time. The design of this
artificial pacemaker circuit requires an equation for vC(t) as a function of
Vmax, R, and C.
To begin the analysis, we assume that the circuit has been in operation
for a long time. Let t = 0 at the instant when the capacitor has completely
discharged and the controller is acting as an open circuit. From the circuit
we find
vC( q ) = Vs,
vC (0) = 0,
t = RC.
245
246
Response of First-Order RL and RC Circuits
Thus, while the capacitor is charging,
vC(t) = Vs (1 - e-t>RC).
Suppose the controller has been programmed to fire an electrical pulse to
stimulate the heart when vC = 0.75Vs . Given values of R and C we can
determine the resulting heart rate in beats per minute as follows:
H =
60
[beats per minute]
- RC ln 0.25
A more realistic design problem requires you to calculate the value of resistance, R, given Vmax as a percentage of Vs , C, and the desired heart rate in
beats per minute. We leave you to develop an equation for resistance in
Problem 7.106.
R
Vs
C
vC
Figure 7.47 The artificial pacemaker circuit at
t = 0, when the capacitor is charging.
NOTE: Assess your understanding of the Practical Perspective by solving Chapter
Problems 7.104–7.107.
Summary
•
A first-order circuit may be reduced to a Thévenin (or
Norton) equivalent connected to either a single equivalent inductor or capacitor. (See page 214.)
•
The natural response is the currents and voltages that
exist when stored energy is released to a circuit that
contains no independent sources. (See page 212.)
•
The solution for either the natural or step response of
both RL and RC circuits involves finding the initial and
final value of the current or voltage of interest and the
time constant of the circuit. Equations 7.59 and 7.60
summarize this approach. (See page 232.)
•
Sequential switching in first-order circuits is analyzed
by dividing the analysis into time intervals corresponding to specific switch positions. Initial values for a particular interval are determined from the solution
corresponding to the immediately preceding interval.
(See page 236.)
•
The time constant of an RL circuit equals the equivalent inductance divided by the Thévenin resistance as
viewed from the terminals of the equivalent inductor.
(See page 216.)
•
The time constant of an RC circuit equals the equivalent capacitance times the Thévenin resistance as
viewed from the terminals of the equivalent capacitor.
(See page 221.)
•
An unbounded response occurs when the Thévenin
resistance is negative, which is possible when the
first-order circuit contains dependent sources. (See
page 240.)
•
The step response is the currents and voltages that
result from abrupt changes in dc sources connected
to a circuit. Stored energy may or may not be present
at the time the abrupt changes take place. (See
page 224.)
•
An integrating amplifier consists of an ideal op amp, a
capacitor in the negative feedback branch, and a resistor in series with the signal source. It outputs the integral of the signal source, within specified limits that
avoid saturating the op amp. (See page 241.)
247
Problems
Problems
Figure P7.3
Section 7.1
30 7.1 The switch in the circuit in Fig. P7.1 has been open
PSPICE
for a long time. At t = 0 the switch is closed.
MULTISIM
a) Determine io(0) and io( q ).
b) Determine io(t) for t Ú 0.
c) How many milliseconds after the switch has been
closed will io equal 100 mA?
90 a
b
60 V
70 t0
v2
i
0.32 H v1
7.4 The switch in the circuit in Fig. P7.4 has been in position 1 for a long time. At t = 0, the switch moves
MULTISIM
instantaneously to position 2. Find vo(t) for t Ú 0+.
PSPICE
Figure P7.1
12 16 20 V
t=0
4
Figure P7.4
io
13 80 mH
t=0
8
7.2 The switch in the circuit in Fig. P7.2 has been closed
PSPICE
for a long time. At t = 0 it is opened.
MULTISIM
a) Write the expression for io(t) for t Ú 0.
b) Write the expression for vo(t) for t Ú 0+.
Figure P7.2
20 t=0
50 3
io
50 V
75 12 1
0.02 H vo
60 15 7.3 In the circuit shown in Fig. P7.3, the switch makes
contact with position b just before breaking contact
with position a. As already mentioned, this is
known as a make-before-break switch and is
designed so that the switch does not interrupt the
current in an inductive circuit. The interval of time
between “making” and “breaking” is assumed to be
negligible. The switch has been in the a position for
a long time. At t = 0 the switch is thrown from position a to position b.
a) Determine the initial current in the inductor.
b) Determine the time constant of the circuit
for t 7 0.
c) Find i, v1, and v2 for t Ú 0.
d) What percentage of the initial energy stored in
the inductor is dissipated in the 90 Æ resistor
1 ms after the switch is thrown from position a to
position b?
90 V
36 mH
2
30 8
6
vo
7.5 For the circuit of Fig. P7.4, what percentage of the
initial energy stored in the inductor is eventually
dissipated in the 6 Æ resistor?
7.6 The two switches in the circuit seen in Fig. P7.6 are
synchronized. The switches have been closed for a
long time before opening at t = 0.
a) How many microseconds after the switches are
open is the energy dissipated in the 4 kÆ resistor 10% of the initial energy stored in the 6 H
inductor?
b) At the time calculated in (a), what percentage of
the total energy stored in the inductor has been
dissipated?
Figure P7.6
t0
105 mA
1 k
6H
4 k
t0
20 k
80 k
7.7 In the circuit in Fig. P7.7, the switch has been closed
for a long time before opening at t = 0.
a) Find the value of L so that vo(t) equals 0.5 vo(0+)
when t = 1 ms.
b) Find the percentage of the stored energy that
has been dissipated in the 10 Æ resistor when
t = 1 ms.
248
Response of First-Order RL and RC Circuits
Figure P7.7
9 k
t0
30 mA
10 vo L
1 k
7.11 In the circuit in Fig. P7.10, let Ig represent the dc
current source, s represent the fraction of initial
energy stored in the inductor that is dissipated in
to seconds, and L represent the inductance.
a) Show that
R =
7.8 The switch in the circuit in Fig. P7.8 has been closed
for a long time before opening at t = 0.
MULTISIM
a) Find i1(0-) and i2(0-).
b) Find i1(0+) and i2(0+).
c) Find i1(t) for t Ú 0.
d) Find i2(t) for t Ú 0+.
e) Explain why i2(0-) Z i2(0+).
80 V
i1
i2
4 k
640 mH
7.9 The switch shown in Fig. P7.9 has been open for a
long time before closing at t = 0.
a) Find io(0-), iL(0-), and vL(0-).
b) Find io(0+), iL(0+), and vL(0+).
c) Find io( q ), iL( q ), and vL( q ).
d) Write the expression for iL(t) for t Ú 0.
e) Write the expression for io(t) for t Ú 0+.
f) Write the expression for vL(t) for t Ú 0+.
Figure P7.9
50 200 io
25 V
iL
50 mH vL
i=0
7.10 The switch in the circuit seen in Fig. P7.10 has been
in position 1 for a long time. At t = 0, the switch
moves instantaneously to position 2. Find the value
of R so that 10% of the initial energy stored in the
10 mH inductor is dissipated in R in 10 ms.
t Ú 0+;
i = 6.4e-10t A,
t Ú 0.
Figure P7.12
i
v
L
R
7.13 a) Use component values from Appendix H to create a first-order RL circuit (see Fig. 7.4) with a
time constant of 1 ms. Use a single inductor and
a network of resistors, if necessary. Draw your
circuit.
b) Suppose the inductor you chose in part (a) has
an initial current of 10 mA. Write an expression
for the current through the inductor for t Ú 0.
c) Using your result from part (b), calculate the
time at which half of the initial energy stored in
the inductor has been dissipated by the resistor.
7.14 The switch in the circuit in Fig. P7.14 has been
closed for a long time before opening at t = 0. Find
MULTISIM
vo(t) for t Ú 0+.
PSPICE
Figure P7.14
Figure P7.10
1
5
2
100 10 mH
t=0
t0
5A
v = 160e-10t V,
Find
a) R.
b) t (in milliseconds).
c) L.
d) the initial energy stored in the inductor.
e) the time (in milliseconds) it takes to dissipate
60% of the initial stored energy.
12 k
t0
.
7.12 In the circuit in Fig. P7.12, the voltage and current
expressions are
Figure P7.8
2to
b) Test the expression derived in (a) by using it to
find the value of R in Problem 7.10.
PSPICE
2 k
L ln [1>(1 - s)]
R
60 V
2.5 vo
vo
20 mH
249
Problems
7.15 The switch in Fig. P7.15 has been closed for a long
time before opening at t = 0. Find
a) iL(t), t Ú 0.
b) vL(t), t Ú 0+.
c) i¢(t), t Ú 0+.
Figure P7.19
30 a
b
io
t0
12 A
150 8
8 mH
2 mH
Figure P7.15
40 120 V
6
60 t0
20 i
iL
250 mH
vL
100 60 i
7.16 What percentage of the initial energy stored in the
inductor in the circuit in Fig. P7.15 is dissipated by
the 60 Æ resistor?
7.17 The two switches shown in the circuit in Fig. P7.17
operate simultaneously. Prior to t = 0 each switch
MULTISIM
has been in its indicated position for a long time. At
t = 0 the two switches move instantaneously to
their new positions. Find
a) vo(t), t Ú 0+.
b) io(t), t Ú 0.
PSPICE
7.20 The 240 V, 2 Æ source in the circuit in Fig. P7.20 is
inadvertently short-circuited at its terminals a, b. At
PSPICE
MULTISIM the time the fault occurs, the circuit has been in
operation for a long time.
a) What is the initial value of the current iab in the
short-circuit connection between terminals a, b?
b) What is the final value of the current iab?
c) How many microseconds after the short circuit
has occurred is the current in the short equal
to 114 A?
Figure P7.20
2
240 V
Figure P7.17
a
10 15 2 mH
6 mH
t0
b
1.25 H
2A
10 10 H
vo
Section 7.2
7.5 k
t0
io
6H
7.18 For the circuit seen in Fig. P7.17, find
a) the total energy dissipated in the 7.5 kÆ resistor.
b) the energy trapped in the ideal inductors.
7.19 In the circuit shown in Fig. P7.19, the switch has
been in position a for a long time. At t = 0, it moves
MULTISIM
instantaneously from a to b.
a) Find io(t) for t Ú 0.
b) What is the total energy delivered to the 8 Æ
resistor?
c) How many time constants does it take to deliver
95% of the energy found in (b)?
PSPICE
7.21 The switch in the circuit in Fig. P7.21 has been in the
left position for a long time. At t = 0 it moves to the
right position and stays there.
a) Find the initial voltage drop across the capacitor.
b) Find the initial energy stored by the capacitor.
c) Find the time constant of this circuit for t 7 0.
d) Write the expression for the capacitor voltage
v(t) for t Ú 0..
Figure P7.21
10 mA
20 k⍀
10 k⍀
10 k⍀
⫹
t=0
400 nF
v
⫺
50 k⍀
75 k⍀
250
Response of First-Order RL and RC Circuits
7.22 The switch shown in Fig. P7.22 has been open for a
long time before closing at t = 0. Write the expression for the capacitor voltage, v(t), for t Ú 0.
7.26 In the circuit shown in Fig. P7.26, both switches
operate together; that is, they either open or close at
the same time. The switches are closed a long time
before opening at t = 0.
a) How many microjoules of energy have been
dissipated in the 12 kÆ resistor 12 ms after the
switches open?
b) How long does it take to dissipate 75% of the
initially stored energy?
Figure P7.22
10 k⍀
20 mA
t=0
60 k⍀ 40 nF
20 k⍀
⫹
v
⫺
Figure P7.26
t0
1.8 k
7.23 The switch in the circuit in Fig. P7.23 has been in the
left position for a long time. At t = 0 it moves to the
right position and stays there.
120 V
t0
10
mF
3
12 k
68 k
a) Write the expression for the capacitor voltage,
v(t), for t Ú 0.
b) Write the expression for the current through the
40 k Æ resistor, i(t), for t Ú 0 + .
7.27 The switch in the circuit in Fig. P7.27 is closed at
t = 0 after being open for a long time.
MULTISIM
a) Find i1(0-) and i2(0-).
b) Find i1(0+) and i2(0+).
c) Explain why i1(0-) = i1(0+).
d) Explain why i2(0-) Z i2(0+).
10 k
e) Find i1(t) for t Ú 0.
f) Find i2(t) for t Ú 0 + .
PSPICE
Figure P7.23
5 k
120 V
40 k
i
10 k
t=0
160 nF
v
25 k
Figure P7.27
100 mA
7.24 What percentage of the initial energy stored in the
capacitor in Fig. P7.23 is dissipated by the 40 k Æ
resistor?
2 mF
20 7.25 The switch in the circuit in Fig. P7.25 has been in
position a for a long time and v2 = 0 V. At t = 0,
the switch is thrown to position b. Calculate
a) i, v1, and v2 for t Ú 0+,
b) the energy stored in the 30 mF capacitor at
t = 0, and
c) the energy trapped in the circuit and the total
energy dissipated in the 2.5 kÆ resistor if the
switch remains in position b indefinitely.
Figure P7.25
b
6 mA
5 k
30 mF
t=0
v1
i
60 mF
v2
i2
i1
3
7.28 The switch in the circuit in Fig. P7.28 has been in
position 1 for a long time before moving to posiMULTISIM
tion 2 at t = 0. Find io(t) for t Ú 0+.
PSPICE
Figure P7.28
5 io
1
t0
2.5 k
2
1V
t0
4.7 k
a
5
15 V
2
io
15 2 mF
Problems
7.29 In the circuit in Fig. P7.29 the voltage and current
expressions are
v = 72e-500t V,
t Ú 0;
i = 9e-500t mA,
t Ú 0+.
Find
a) R.
b) C.
c) t (in milliseconds).
d) the initial energy stored in the capacitor.
e) how many microseconds it takes to dissipate
68% of the initial energy stored in the capacitor.
251
7.32 a) In Problem 7.31, how many microjoules of
energy are generated by the dependent current
source during the time the capacitor discharges
to 0 V?
b) Show that for t Ú 0 the total energy stored and
generated in the capacitive circuit equals the
total energy dissipated.
7.33 After the circuit in Fig. P7.33 has been in operation
for a long time, a screwdriver is inadvertently conMULTISIM
nected across the terminals a, b. Assume the resistance of the screwdriver is negligible.
PSPICE
a) Find the current in the screwdriver at t = 0+ and
t = q.
b) Derive the expression for the current in the
screwdriver for t Ú 0+.
Figure P7.29
i
Figure P7.33
C
v
a
R
80 75 mA
7.30 a) Use component values from Appendix H to create a first-order RC circuit (see Fig. 7.11) with a
time constant of 50 ms. Use a single capacitor
and a network of resistors, if necessary. Draw
your circuit.
b) Suppose the capacitor you chose in part (a) has an
initial voltage drop of 50 V.Write an expression for
the voltage drop across the capacitor for t Ú 0.
c) Using you result from part (b), calculate the
time at which the voltage drop across the capacitor has reached 10 V.
7.31 The switch in the circuit seen in Fig. P7.31 has been
in position x for a long time. At t = 0, the switch
moves instantaneously to position y.
a) Find a so that the time constant for t 7 0 is
40 ms.
b) For the a found in (a), find v¢.
25 mF
7.34 At the time the switch is closed in the circuit in
PSPICE
Fig. P7.34, the voltage across the parallel capacitors is
MULTISIM
50 V and the voltage on the 250 nF capacitor is 40 V.
a) What percentage of the initial energy stored in
the three capacitors is dissipated in the 24 kÆ
resistor?
b) Repeat (a) for the 400 Æ and 16 kÆ resistors.
c) What percentage of the initial energy is trapped
in the capacitors?
Figure P7.34
250 nF
20 k
x
y
3.6 k
t0
0.8 mF
50 V
400 40 V t 0
24 k
16 k
800 nF
Section 7.3
av
5 mA
400 b
200 nF
Figure P7.31
50 mF
200 v 5 k
7.35 After the switch in the circuit of Fig. P7.35 has been
open for a long time, it is closed at t = 0. Calculate
(a) the initial value of i; (b) the final value of i;
(c) the time constant for t Ú 0; and (d) the numerical expression for i(t) when t Ú 0.
252
Response of First-Order RL and RC Circuits
Figure P7.35
20 k 200 mH
75 k
i
5 k
50 mA
50 k
t=0
75 V
7.36 The switch in the circuit shown in Fig. P7.36 has
been in position a for a long time before moving to
MULTISIM
position b at t = 0.
a) Find the numerical expressions for iL(t) and
vo(t) for t Ú 0.
b) Find the numerical values of vL(0+) and vo(0+).
PSPICE
Figure P7.36
12 iL
5 mH
vo
32 V
b
vL a
t=0
8
6A
7.37 The switch in the circuit shown in Fig. P7.37 has
been in position a for a long time. At t = 0, the
MULTISIM
switch moves instantaneously to position b.
a) Find the numerical expression for io(t) when
t Ú 0.
b) Find the numerical expression for vo(t) for
t Ú 0+.
PSPICE
7.40 a) Use component values from Appendix H to
create a first-order RL circuit (see Fig. 7.16)
with a time constant of 8 ms. Use a single inductor and a network of resistors, if necessary.
Draw your circuit.
b) Suppose the inductor you chose in part (a) has
no initial stored energy. At t = 0, a switch connects a voltage source with a value of 25 V in
series with the inductor and equivalent resistance. Write an expression for the current
through the inductor for t Ú 0.
c) Using your result from part (b), calculate the
time at which the current through the inductor
reaches 75% of its final value.
7.41 The switch in the circuit shown in Fig. P7.41 has
been closed for a long time. The switch opens at
t = 0. For t Ú 0+:
a) Find vo(t) as a function of Ig, R1, R2, and L.
b) Explain what happens to vo(t) as R2 gets larger
and larger.
c) Find vSW as a function of Ig, R1, R2, and L.
d) Explain what happens to vSW as R2 gets larger
and larger.
Figure P7.41
t0
R2
vsw Figure P7.37
t=0
5
a
b
60 45 A
5
240 V
io
vo
20 10 mH
7.38 Repeat Problem 7.37 assuming that the switch in
the circuit in Fig. P7.37 has been in position b for a
long time and then moves to position a at t = 0 and
stays there.
7.39 The current and voltage at the terminals of the
inductor in the circuit in Fig. 7.16 are
i(t) = (4 + 4e
) A,
-40t
v(t) = - 80e -40t V,
Ig
L vo(t)
R1
7.42 The switch in the circuit in Fig. P7.42 has been
closed for a long time. A student abruptly opens the
switch and reports to her instructor that when the
switch opened, an electric arc with noticeable persistence was established across the switch, and at
the same time the voltmeter placed across the coil
was damaged. On the basis of your analysis of the
circuit in Problem 7.41, can you explain to the student why this happened?
Figure P7.42
R
t0
t Ú 0;
t Ú 0+.
a) Specify the numerical values of Vs, R, Io, and L.
b) How many milliseconds after the switch has
been closed does the energy stored in the inductor reach 9 J?
Vbb
d'Arsonval
voltmeter
L
7.43 a) Derive Eq. 7.47 by first converting the Thévenin
equivalent in Fig. 7.16 to a Norton equivalent
Problems
and then summing the currents away from the
upper node, using the inductor voltage v as the
variable of interest.
b) Use the separation of variables technique to find
the solution to Eq. 7.47. Verify that your solution
agrees with the solution given in Eq. 7.42.
7.47 The switch in the circuit in Fig. P7.47 has been in
position 1 for a long time. At t = 0 it moves instanMULTISIM
taneously to position 2. How many milliseconds
after the switch operates does vo equal 100 V?
PSPICE
Figure P7.47
7.44 The switch in the circuit in Fig. P7.44 has been open
PSPICE
a long time before closing at t = 0. Find io(t)
MULTISIM
for t Ú 0.
10 50 V
2
1
t0
3H
1.5 H vo
Figure P7.44
40 20 40 mH
40 vf io(t)
50 t=0
10 0.1vf
15 10 A
140 V
7.45 The switch in the circuit in Fig. P7.45 has been
PSPICE
open a long time before closing at t = 0. Find vo(t)
MULTISIM
for t Ú 0+.
Figure P7.45
10 253
t0
5
7.48 For the circuit in Fig. P7.47, find (in joules):
a) the total energy dissipated in the 40 Æ resistor;
b) the energy trapped in the inductors, and
c) the initial energy stored in the inductors.
7.49 The make-before-break switch in the circuit of
Fig. P7.49 has been in position a for a long time. At
MULTISIM t = 0, the switch moves instantaneously to position b. Find
a) vo(t), t Ú 0+.
b) i1(t), t Ú 0.
c) i2(t), t Ú 0.
PSPICE
20 mA
i
15 vo 4 mH
8 9i
Figure P7.49
50 mA
a
b
t0
7.46 The switch in the circuit in Fig. P7.46 has been
PSPICE
open a long time before closing at t = 0. Find vo(t)
MULTISIM
for t Ú 0+.
vo 45 mH
15 mH
25 mA
15 i1
60 mH i2
40 mH vo 120 50 mA
Figure P7.46
t=0
2 k
3 k
80 V
10 mA
7.50 There is no energy stored in the inductors L1 and L2
at the time the switch is opened in the circuit shown
in Fig. P7.50.
a) Derive the expressions for the currents i1(t) and
i2(t) for t Ú 0.
b) Use the expressions derived in (a) to find i1( q )
and i2( q ).
254
Response of First-Order RL and RC Circuits
Figure P7.53
Figure P7.50
Ig
t0
i1(t)
Rg
10 k
L1
i2(t)
b
L2
t0
3 k
120 V
7.51 Assume that the switch in the circuit of Fig. P7.51
has been in position a for a long time and that at
t = 0 it is moved to position b. Find (a) vC(0+);
(b) vC( q ); (c) t for t 7 0; (d) i(0+); (e) vC, t Ú 0;
and (f) i, t Ú 0+.
40 k
a
9 k
20 nF
1.5 mA
vC
7.54 The switch in the circuit seen in Fig. P7.54 has been in
position a for a long time. At t = 0, the switch moves
MULTISIM
instantaneously to position b. For t Ú 0+, find
a) vo(t).
b) io(t).
PSPICE
Figure P7.51
40 k a
120 V
b 2.5 k
50 k
t=0
150 k
25 nF
i vC
200 V
Figure P7.54
7.52 a) The switch in the circuit in Fig. P7.52 has been in
position a for a long time. At t = 0, the switch
moves instantaneously to position b and stays
there. Find the initial and final values of the
capacitor voltage, the time constant for t Ú 0,
and the expression for the capacitor voltage for
t Ú 0.
b) Now suppose the switch in the circuit in
Fig. P7.52 has been in position b for a long time.
At t = 0, the switch moves instantaneously to
position a and stays there. Find the initial and
final values of the capacitor voltage, the time
constant for t Ú 0, and the expression for the
capacitor voltage for t Ú 0.
75 V
⫹
⫺
10 k⍀
10 k⍀
⫹
t=0
io(t)
40 nF
40 k⍀
250 ⍀
b
⫺
⫺ 100 V
⫹
Figure P7.55
30 k
50 k
100 ⍀
t=0
⫹
vC
⫺
vo(t)
7.55 The switch in the circuit seen in Fig. P7.55 has been
in position a for a long time. At t = 0, the switch
MULTISIM
moves instantaneously to position b. Find vo(t) and
io(t) for t Ú 0+.
Figure P7.52
⫹
⫺
b
PSPICE
10 mA
10 V
a
5 k⍀
a
io(t)
vo(t)
20 k
15 mA
16 nF
t0
b
25 mF
400 ⍀
a
15 mA
7.56 The circuit in Fig. P7.56 has been in operation for a
long time. At t = 0, the voltage source reverses
MULTISIM
polarity and the current source drops from 3 mA to
2 mA. Find vo(t) for t Ú 0.
PSPICE
7.53 The switch in the circuit of Fig. P7.53 has been in
position a for a long time. At t = 0 the switch is
moved to position b. Calculate (a) the initial voltage
on the capacitor; (b) the final voltage on the capacitor; (c) the time constant (in microseconds) for
t 7 0; and (d) the length of time (in microseconds)
required for the capacitor voltage to reach zero
after the switch is moved to position b.
Figure P7.56
10 k
80 V 4 k
40 k
3 mA
24 k
0.05 mF
vo
Problems
7.57 The switch in the circuit in Fig. P7.57 has been in
position a for a long time. At t = 0, the switch
MULTISIM
moves instantaneously to position b. At the instant
the switch makes contact with terminal b, switch 2
opens. Find vo(t) for t Ú 0.
PSPICE
7.61 The switch in the circuit shown in Fig. P7.61 has
been in the OFF position for a long time. At t = 0,
MULTISIM
the switch moves instantaneously to the ON position. Find vo(t) for t Ú 0.
PSPICE
Figure P7.61
Figure P7.57
40 k
50 V
255
a
1
t0
vo
60 k
250 nF
10 103iΔ
25 k
b
t0
5 mA
2
20 k
6 mA
OFF
ON
25 nF
vo
15 k
7.58 The current and voltage at the terminals of the
capacitor in the circuit in Fig. 7.21 are
t=0
90 k
iΔ
30 k
300 V
7.62 Assume that the switch in the circuit of Fig. P7.61
has been in the ON position for a long time before
MULTISIM
switching instantaneously to the OFF position at
t = 0. Find vo(t) for t Ú 0.
PSPICE
i(t) = 3e-2500t mA,
t Ú 0+;
v(t) = (40 - 24e-2500t) V,
t Ú 0.
a) Specify the numerical values of Is, Vo, R, C,
and t.
b) How many microseconds after the switch has
been closed does the energy stored in the capacitor reach 81% of its final value?
7.59 a) Use component values from Appendix H to create a first-order RC circuit (see Fig. 7.21) with a
time constant of 250 ms. Use a single capacitor
and a network of resistors, if necessary. Draw
your circuit.
b) Suppose the capacitor you chose in part (a) has an
initial voltage drop of 100 V.At t = 0, a switch connects a current source with a value of 1 mA in parallel with the capacitor and equivalent resistance.
Write an expression for the voltage drop across
the capacitor for t Ú 0.
c) Using your result from part (b), calculate the
time at which the voltage drop across the capicitor reaches 50 V.
7.63 a) Derive Eq. 7.52 by first converting the Norton
equivalent circuit shown in Fig. 7.21 to a Thévenin
equivalent and then summing the voltages around
the closed loop, using the capacitor current i as the
relevant variable.
b) Use the separation of variables technique to find
the solution to Eq. 7.52. Verify that your solution
agrees with that of Eq. 7.53.
7.64 The switch in the circuit in Fig. P7.64 has been in
position x for a long time. The initial charge on the
60 nF capacitor is zero. At t = 0, the switch moves
instantaneously to position y.
a) Find vo(t) for t Ú 0+.
b) Find v1(t) for t Ú 0.
Figure P7.64
7.60 The switch in the circuit shown in Fig. P7.60 opens at
PSPICE
t = 0 after being closed for a long time. How many
MULTISIM
milliseconds after the switch opens is the energy
stored in the capacitor 36% of its final value?
Figure P7.60
ib
120 mA
33 k
t0
47 k
4 mA
y
30 k
t=0
30 nF
60 nF
120 k
v1 v0 60 k
90 V
7.65 The switch in the circuit of Fig. P7.65 has been in
position a for a long time. At t = 0, it moves instanMULTISIM
taneously to position b. For t Ú 0+, find
a) vo(t).
16 k 0.25 mF
b) io(t).
c) v1(t).
PSPICE
25ib
x
15 k
256
Response of First-Order RL and RC Circuits
d) v2(t).
e) the energy trapped in the capacitors as t : q .
Figure P7.68
250 io
Figure P7.65
10 V
2.2 k
a
b
6.25 k
0.2 mF
0.8 mF
v1
v2
vo
80 V
0.25 H
i2
i1
7.69 There is no energy stored in the circuit in Fig. P7.69
at the time the switch is closed.
MULTISIM
a) Find i(t) for t Ú 0.
b) Find v1(t) for t Ú 0+.
c) Find v2(t) for t Ú 0.
PSPICE
d) Do your answers make sense in terms of known
circuit behavior?
Figure P7.69
4.5 k
v1(t) 20 mH
t=0
i (t)
90 V
Figure P7.66
15 mH
40 mH v2(t)
Rg
vo
7.66 There is no energy stored in the capacitors C1 and C2
at the time the switch is closed in the circuit seen in
Fig. P7.66.
a) Derive the expressions for v1(t) and v2(t) for
t Ú 0.
b) Use the expressions derived in (a) to find v1( q )
and v2( q ).
Vg
0.25 H
0.5 H
io
t0
40 V
t0
t0
C1
C2
v1(t)
v2(t)
Section 7.4
7.67 Repeat (a) and (b) in Example 7.10 if the mutual
inductance is reduced to zero.
7.68 There is no energy stored in the circuit in Fig. P7.68
at the time the switch is closed.
MULTISIM
a) Find io(t) for t Ú 0.
b) Find vo(t) for t Ú 0+.
c) Find i1(t) for t Ú 0.
d) Find i2(t) for t Ú 0.
e) Do your answers make sense in terms of known
circuit behavior?
7.70 Repeat Problem 7.69 if the dot on the 40 mH coil is
at the bottom of the coil.
PSPICE
MULTISIM
7.71 There is no energy stored in the circuit of Fig. P7.71
at the time the switch is closed.
a) Find io(t) for t Ú 0.
b) Find vo(t) for t Ú 0 + .
c) Find i1(t) for t Ú 0.
d) Find i2(t) for t Ú 0.
e) Do your answers make sense in terms of known
circuit behavior?
PSPICE
Figure P7.71
20 80 V
io
t0
vo
i2
5H
i1
5H
10 H
Problems
Section 7.5
7.72 The action of the two switches in the circuit seen in
Fig. P7.72 is as follows. For t 6 0, switch 1 is in posiMULTISIM
tion a and switch 2 is open. This state has existed for
a long time. At t = 0, switch 1 moves instantaneously from position a to position b, while switch 2
remains open. Ten milliseconds after switch 1 operates, switch 2 closes, remains closed for 10 ms and
then opens. Find vo(t) 25 ms after switch 1 moves to
position b.
257
c) i(6 ms).
d) v(1- ms).
e) v(1+ ms).
PSPICE
Figure P7.75
40 a
b
40 20 A
i
c
v 80 mH
60 120 Figure P7.72
5
a 1
15 A
10 b t0
0 10 ms
2
vo
7.76 The capacitor in the circuit seen in Fig. P7.76 has
been charged to 300 V. At t = 0, switch 1 closes,
MULTISIM
causing the capacitor to discharge into the resistive
network. Switch 2 closes 200 ms after switch 1
closes. Find the magnitude and direction of the current in the second switch 300 ms after switch 1
closes.
PSPICE
50 mH 20 7.73 For the circuit in Fig. P7.72, how many milliseconds
after switch 1 moves to position b is the energy
stored in the inductor 4% of its initial value?
7.74 In the circuit in Fig. P7.74, switch A has been open
and switch B has been closed for a long time. At
MULTISIM
t = 0, switch A closes. Twenty-five milliseconds after
switch A closes, switch B opens. Find iL(t) for t Ú 0.
Figure P7.76
1
t0
30 k
PSPICE
300 V
60 k
2
10
nF
3
t 0 200 ms
120 k
Figure P7.74
40 k
iL(t)
t = 25 ms
B
75 mA
A
200 t=0
500 10 mH
7.75 The switch in the circuit shown in Fig. P7.75 has
been in position a for a long time. At t = 0, the
MULTISIM
switch is moved to position b, where it remains for
1 ms. The switch is then moved to position c, where
it remains indefinitely. Find
a) i(0+).
b) i(200 ms).
7.77 There is no energy stored in the capacitor in the circuit in Fig. P7.77 when switch 1 closes at t = 0.
MULTISIM
Switch 2 closes 2.5 milliseconds later. Find vo(t) for
t Ú 0.
PSPICE
Figure P7.77
500 1
PSPICE
2
20 V
t=0
vo
t = 2.5 ms
8 mF
2 k
25 mA
258
Response of First-Order RL and RC Circuits
7.78 In the circuit in Fig. P7.78, switch 1 has been in position a and switch 2 has been closed for a long time.At
t = 0, switch 1 moves instantaneously to position b.
Two hundred microseconds later, switch 2 opens,
remains open for 600 ms, and then recloses. Find vo
1 ms after switch 1 makes contact with terminal b.
7.82 The voltage waveform shown in Fig. P7.82(a) is
applied to the circuit of Fig. P7.82(b). The initial
MULTISIM
current in the inductor is zero.
a) Calculate vo(t).
b) Make a sketch of vo(t) versus t.
c) Find io at t = 5 ms.
PSPICE
Figure P7.78
0 + 0.2ms
5 k
10 V
a
1
5 k
t=0
vo
vs (V)
0 + 0.8 ms
b
25 nF
10 k
Figure P7.82
2
20 80
30 k
vs
io
40 mH
7.79 For the circuit in Fig. P7.78, what percentage of the
initial energy stored in the 25 nF capacitor is dissiMULTISIM
pated in the 30 kÆ resistor?
0
2.5
(a)
PSPICE
7.80 The switch in the circuit in Fig. P7.80 has been in
position a for a long time. At t = 0, it moves instanMULTISIM
taneously to position b, where it remains for five seconds before moving instantaneously to position c.
Find vo for t Ú 0.
PSPICE
Figure P7.80
b
3.3 k
t0
t5s
a
5 mA
t (ms)
(b)
7.83 The voltage signal source in the circuit in Fig. P7.83(a)
is generating the signal shown in Fig. P7.83(b).There is
MULTISIM
no stored energy at t = 0.
a) Derive the expressions for vo(t) that apply
0 … t … 25 ms;
in the intervals t 6 0;
25 ms … t … 50 ms; and 50 ms … t 6 q.
b) Sketch vo and vs on the same coordinate axes.
c) Repeat (a) and (b) with R reduced to 800 Æ.
PSPICE
c
vo
1 k
vo
100 mF
Figure P7.83
100 k
R = 4 k
vs
7.81 The current source in the circuit in Fig. P7.81(a)
PSPICE
generates the current pulse shown in Fig. P7.81(b).
MULTISIM
There is no energy stored at t = 0.
a) Derive the numerical expressions for vo(t) for
the time intervals t 6 0, 0 … t … 25 ms, and
25 ms … t 6 q .
b) Calculate vo (25- ms) and vo (25+ ms).
c) Calculate io (25- ms) and io (25+ ms).
+
vo
50 nF
−
(a)
vs (V)
10
0
25
50
t (ms)
−10
Figure P7.81
(b)
is (mA)
io
is
3 k
vo
(a)
50
7.84 The voltage waveform shown in Fig. P7.84(a) is
applied to the circuit of Fig. P7.84(b). The initial
MULTISIM
voltage on the capacitor is zero.
a) Calculate vo(t).
b) Make a sketch of vo(t) versus t.
PSPICE
75 mH
0
25 t (ms)
(b)
Problems
Figure P7.84
vs (V)
equals or exceeds 20 kV, how long does it take to
rupture the capacitor?
10 nF
50
vs
1
Figure P7.87
vo
400 k
0
12 104 i
(b)
80 k
t (ms)
(a)
259
2.5 mF
20 V
t0
i
20 k
Section 7.6
7.85 The inductor current in the circuit in Fig. P7.85 is
25 mA at the instant the switch is opened. The
MULTISIM
inductor will malfunction whenever the magnitude
of the inductor current equals or exceeds 5 A. How
long after the switch is opened does the inductor
malfunction?
PSPICE
7.88 The switch in the circuit in Fig. P7.88 has been
closed for a long time. The maximum voltage rating
MULTISIM
of the 1.6 mF capacitor is 14.4 kV. How long after
the switch is opened does the voltage across the
capacitor reach the maximum voltage rating?
PSPICE
Figure P7.88
Figure P7.85
1 k
2 k
10 H
25 mA
vf t0
2 103 vf
4 k
7.86 The gap in the circuit seen in Fig. P7.86 will arc over
PSPICE
whenever the voltage across the gap reaches 30 kV.
MULTISIM
The initial current in the inductor is zero. The value
of b is adjusted so the Thévenin resistance with
respect to the terminals of the inductor is -4 kÆ.
a) What is the value of b ?
b) How many microseconds after the switch has
been closed will the gap arc over?
Figure P7.86
4 k
t=0
40 V
is
12 k
bis
80 mH
Gap
7.87 The capacitor in the circuit shown in Fig. P7.87 is
charged to 20 V at the time the switch is closed. If
MULTISIM
the capacitor ruptures when its terminal voltage
PSPICE
4 i
2 k
1.6 mF i
4 k
t0
5 mA
7.89 The circuit shown in Fig. P7.89 is used to close the
switch between a and b for a predetermined length
of time. The electric relay holds its contact arms
down as long as the voltage across the relay coil
exceeds 5 V. When the coil voltage equals 5 V, the
relay contacts return to their initial position by a
mechanical spring action. The switch between a and
b is initially closed by momentarily pressing the
push button. Assume that the capacitor is fully
charged when the push button is first pushed down.
The resistance of the relay coil is 25 kÆ, and the
inductance of the coil is negligible.
a) How long will the switch between a and b
remain closed?
b) Write the numerical expression for i from the
time the relay contacts first open to the time the
capacitor is completely charged.
c) How many milliseconds (after the circuit
between a and b is interrupted) does it take the
capacitor to reach 85% of its final value?
260
Response of First-Order RL and RC Circuits
Figure P7.89
Figure P7.92
vg (mV)
Push button
a
75
b
4
0
4 k
i
2
t (s)
−75
2 mF
Electric
relay
25 k
80 V
(a)
250 nF
Section 7.7
10 V
80 k
7.90 The energy stored in the capacitor in the circuit
shown in Fig. P7.90 is zero at the instant the switch
MULTISIM
is closed. The ideal operational amplifier reaches
saturation in 15 ms. What is the numerical value of
R in kilo-ohms?
t=0
PSPICE
−10 V
vg
vo
Figure P7.90
(b)
500 nF
R
t0
4V
7.93 Repeat Problem 7.92 with a 4 MÆ resistor placed
across the 250 nF feedback capacitor.
10 V
PSPICE
MULTISIM
10 V
7.94 There is no energy stored in the capacitors in the
circuit shown in Fig. P7.94 at the instant the two
MULTISIM
switches close. Assume the op amp is ideal.
a) Find vo as a function of va, vb, R, and C.
b) On the basis of the result obtained in (a),
describe the operation of the circuit.
c) How long will it take to saturate the amplifier
if va = 40 mV; vb = 15 mV; R = 50 kÆ;
C = 10 nF; and VCC = 6 V?
PSPICE
vo
5.1 k
7.91 At the instant the switch is closed in the circuit of
Fig. P7.90, the capacitor is charged to 6 V, positive
MULTISIM
at the right-hand terminal. If the ideal operational amplifier saturates in 40 ms, what is the
value of R?
PSPICE
7.92 The voltage pulse shown in Fig. P7.92(a) is
PSPICE
applied to the ideal integrating amplifier shown
MULTISIM
in Fig. P7.92(b). Derive the numerical expressions for vo(t) when vo(0) = 0 for the time
intervals
Figure P7.94
va
t0
a) t 6 0.
R
b) 0 … t … 2 s.
c) 2 s … t … 4 s.
d) 4 s … t
C
R
vb
t0
C
VCC
VCC
vo
Problems
7.95 At the instant the switch of Fig. P7.95 is closed, the
voltage on the capacitor is 56 V. Assume an ideal
MULTISIM
operational amplifier. How many milliseconds
after the switch is closed will the output voltage vo
equal zero?
PSPICE
vo
Figure P7.97
56 V 2.5 mF
47 k
t0
14 V
20 k
45 V
25 V
25 V
80 k
Sections 7.1–7.7
7.97 The circuit shown in Fig. P7.97 is known as a
monostable multivibrator. The adjective monostable
MULTISIM
is used to describe the fact that the circuit has one
stable state. That is, if left alone, the electronic
switch T2 will be ON, and T1 will be OFF. (The operation of the ideal transistor switch is described in
detail in Problem 7.99.) T2 can be turned OFF by
momentarily closing the switch S. After S returns to
its open position, T2 will return to its ON state.
a) Show that if T2 is ON, T1 is OFF and will stay OFF.
b) Explain why T2 is turned OFF when S is momentarily closed.
c) Show that T2 will stay OFF for RC ln 2 s.
Figure P7.95
33 k
7.96 The voltage source in the circuit in Fig. P7.96(a) is
generating the triangular waveform shown in
MULTISIM
Fig. P7.96(b). Assume the energy stored in the
capacitor is zero at t = 0 and the op amp is ideal.
a) Derive the numerical expressions for vo(t) for
the following time intervals: 0 … t … 1 ms;
1 ms … t … 3 ms; and 3 ms … t … 4 ms.
b) Sketch the output waveform between 0 and 4 ms.
c) If the triangular input voltage continues to repeat
itself for t 7 4 ms, what would you expect the
output voltage to be? Explain.
PSPICE
RL
PSPICE
Figure P7.96
800 pF
1 k
t0
vg
15 V
15 V
vo
(a)
vg (V)
2
1
2
2
(b)
3
4
t (ms)
R
C
ib1
S
c1
b1
T1
e1
VCC
ib2
b2
vbe1
RL
R1
vbe2
c2
T2
e2
vce2
7.98 The parameter values in the circuit in Fig. P7.97
are
VCC = 6 V;
R1 = 5.0 kÆ;
RL = 20 kÆ;
C = 250 pF; and R = 23,083 Æ.
a) Sketch vce2 versus t, assuming that after S is
momentarily closed, it remains open until the
circuit has reached its stable state. Assume S is
closed at t = 0. Make your sketch for the interval - 5 … t … 10 ms.
b) Repeat (a) for ib2 versus t.
7.99 The circuit shown in Fig. P7.99 is known as an
astable multivibrator and finds wide application in
MULTISIM
pulse circuits. The purpose of this problem is to
relate the charging and discharging of the capacitors to the operation of the circuit. The key to analyzing the circuit is to understand the behavior of
the ideal transistor switches T1 and T2. The circuit is
designed so that the switches automatically alternate between ON and OFF. When T1 is OFF, T2 is ON
and vice versa. Thus in the analysis of this circuit, we
assume a switch is either ON or OFF. We also assume
that the ideal transistor switch can change its state
instantaneously. In other words, it can snap from
OFF to ON and vice versa. When a transistor switch is
PSPICE
0
261
262
Response of First-Order RL and RC Circuits
ON, (1) the base current ib is greater than zero,
(2) the terminal voltage vbe is zero, and (3) the terminal voltage vce is zero. Thus, when a transistor
switch is ON, it presents a short circuit between the
terminals b,e and c,e. When a transistor switch is
OFF, (1) the terminal voltage vbe is negative, (2) the
base current is zero, and (3) there is an open circuit
between the terminals c,e. Thus when a transistor
switch is OFF, it presents an open circuit between
the terminals b,e and c,e. Assume that T2 has been
ON and has just snapped OFF, while T1 has been OFF
and has just snapped ON. You may assume that at
this instance, C2 is charged to the supply voltage
VCC, and the charge on C1 is zero. Also assume
C1 = C2 and R1 = R2 = 10RL.
a) Derive the expression for vbe2 during the interval that T2 is OFF.
b) Derive the expression for vce2 during the interval that T2 is OFF.
c) Find the length of time T2 is OFF.
d) Find the value of vce2 at the end of the interval
that T2 is OFF.
e) Derive the expression for ib1 during the interval
that T2 is OFF.
f) Find the value of ib1 at the end of the interval
that T2 is OFF.
g) Sketch vce2 versus t during the interval that T2
is OFF.
h) Sketch ib1 versus t during the interval that T2
is OFF.
Figure P7.99
RL
i b1
c1 b1
vce1 T1
e1
R1
R2
C2
RL
C1
i b2
b2 c2
vbe1
vbe2
e2
T2
vce2
VCC
7.100 The component values in the circuit of Fig. P7.99
are VCC = 9 V; RL = 3 kÆ; C1 = C2 = 2 nF; and
R1 = R2 = 18 kÆ.
a) How long is T2 in the OFF state during one cycle
of operation?
b) How long is T2 in the ON state during one cycle
of operation?
c) Repeat (a) for T1.
d) Repeat (b) for T1.
e) At the first instant after T1 turns ON, what is the
value of ib1?
f) At the instant just before T1 turns OFF, what is
the value of ib1?
g) What is the value of vce2 at the instant just
before T2 turns ON?
7.101 Repeat Problem 7.100 with C1 = 3 nF and
C2 = 2.8 nF. All other component values are
unchanged.
7.102 The astable multivibrator circuit in Fig. P7.99 is to
satisfy the following criteria: (1) One transistor
switch is to be ON for 48 ms and OFF for 36 ms for
each cycle; (2) RL = 2 kÆ; (3) VCC = 5 V;
(4) R1 = R2; and (5) 6RL … R1 … 50RL. What are
the limiting values for the capacitors C1 and C2?
7.103 The relay shown in Fig. P7.103 connects the 30 V
PRACTICAL dc generator to the dc bus as long as the relay curPERSPECTIVE
rent is greater than 0.4 A. If the relay current
drops to 0.4 A or less, the spring-loaded relay
immediately connects the dc bus to the 30 V
standby battery. The resistance of the relay winding is 60 Æ. The inductance of the relay winding is
to be determined.
a) Assume the prime motor driving the 30 V dc
generator abruptly slows down, causing the
generated voltage to drop suddenly to 21 V.
What value of L will assure that the standby
battery will be connected to the dc bus in
0.5 seconds?
b) Using the value of L determined in (a), state
how long it will take the relay to operate if the
generated voltage suddenly drops to zero.
Problems
7.105 Use an expression similar to the one derived in
Problem 7.104 to calculate the heart rate in beats
per minute for R = 150 k Æ , C = 6 mF, if the capacitor discharges when its voltage reaches 60% of the
source voltage Vs.
Figure P7.103
30 V
30 V
dc
gen
relay
coil
(R, L)
DC BUS
Compressed
springs
7.106 Show that the resistance required to achieve a heart
rate H, in beats per minute, is given by the equation
DC loads
7.104 Derive the expression for heart rate in beats per
minute given the values of R and C and assuming
that the capacitor discharges when its voltage
reaches 75% of the source voltage Vs. The expression, given in the Practical Perspective, is repeated
here for convenience:
H =
60
-RC ln 0.25
263
[beats per minute].
R =
- 60
HC ln a1 -
Vmax
b
Vs
,
where C is the capacitance, Vs. is the source voltage, and Vmax is the capacitor voltage at which
discharge occurs .
7.107 Use the expression derived in Problem 7.106 to
calculate the resistance required to achieve a
heart rate of 70 beats per minute using a capacitance of 2.5 mF and assuming that the capacitor
discharges when its voltage reaches 68% of the
source voltage.
CHAPTER
8
CHAPTER CONTENTS
8.1 Introduction to the Natural Response of a
Parallel RLC Circuit p. 266
8.2 The Forms of the Natural Response of a
Parallel RLC Circuit p. 270
8.3 The Step Response of a Parallel
RLC Circuit p. 280
8.4 The Natural and Step Response of a Series
RLC Circuit p. 285
8.5 A Circuit with Two Integrating
Amplifiers p. 289
CHAPTER OBJECTIVES
1 Be able to determine the natural response and
the step response of parallel RLC circuits.
2 Be able to determine the natural response and
the step response of series RLC circuits.
264
Natural and Step
Responses of RLC Circuits
In this chapter, discussion of the natural response and step
response of circuits containing both inductors and capacitors is
limited to two simple structures: the parallel RLC circuit and the
series RLC circuit. Finding the natural response of a parallel RLC
circuit consists of finding the voltage created across the parallel
branches by the release of energy stored in the inductor or capacitor or both. The task is defined in terms of the circuit shown in
Fig. 8.1 on page 266. The initial voltage on the capacitor, V0, represents the initial energy stored in the capacitor. The initial current
through the inductor, I0, represents the initial energy stored in the
inductor. If the individual branch currents are of interest, you can
find them after determining the terminal voltage.
We derive the step response of a parallel RLC circuit by using
Fig. 8.2 on page 266. We are interested in the voltage that appears
across the parallel branches as a result of the sudden application
of a dc current source. Energy may or may not be stored in the
circuit when the current source is applied.
Finding the natural response of a series RLC circuit consists
of finding the current generated in the series-connected elements
by the release of initially stored energy in the inductor, capacitor,
or both. The task is defined by the circuit shown in Fig. 8.3 on
page 266. As before, the initial inductor current, I0, and the initial
capacitor voltage, V0, represent the initially stored energy. If any
of the individual element voltages are of interest, you can find
them after determining the current.
We describe the step response of a series RLC circuit in terms
of the circuit shown in Fig. 8.4 on page 266. We are interested in
the current resulting from the sudden application of the dc voltage source. Energy may or may not be stored in the circuit when
the switch is closed.
If you have not studied ordinary differential equations, derivation of the natural and step responses of parallel and series
RLC circuits may be a bit difficult to follow. However, the results
are important enough to warrant presentation at this time. We
begin with the natural response of a parallel RLC circuit and
cover this material over two sections: one to discuss the solution
of the differential equation that describes the circuit and one to
present the three distinct forms that the solution can take.