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3
AMPLIFIERS AND SIGNAL
PROCESSING
John G.Webster
Most bioelectric signals are small and require ampliﬁcation.Ampliﬁers are
also used for interfacing sensors that sense body motions,temperature,and
chemical concentrations.In addition to simple ampliﬁcation,the ampliﬁer may
also modify the signal to produce frequency ﬁltering or nonlinear effects.This
chapter emphasizes the operational ampliﬁer (op amp),which has revolution
ized electronic circuit design.Most circuit design was formerly performed with
discrete components,requiring laborious calculations,many components,and
large expense.Nowa 20cent op amp,a fewresistors,and knowledge of Ohm’s
law are all that is needed.
3.1 IDEAL OP AMPS
An op amp is a highgain dc differential ampliﬁer.It is normally used in circuits
that have characteristics determined by external negativefeedback networks.
The best way to approach the design of a circuit that uses op amps is ﬁrst to
assume that the op amp is ideal.After the initial design,the circuit is checked
to determine whether the nonideal characteristics of the op amp are important.
If they are not,the design is complete;if they are,another design check is
made,which may require additional components.
IDEAL CHARACTERISTICS
Figure3.1shows theequivalent circuit for anonideal opamp.It is a dc differential
ampliﬁer,which means that any differential voltage,v
d
¼ ðv
2
v
1
Þ,is multiplied
by the very high gain A to produce the output voltage v
o
.
To simplify calculations,we assume the following characteristics for an
ideal op amp:
1.A ¼
1
(gain is inﬁnity)
2.v
o
¼ 0,when v
1
¼ v
2
(no offset voltage)
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3.R
d
¼
1
(input impedance is inﬁnity)
4.R
o
¼ 0 (output impedance is zero)
5.Bandwidth =
1
(no frequencyresponse limitations) and no phase shift
Later in the chapter we shall examine the effect on the circuit of character
istics that are not ideal.
Figure 3.2 shows the opamp circuit symbol,which includes two differen
tial input terminals and one output terminal.All these voltages are measured
with respect to the ground shown.Power supplies,usually 15 V,must be
connected to terminals indicated on the manufacturer’s speciﬁcation sheet
(Jung,1986;Horowitz and Hill,1989).
TWO BASIC RULES
Throughout this chapter we shall use two basic rules (or input terminal
restrictions) that are very helpful in designing opamp circuits.
RULE 1 When the opamp output is in its linear range,the two input terminals
are at the same voltage.
This is true because if the two input terminals were not at the same voltage,the
differential input voltage would be multiplied by the inﬁnite gain to yield an
inﬁnite output voltage.This is absurd;most op amps use a power supply of 15 V,
so v
o
is restricted to this range.Actually the opamp speciﬁcations guarantee a
Figure 3.1 Opamp equivalent circuit
The two inputs are v
l
and v
2
.A
differential voltage between themcauses current ﬂow through the differential
resistance R
d
.The differential voltage is multiplied by A,the gain of the op
amp,to generate the outputvoltage source.Any current ﬂowing to the output
terminal v
o
must pass through the output resistance R
o
.
Figure 3.2 Opamp circuit symbol
A voltage at v
1
,the inverting input,is
greatly ampliﬁed and inverted to yield v
o
.A voltage at v
2
,the noninverting
input,is greatly ampliﬁed to yield an inphase output at v
o
.
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linear output range of only 10 V,although some saturate at about 13 V.A
single supply is adequate with some op amps,such as the LM358 (Horowitz and
Hill,1989).
RULE 2 No current ﬂows into either input terminal of the op amp.
This is true because we assume that the input impedance is inﬁnity,and no
current ﬂows into an inﬁnite impedance.Even if the input impedance were
ﬁnite,Rule 1 tells us that there is no voltage drop across R
d
;so therefore,no
current ﬂows.
3.2 INVERTING AMPLIFIERS
CIRCUIT
Figure 3.3(a) shows the basic invertingampliﬁer circuit.It is widely used in
instrumentation.Note that a portion of v
o
is fed back via R
f
to the negative
input of the op amp.This provides the inverting ampliﬁer with the many
advantages associated with the use of negative feedback—increased band
width,lower output impedance,and so forth.If v
o
is ever fed back to the
positive input of the op amp,examine the circuit carefully.Either there is a
mistake,or the circuit is one of the rare ones in which a regenerative action is
desired.
EQUATION
Note that the positive input of the op amp is at 0 V.Therefore,by Rule 1,the
negative input of the op amp is also at 0 V.Thus no matter what happens to the
rest of the circuit,the negative input of the op amp remains at 0 V,a condition
known as a virtual ground.
Because the right side of R
i
,is at 0 Vand the left side is v
i
,by Ohm’s lawthe
current i through R
i
,is i ¼ v
i
=R
i
.By Rule 2,no current can enter the op amp;
therefore i must also ﬂowthrough R
f
.This produces a voltage drop across R
f
of
iR
f
.Because the left end of R
f
is at 0 V,the right end must be
v
o
¼ iR
f
¼ v
i
R
f
R
i
or
v
o
v
i
¼
R
f
R
i
(3.1)
Thus the circuit inverts,and the invertingampliﬁer gain (not the opamp gain)
is given by the ratio of R
f
to R
i
.
LEVER ANALOGY
Figure 3.3(b) shows an easy way to visualize the circuit’s behavior.A lever is
formed with arm lengths proportional to resistance values.Because the
3.2 I N V E R T I N G A M P L I F I E R S
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negative input is at 0 V,the fulcrum is placed at 0 V,as shown.If R
f
is three
times R
i
,as shown,any variation of v
i
results in a threetimesbigger variation
of v
o
.The circuit in Figure 3.3(a) is a voltagecontrolled current source (VCCS)
for any load R
f
(Jung,1986).The current i through R
f
is v
i
/R
i
,so v
i
controls i.
Current sources are useful in electrical impedance plethysmography for
passing a ﬁxed current through the body (Section 8.7).
INPUT–OUTPUT CHARACTERISTIC
Figure 3.3(c) shows that the circuit is linear only over a limited range of v
i
.
When v
o
exceeds about 13 V,it saturates (limits),and further increases in v
i
produce no change in the output.The linear swing of v
o
is about 4 V less than
the difference in powersupply voltages.Although op amps usually have
Figure 3.3
(a) An inverting ampliﬁer.Current ﬂowing through the input
resistor R
i
also ﬂows through the feedback resistor R
f
.(b) A lever with arm
lengths proportional to resistance values enables the viewer to visualize the
input–output characteristics easily.(c) The input–output plot shows a slope of
R
f
=R
i
in the central portion,but the output saturates at about 13 V.
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powersupply voltages set at 15 V,reduced powersupply voltages may be
used,with a corresponding reduction in the saturation voltages and the linear
swing of v
o
.
SUMMING AMPLIFIER
The inverting ampliﬁer may be extended to form a circuit that yields the
weighted sum of several input voltages.Each input voltage v
i1
,v
i2
,...,v
ik
is
connected to the negative input of the op amp by an individual resistor the
conductance of which ð1=R
ik
Þ is proportional to the desired weighting.
EXAMPLE 3.1 The output of a biopotential preampliﬁer that measures
the electrooculogram (EOG) (Section 4.7) is an undesired dc voltage of
5 V due to electrode halfcell potentials (Section 5.1),with a desired
signal of 1 V superimposed.Design a circuit that will balance the dc
voltage to zero and provide a gain of 10 for the desired signal without
saturating the op amp.
ANSWER Figure E3.1(a) shows the design.We assume that v
b
,the balancing
voltage available from the 5kV potentiometer,is 10V.The undesired
voltage at v
i
¼ 5V.For v
o
¼ 0,the current through R
f
is zero.Therefore
the sum of the currents through R
i
and R
b
,is zero.
v
i
R
i
þ
v
b
R
b
¼ 0
R
b
¼
R
i
v
b
v
i
¼
10
4
ð10Þ
5
¼ 2 10
4
V
Figure E3.1
(a) This circuit sums the input voltage v
i
plus onehalf of the
balancing voltage v
b
.Thus the output voltage v
o
can be set to zero even when v
i
has a nonzero dc component,(b) The three waveforms show v
i
,the input
voltage;ðv
i
þv
b
=2Þ,the balancedout voltage;and v
o
,the ampliﬁed output
voltage.If v
i
were directly ampliﬁed,the op amp would saturate.
3.2 I N V E R T I N G A M P L I F I E R S
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For a gain of 10,(3.1) requires R
f
=R
i
;¼ 10;or R
f
;¼ 100 kV.The circuit
equation is
v
o
¼ R
f
v
i
R
i
þ
v
b
R
b
v
o
¼ 10
5
v
i
10
4
þ
v
b
2 10
4
v
o
¼ 10 v
i
þ
v
b
2
The potentiometer can balance out any undesired voltage in the range 5V,as
shown by Figure E3.1(b).Here we have selected resistors of 10 kV to 100 kV
fromthe common resistors used in electronic circuits that have values between
10 V and 22 MV.
3.3 NONINVERTING AMPLIFIERS
FOLLOWER
Figure 3.4(a) shows the circuit for a unitygain follower.Because v
i
exists at the
positive input of the op amp,by Rule 1 v
i
must also exist at the negative input.
But v
o
is also connected to the negative input.Therefore v
o
¼ v
i
,or the output
voltage follows the input voltage.At ﬁrst glance it seems nothing is gained by
using this circuit;the output is the same as the input.However,the circuit is very
useful as a buffer,to prevent a high source resistance frombeing loaded down
by a lowresistance load.By Rule 2,no current ﬂows into the positive input,and
therefore the source resistance in the external circuit is not loaded at all.
NONINVERTING AMPLIFIER
Figure 3.4(b) shows how the follower circuit can be modiﬁed to produce gain.
By Rule 1,v
i
appears at the negative input of the op amp.This causes current
i ¼ v
i
=R
i
,to ﬂow to ground.By Rule 2,none of i can come from the negative
input;therefore all must ﬂowthrough R
f
.We can then calculate v
o
¼ iðR
f
þR
i
Þ
and solve for the gain.
v
o
v
i
¼
iðR
f
þR
i
Þ
iR
i
¼
R
f
þR
i
R
i
(3.2)
We note that the circuit gain(not the opampgain) is positive,always greater than
or equal to 1;and that if R
i
=
1
(open circuit),the circuit reduces to Figure 3.4(a).
Figure 3.4(c) shows howa lever makes possible an easy visualization of the
input–output characteristics.The fulcrumis placed at the left end,because R
i
is
grounded at the left end.v
i
appears between the two resistors,so it provides an
input at the central part of the diagram.v
o
travels through an output excursion
determined by the lever arms.
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Figure 3.4(d),the input–output characteristic,shows that a oneopamp
circuit can have a positive ampliﬁer gain.Again saturation is evident.
3.4 DIFFERENTIAL AMPLIFIERS
ONEOPAMP DIFFERENTIAL AMPLIFIER
The right side of Figure 3.5(a) shows a simple oneopamp differential ampliﬁer.
Current ﬂows fromv
4
through R
3
and R
4
to ground.By Rule 2,no current ﬂows
into the positive input of the op amp.Hence R
3
and R
4
,act as a simple voltage
divider attenuator,which is unaffected by having the op amp attached or by any
other changes in the circuit.The voltages in this part of the circuit are visualized
in Figure 3.5(b) by the single lever that is attached to the fulcrum (ground).
By Rule 1,whatever voltage appears at the positive input also appears at
the negative input.Once this voltage is ﬁxed,the top half of the circuit
Figure 3.4
(a) A follower,v
o
¼ v
i
.(b) A noninverting ampliﬁer,v
i
appears
across R
i
,producing a current through R
i
that also ﬂows through R
f
.(c) Alever
with arm lengths proportional to resistance values makes possible an easy
visualization of input–output characteristics.(d) The input–output plot shows
a positive slope of ðR
f
þR
i
Þ=R
i
in the central portion,but the output saturates
at about 13 V.
3.4 D I F F E R E N T I A L A M P L I F I E R S
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behaves like an inverting ampliﬁer.For example,if v
4
is 0 V,the positive
input of the op amp is 0 V and the v
3
–v
o
circuit behaves exactly like an
inverting ampliﬁer.For other values of v
4
,an inverting relation is obtained
about some voltage intermediate between v
4
and 0 V.The relationship can
be visualized in Figure 3.5(b) by noting that the two levers behave like a pair
of scissors.The thumb and ﬁnger holes are v
4
and v
3
,and the points are at v
o
and 0 V.
We solve for the gain by ﬁnding v
5
.
v
5
¼
v
4
R
4
R
3
þR
4
(3.3)
Figure 3.5
(a) The right side shows a oneopamp differential ampliﬁer,but it
has low input impedance.The left side shows how two additional op amps can
provide high input impedance and gain.(b) For the oneopamp differential
ampliﬁer,two levers with arm lengths proportional to resistance values make
possible an easy visualization of input–output characteristics.
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Then,solving for the current in the top half,we get
i ¼
v
3
v
5
R
3
¼
v
5
v
o
R
4
(3.4)
Substituting (3.3) into (3.4) yields
v
o
¼
ðv
4
v
3
ÞR
4
R
3
(3.5)
This is the equation for a differential ampliﬁer.If the two inputs are hooked
together and driven by a common source,with respect to ground,then the
commonmode voltage v
c
is v
3
¼ v
4
.Equation (3.5) shows that the ideal output
is 0.The differential ampliﬁercircuit (not opamp) commonmode gain G
c
is 0.
In Figure 3.5(b),imagine the scissors to be closed.No matter how the inputs
are varied,v
o
¼ 0.
If on the other hand v
3
6
¼v
4
,then the differential voltage ðv
4
v
3
Þ
produces an ampliﬁercircuit (not opamp) differential gain G
d
that from
(3.5) is equal to R
4
=R
3
.This result can be visualized in Figure 3.5(b) by noting
that as the scissors open,v
o
is geometrically related to ðv
4
v
3
Þ in the same
ratio as the lever arms,R
4
=R
3
.
No differential ampliﬁer perfectly rejects the commonmode voltage.To
quantify this imperfection,we use the term commonmode rejection ratio
(CMRR),which is deﬁned as
CMRR ¼
G
d
G
c
(3.6)
This factor may be lower than 100 for some oscilloscope differential ampliﬁers
and higher than 10,000 for a highquality biopotential ampliﬁer.
EXAMPLE 3.2 A bloodpressure sensor uses a fouractivearm Wheatstone
strain gage bridge excited with dc.At full scale,each arm changes resistance
by 0.3%.Design an ampliﬁer that will provide a fullscale output over the
op amp’s full range of linear operation.Use the minimal number of
components.
ANSWER From (2.6),Dv
o
¼ v
i
DR=R ¼ 5Vð0:003Þ ¼ 0:015V.Gain ¼ 20=
0:015 ¼ 1333.Assume R ¼ 120 V.Then the Thevenin source impedance
¼ 60 V.Use this to replace R
3
of Figure 3.5(a) right side.Then R
4
¼
R
3
ðgainÞ ¼ 60 Vð1333Þ ¼ 80kV.
THREEOPAMP DIFFERENTIAL AMPLIFIER
The oneopamp differential ampliﬁer is quite satisfactory for lowresistance
sources,such as straingage Wheatstone bridges (Section 2.3).But the input
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resistance is too lowfor highresistance sources.Our ﬁrst recourse is to add the
simple follower shown in Figure 3.4(a) to each input.This provides the
required buffering.Because this solution uses two additional op amps,we
can also obtain gain from these buffering ampliﬁers by using a noninverting
ampliﬁer,as shown in Figure 3.4(b).However,this solution ampliﬁes the
commonmode voltage,as well as the differential voltage,so there is no
improvement in CMRR.
A superior solution is achieved by hooking together the two R
i
’s of the
noninverting ampliﬁers and eliminating the connection to ground.The result is
shown on the left side of Figure 3.5(a).To examine the effects of commonmode
voltage,assume that v
1
¼ v
2
.By Rule 1,v
1
,appears at bothnegative inputs tothe
op amps.This places the same voltage at both ends of R
1
.Hence current through
R
1
is 0.By Rule 2,no current can ﬂowfromthe opampinputs.Hence the current
through both R
2
’s is 0,so v
1
appears at both opamp outputs and the G
c
is 1.
To examine the effects when v
1
6
¼ v
2
,we note that v
1
v
2
appears across
R
1
.This causes a current to ﬂowthrough R
1
that also ﬂows through the resistor
string R
2
,R
1
,R
2
.Hence the output voltage
v
3
v
4
¼ iðR
2
þR
1
þR
2
Þ
whereas the input voltage
v
1
v
2
¼ iR
1
The differential gain is then
G
d
¼
v
3
v
4
v
1
v
2
¼
2R
2
þR
1
R
1
(3.7)
Since the G
c
is 1,the CMRR is equal to the G
d
,which is usually much greater
than 1.When the left and right halves of Figure 3.5(a) are combined,the
resulting threeopamp ampliﬁer circuit is frequently called an instrumentation
ampliﬁer.It has high input impedance,a high CMRR,and a gain that can be
changed by adjusting R
1
.This circuit ﬁnds wide use in measuring biopotentials
(Section 6.7),because it rejects the large 60 Hz commonmode voltage that
exists on the body.
3.5 COMPARATORS
SIMPLE
Acomparator is a circuit that compares the input voltage with some reference
voltage.The comparator’s output ﬂips fromone saturation limit to the other,as
the negative input of the op amp passes through 0 V.For v
i
greater than the
comparison level,v
o
¼ 13V.For v
i
less than the comparison level,
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v
o
¼ þ13V.Thus this circuit performs the same function as a Schmitt trigger,
which detects an analog voltage level and yields a logic level output.The
simplest comparator is the op amp itself,as shown in Figure 3.2.If a reference
voltage is connected to the positive input and v
i
is connected to the negative
input,the circuit is complete.The inputs may be interchanged to invert
the output.The input circuit may be expanded by adding the two R
1
resistors
shown in Figure 3.6(a).This provides a known input resistance for the circuit
and minimizes overdriving the opamp input.Figure 3.6(b) shows that the
comparator ﬂips when v
i
¼ v
ref
.To avoid building a separate power supply
for v
ref
,we can connect v
ref
to the 15 Vpower supply and adjust the values of
the input resistors so that the negative input of the op amp is at 0 Vwhen v
i
is at
the desired positive comparison level.When negative comparison levels are
desired,v
ref
is connected to the þ15 V power supply.
WITH HYSTERESIS
For a simple comparator,if v
i
is at the comparison level and there is noise on v
i
,
then v
o
ﬂuctuates wildly.To prevent this,we can add hysteresis to the
comparator by adding R
2
and R
3
,as shown in Figure 3.6(a).The effect of
this positive feedback is illustrated by the input–output characteristics shown
in Figure 3.6(b).To analyze this circuit,ﬁrst assume that v
ref
¼ 5V and
v
i
¼ þ10 V.Then,because the op amp inverts and saturates,v
o
¼ 13V.
Divide v
o
by R
2
and R
3
so that the positive input is at,say,1 V.As v
i
is
lowered,the comparator does not ﬂip until v
i
reaches þ3 V,which makes the
negative input equal to the positive input,1 V.At this point,v
o
ﬂips to þ13 V,
causing the positive input to change to þ1 V.Noise on v
i
cannot cause v
o
to ﬂip
back,because the negative input must be raised to þ1 Vto cause the next ﬂip.
This requires v
i
to be raised to þ7 V,at which level the circuit can ﬂip back to
its original state.Fromthis example,we see that the width of the hysteresis is
four times as great as the magnitude of the voltage across R
3
.The width of the
hysteresis loop can be varied by replacing R
3
by a potentiometer.
Figure 3.6
(a) Comparator.When R
3
¼ 0;v
o
indicates whether ðv
I
þv
ref
Þ is
greater or less than 0 V.When R
3
is larger,the comparator has hysteresis,as
shown in,(b) the input–output characteristic.
3.5 C O M P A R A T O R S
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3.6 RECTIFIERS
Simple resistor–diode rectiﬁers do not work well for voltages below 0.7 V,
because the voltage is not sufﬁcient to overcome the forward voltage drop of
the diode.This problem can be overcome by placing the diode within the
feedback loop of an op amp,thus reducing the voltage limitation by a factor
equal to the gain of the op amp.
Figure 3.7(a) shows the circuit for a fullwave precision rectiﬁer (Graeme,
1974b).For v
i
>0,D
2
and D
3
conduct,whereas D
1
and D
4
are reversebiased.
The top op amp is a noninverting ampliﬁer with a gain of 1/x,where x is a
fraction corresponding to the potentiometer setting.Because D
4
is not con
ducting,the lower op amp does not contribute to the output.
Figure 3.7
(a) Fullwave precision rectiﬁer.For v
i
>0,the noninverting ampli
ﬁer at the top is active,making v
o
>0.For v
i
<0,the inverting ampliﬁer at the
bottomis active,making v
o
>0.Circuit gainmay be adjustedwitha single pot.(b)
Input–output characteristics showsaturation when v
o
> þ13V.(Reprinted with
permission fromElectronics Magazine,copyright
#
December 12,1974;Penton
Publishing,Inc.) (c) Oneopamp fullwave rectiﬁer.For v
i
< 0,the circuit
behaves like the inverting ampliﬁer rectiﬁer with a gain of þ0.5.For v
i
>0,
the op amp disconnects and the passive resistor chain yields a gain of þ0.5.
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For v
i
<0;D
1
and D
4
conduct,while D
2
and D
3
are reversebiased.At the
potentiometer wiper v
i
serves as the input to the lower opamp inverting
ampliﬁer,which has a gain of 1=x.Because D
2
is not conducting,the upper op
amp does not contribute to the output.And because the polarity of the gain
switches with the polarity of v
i
,v
o
¼ jv
i
=xj.
The advantage of this circuit over other fullwave rectiﬁer circuits (Wait,
1975,p.173) is that the gain can be varied with a single potentiometer and the
input resistance is very high.If only a halfwave rectiﬁer is needed,either the
noninverting ampliﬁer or the inverting ampliﬁer can be used separately,thus
requiring only one op amp.The perfect rectiﬁer is frequently used with an
integrator to quantify the amplitude of electromyographic signals (Section 6.8).
Figure 3.7(c) shows a oneopamp fullwave rectiﬁer (Tompkins and
Webster,1988).Unlike other fullwave rectiﬁers,it requires the load to remain
constant,because the gain is a function of load.
3.7 LOGARITHMIC AMPLIFIERS
The logarithmic ampliﬁer makes use of the nonlinear volt–ampere relation of
the silicon planar transistor (Jung,1986).
V
BE
¼ 0:060 log
I
C
I
S
(3.8)
where
V
BE
¼ base emitter voltage
I
C
¼ collector current
I
S
¼ reversesaturationcurrent;10
13
Aat 27
C
The transistor is placed in the transdiode conﬁguration shown in Figure
3.8(a),in which I
C
¼ v
i
=R
i
.Then the output v
o
¼ V
BE
is logarithmically related
to v
i
as given by (3.8) over the approximate range 10
7
A< I
C
<10
2
A.The
approximate range of v
o
is 0.36 to 0.66 V,so larger ranges of v
o
are
sometimes obtained by the alternate switch position shown in Figure 3.8(a).
The resistor network feeds back only a fraction of v
o
in order to boost v
o
and
uses the same principle as that used in the noninverting ampliﬁer.Figure 3.8(b)
shows the input–output characteristics for each of these circuits.
Because semiconductors are temperature sensitive,accurate circuits re
quire temperature compensation.Antilog (exponential) circuits are made by
interchanging the resistor and semiconductor.These log and antilog circuits
are used to multiply a variable,divide it,or raise it to a power;to compress
large dynamic ranges into small ones;and to linearize the output of devices
with logarithmic or exponential inputoutput relations.In the photometer
3.7 L O G A R I T H M I C A M P L I F I E R S
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(Section 11.1),the logarithmic converter can be used to convert transmittance
to absorbance.
3.8 INTEGRATORS
So far in this chapter,we have considered only circuits with a ﬂat gainversus
frequency characteristic.Now let us consider circuits that have a deliberate
change in gain with frequency.The ﬁrst such circuit is the integrator.Figure 3.9
Figure 3.8
(a) Alogarithmic ampliﬁer makes use of the fact that a transistor’s
V
BE
is related to the logarithmof its collector current.With the switch thrown
in the alternate position,the circuit gain is increased by 10.(b) Input–output
characteristics show that the logarithmic relation is obtained for only one
polarity;1 and 10 gains are indicated.
Figure 3.9 A threemode integrator
With S
1
open and S
2
closed,the dc
circuit behaves as an inverting ampliﬁer.Thus v
o
¼ v
ic
and v
o
can be set to any
desired initial condition.With S
1
closed and S
2
open,the circuit integrates.
With both switches open,the circuit holds v
o
constant,making possible a
leisurely readout.
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shows the circuit for an integrator,which is obtained by closing switch S
i
.The
voltage across an initially uncharged capacitor is given by
v ¼
1
C
Z
t
1
0
idt (3.9)
where i is the current through C and t
1
is the integration time.For the
integrator,for v
i
positive,the input current i ¼ v
i
=R ﬂows through C in a
direction to cause v
o
to move in a negative direction.Thus
v
o
¼
1
RC
Z
t
1
0
v
i
dt þv
ic
(3.10)
This shows that v
o
is equal to the negative integral of v
i
,scaled by the factor 1/
RC and added to v
ic
,the voltage due to the initial condition.For v
o
¼ 0 and
v
i
¼ constant;v
o
¼ v
i
after an integration time equal to RC.Because any real
integrator eventually drifts into saturation,a means must be provided to
restore v
o
to any desired initial condition.If an initial condition of v
o
¼ 0V
is desired,a simple switch to short out Cis sufﬁcient.For more versatility,S
1
is
opened and S
2
closed.This dc circuit then acts as an inverting ampliﬁer,which
makes v
o
¼ v
ic
.During integration,S
1
is closed and S
2
open.After the
integration,both switches may be opened to hold the output at the ﬁnal
calculated value,thus permitting time for a readout.The circuit is useful for
computing the area under a curve,as technicians do when they calculate
cardiac output (Section 8.2).
The frequency response of an integrator is easily analyzed because the
formula for the inverting ampliﬁer gain (3.1) can be generalized to any input
and feedback impedances.Thus for Figure 3.9,with S
1
closed,
V
o
ð jvÞ
V
i
ð jvÞ
¼
Z
f
Z
i
¼
1=jvC
R
¼
1
jvRC
¼
1
jvt
(3.11)
where t ¼ RC;v ¼ 2pf,and f = frequency.Equation (3.11) shows that the
circuit gain decreases as Rincreases,Figure 3.10 shows the frequency response,
and (3.11) shows that the circuit gain is 1 when vt ¼ 1.
EXAMPLE 3.3 The output of the piezoelectric sensor shown in Figure
2.11(b) may be fed directly into the negative input of the integrator shown in
Figure 3.9,as shown in Figure E3.2.Analyze the circuit of this charge
ampliﬁer and discuss its advantages.
ANSWER Because the FETopamp negative input is a virtual ground,i
sC
¼
i
sR
¼ 0.Hence long cables may be used without changing sensor sensitivity or
3.8 I N T E G R A T O R S
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time constant,as is the case with voltage ampliﬁers.From Figure E3.2,current
generated by the sensor,i
s
¼ Kdx=dt,all ﬂows into C,so,using (3.10),we ﬁnd
that v
o
is
v
o
¼ v
1
C
Z
t
1
0
Kdx
dt
dt ¼
kX
C
which shows that v
o
is proportional to x,even down to dc.Like the
integrator,the charge ampliﬁer slowly drifts with time because of bias
Figure E3.2
The charge ampliﬁer transfers charge generated from a piezo
electric sensor to the opamp feedback capacitor C.
Figure 3.10 Bode plot (gain versus frequency) for various ﬁlters
Integrator
(I);differentiator (D);low pass (LP),1,2,3 section (pole);high pass (HP);
bandpass (BP).Corner frequencies f
c
for highpass,lowpass,and bandpass
ﬁlters.
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currents required by the opamp input.A large feedback resistance R
must therefore be added to prevent saturation.This causes the circuit to
behave as a highpass ﬁlter,with a time constant t ¼ RC.It then responds
only to frequencies above f
c
¼ 1=ð2pRCÞ and has no frequencyresponse
improvement over the voltage ampliﬁer.Common capacitor values are
10 pF to 1 mF.
3.9 DIFFERENTIATORS
Interchanging the integrator’s R and C yields the differentiator shown in
Figure 3.11.The current through a capacitor is given by
i ¼ C
dv
dt
(3.12)
If dv
i
=dt is positive,i ﬂows through Rin a direction such that it yields a negative
v
o
.Thus
v
o
¼ RC
dv
i
dt
(3.13)
The frequency response of a differentiator is given by the ratio of feedback
to input impedance.
V
o
ð jvÞ
V
i
ð jvÞ
¼
Z
f
Z
i
¼
R
1=jvC
¼ jvRC ¼ jvt
(3.14)
Equation (3.14) shows that the circuit gain increases as f increases and that it is
equal to unity when vt ¼ 1.Figure 3.10 shows the frequency response.
Unless speciﬁc preventive steps are taken,the circuit tends to oscillate.
The output also tends to be noisy,because the circuit emphasizes high
frequencies.Adifferentiator followed by a comparator is useful for detecting
Figure 3.11 Adifferentiator
The dashed lines indicate that a small capacitor
must usually be added across the feedback resistor to prevent oscillation.
3.9 D I F F E R E N T I A T O R S
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an event the slope of which exceeds a given value—for example,detection of
the R wave in an electrocardiogram.
3.10 ACTIVE FILTERS
LOWPASS FILTER
Figure 1.9(a) shows a lowpass ﬁlter that is useful for attenuating high
frequency noise.A lowpass active ﬁlter can be obtained by using the one
opamp circuit shown in Figure 3.12(a).The advantages of this circuit are that
Figure 3.12 Active ﬁlters
(a) A lowpass ﬁlter attenuates high frequencies.
(b) Ahighpass ﬁlter attenuates lowfrequencies and blocks dc.(c) Abandpass
ﬁlter attenuates both low and high frequencies.
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it is capable of gain and that it has a very lowoutput impedance.The frequency
response is given by the ratio of feedback to input impedance.
V
o
ð jvÞ
V
i
ð jvÞ
¼
Z
f
Z
i
¼
ðR
f
=jvC
f
Þ
½ð1=jvC
f
Þ þR
f
R
i
¼
R
f
ð1 þ jvR
f
C
f
ÞR
i
¼
R
f
R
i
1
1 þ jvt
(3.15)
where t ¼ R
f
C
f
.Note that (3.15) has the same formas (1.23).Figure 3.10 shows
the frequency response,which is similar to that shown in Figure 1.8(d).For
v1=t,the circuit behaves as an inverting ampliﬁer (Figure 3.3),because the
impedance of C
f
is large compared with R
f
.For v1=t,the circuit behaves as
an integrator (Figure 3.9),because C
f
is the dominant feedback impedance.
The corner frequency f
c
,which is deﬁned by the intersection of the two
asymptotes shown,is given by the relation vt ¼ 2pf
c
t ¼ 1.When a designer
wishes to limit the frequency of a widebandwideband ampliﬁer,it is not
necessary to add a separate stage,as shown in Figure 3.12(a),but only to add
the correct size C
f
to the existing wideband ampliﬁer.
HIGHPASS FILTER
Figure 3.12(b) shows a oneopamp highpass ﬁlter.Such a circuit is useful for
amplifying a small ac voltage that rides on top of a large dc voltage,because C
i
,
blocks the dc.The frequencyresponse equation is
V
o
ð jvÞ
V
i
ð jvÞ
¼
Z
f
Z
i
¼
R
f
1=jvC
i
þR
i
¼
jvR
f
C
i
1 þ jvC
i
R
i
¼
R
f
R
i
jvt
1 þ jvt
(3.16)
where t ¼ R
i
C
i
.Figure 3.10 shows the frequency response.For v1=t,the
circuit behaves as a differentiator (Figure 3.11),because C
i
:is the dominant
input impedance.For v1=t,the circuit behaves as an inverting ampliﬁer,
because the impedance of R
i
is large compared with that of C
i
.The corner
frequency f
c
,which is deﬁned by the intersection of the two asymptotes shown,
is given by the relation vt ¼ 2pf
c
t ¼ 1.
BANDPASS FILTER
Aseries combination of the lowpass ﬁlter and the highpass ﬁlter results in a
bandpass ﬁlter,which ampliﬁes frequencies over a desired range and attenu
ates higher and lower frequencies.Figure 3.12(c) shows that the bandpass
function can be achieved with a oneopamp circuit.Figure 3.10 shows the
frequency response.The corner frequencies are deﬁned by the same relations
as those for the lowpass and the highpass ﬁlters.This circuit is useful for
3.1 0 A C T I V E F I L T E R S
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amplifying a certain band of frequencies,such as those required for recording
heart sounds or the electrocardiogram.
3.11 FREQUENCY RESPONSE
Up until now,we have found it useful to consider the op amp as ideal.Nowwe
shall examine the effects of several nonideal characteristics,starting with that
of frequency response.
OPENLOOP GAIN
Because the op amp requires very high gain,it has several stages.Each of these
stages has stray or junction capacitance that limits its highfrequency response
in the same way that a simple RC lowpass ﬁlter reduces highfrequency gain.
At high frequencies,each stage has a 1 slope on a log–log plot of gain versus
frequency,and each has a 908 phase shift.Thus a threestage op amp,such as
type 709,reaches a slope of 3,as shown by the dashed curve in Figure 3.13.
Figure 3.13 Opampfrequencycharacteristics
Early op amps (such as the 709)
were uncompensated,hada gain greater than 1 when the phase shift was equal to
1808,and therefore oscillated unless compensation was added externally.A
popular op amp,the 411,is compensated internally;so for a gain greater than 1,
the phase shift is limited to 908.When feedback resistors are added to build an
ampliﬁer circuit,the loop gain on this log–log plot is the difference between the
opamp gain and the ampliﬁer–circuit gain.
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The phase shift reaches 2708,which is quite satisfactory for a comparator,
because feedback is not employed.For an ampliﬁer,if the gain is greater than 1
when the phase shift is equal to 1808 (the closedloop condition for oscilla
tion),there is undesirable oscillation.
COMPENSATION
Adding an external capacitor to the terminals indicated on the speciﬁcation
sheet moves one of the RC ﬁlter corner frequencies to a very low frequency.
This compensates the uncompensated op amp,resulting in a slope of 1 and a
maximal phase shift of 908.This is done with an internal capacitor in the 411,
resulting in the solid curve shown in Figure 3.13.This op amp does not
oscillate for any ampliﬁer we have described.This op amp has very high dc
gain,but the gain is progressively reduced at higher frequencies,until it is only
1 at 4 MHz.
CLOSEDLOOP GAIN
It might appear that the op amp has very poor frequency response,because its
gain is reduced for frequencies above 40 Hz.However,an ampliﬁer circuit is
never built using the opamp open loop,so we shall therefore discuss only the
circuit closedloop response.For example,if we build an ampliﬁer circuit with
a gain of 10,as shown in Figure 3.13,the frequency response is ﬂat up to 400
kHz and is reduced above that frequency only because the ampliﬁercircuit
gain can never exceed the opamp gain.We ﬁnd this an advantage of using
negative feedback,in that the frequency response is greatly extended.
LOOP GAIN
The loop gain for an ampliﬁer circuit is obtained by breaking the feedback loop
at any point in the loop,injecting a signal,and measuring the gain around the
loop.For example,in a unitygain follower [Figure 3.4(a)] we break the
feedback loop and then the injected signal enters the negative input,after
which it is ampliﬁed by the opamp gain.Therefore,the loop gain equals the
opamp gain.To measure loop gain in an inverting ampliﬁer with a gain of 1
[Figure 3.3(a)],assume that the ampliﬁercircuit input is grounded.The
injected signal is divided by 2 by the attenuator formed of R
f
and R
i
,and is
then ampliﬁed by the opamp gain.Thus the loop gain is equal to (opamp
gain)/2.
Figure 3.13 shows the loopgain concept for a noninverting ampliﬁer.The
ampliﬁercircuit gain is 10.On the log–log plot,the difference between the op
amp gain and the ampliﬁercircuit gain is the loop gain.At lowfrequencies,the
loop gain is high and the closedloop ampliﬁercircuit characteristics are
determined by the feedback resistors.At high frequencies,the loop gain is
low and the ampliﬁercircuit characteristics follow the opamp characteristics.
High loop gain is good for accuracy and stability,because the feedback
resistors can be made much more stable than the opamp characteristics.
3.1 1 F R E Q U E N C Y R E S P O N S E
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GAIN–BANDWIDTH PRODUCT
The gain–bandwidth product of the op amp is equal to the product of gain and
bandwidth at a particular frequency.Thus in Figure 3.13 the unitygain–
bandwidth product is 4 MHz,a typical value for op amps.Note that along
the entire curve with a slope of 1,the gainbandwidth product is still constant,
at 4 MHz.Thus,for any ampliﬁer circuit,we can obtain its bandwidth by
dividing the gain–bandwidth product by the ampliﬁercircuit gain.For higher
frequency applications,op amps such as the OP37E are available with gain–
bandwidth products of 60 MHz.
SLEWRATE
Smallsignal response follows the ampliﬁercircuit frequency response pre
dicted by Figure 3.13.For large signals there is an additional limitation.When
rapid changes in output are demanded,the capacitor added for compensation
must be charged up froman internal source that has limited current capability
I
max
.The change in voltage across the capacitor is then limited,dv=dt ¼I
max
=C,
and dv
o
=dt is limited to a maximal slew rate (15 V/ms for the 411).If this slew
rate S
r
is exceeded by a largeamplitude,highfrequency sine wave,distortion
occurs.Thus there is a limitation on the sinewave fullpower response,or
maximal frequency for rated output,
f
p
¼
S
r
2pV
or
(3.17)
where V
or
is the rated output voltage (usually 10 V).If the slewrate is too slow
for fast switching of a comparator,an uncompensated op amp can be used,
because comparators do not contain the negativefeedback path that may
cause oscillations.
3.12 OFFSET VOLTAGE
Another nonideal characteristic is that of offset voltage.The two opamp
inputs drive the bases of transistors,and the basetoemitter voltage drop may
be slightly different for each.Thus,so that we can obtain v
o
¼ 0,the voltage
ðv
1
v
2
Þ must be a few millivolts.This offset voltage is usually not important
when v
i
is 1 to 10 V.But when v
i
is on the order of millivolts,as when
amplifying the output from thermocouples or strain gages,the offset voltage
must be considered.
NULLING
The offset voltage may be reduced to zero by adding an external nulling pot to
the terminals indicated on the speciﬁcation sheet.Adjustment of this pot
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increases emitter current through one of the input transistors and lowers it
through the other.This alters the basetoemitter voltage of the two transistors
until the offset voltage is reduced to zero.
DRIFT
Even though the offset voltage may be set to 0 at 25 8C,it does not remain
there if temperature is not constant.Temperature changes that affect the
basetoemitter voltages may be due to either environmental changes or to
variations in the dissipation of power in the chip that result from ﬂuctuating
output voltage.The effects of temperature may be speciﬁed as a maximal
offset voltage change in volts per degree Celsius or a maximal offset voltage
change over a given temperature range,say 25 8Cto þ85 8C.If the drift of an
inexpensive op amp is too high for a given application,tighter speciﬁcations
ð0:1 mV/
CÞ are available with temperaturecontrolled chips.An alternative
technique modulates the dc as in chopperstabilized and varactor op amps
(Tobey et al.,1971).
NOISE
All semiconductor junctions generate noise,which limits the detection of small
signals.Op amps have transistor input junctions,which generate both noise
voltage sources and noisecurrent sources.These can be modeled as shown in
Figure 3.14.For lowsource impedances,only the noise voltage v
n
is important;
it is large compared with the i
n
Rdrop caused by the current noise i
n
.The noise
is random,but the amplitude varies with frequency.For example,at low
Figure 3.14 Noise sources in an op amp
The noisevoltage source v
n
is in
series with the input and cannot be reduced.The noise added by the noise
current sources in can be minimized by using small external resistances.
3.1 2 O F F S E T V O L T A G E
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frequencies the noise power density varies as 1=f (ﬂicker noise),so a large
amount of noise is present at lowfrequencies.At the midfrequencies,the noise
is lower and can be speciﬁed in rootmeansquare (rms) units of VHz
1=2
.In
addition,some silicon planardiffused bipolar integratedcircuit op amps
exhibit bursts of noise,called popcorn noise (Wait et al.,1975).
3.13 BIAS CURRENT
Because the twoopamp inputs drive transistors,base or gate current must ﬂow
all the time to keep the transistors turned on.This is called bias current,which
for the 411 is about 200 pA.This bias current must ﬂow through the feedback
network.It causes errors proportional to feedbackelement resistances.To
minimize these errors,small feedback resistors,such as those with resistances
of 10 kV,are normally used.Smaller values should be used only after a check to
determine that the current ﬂowing through the feedback resistor,plus the
current ﬂowing through all load resistors,does not exceed the opamp output
current rating (20 mA for the 411).
DIFFERENTIAL BIAS CURRENT
The difference between the two input bias currents is much smaller than either
of the bias currents alone.A degree of cancellation of the effects of bias
current can be achieved by having each bias current ﬂow through the same
equivalent resistance.This is accomplished for the inverting ampliﬁer and the
noninverting ampliﬁer by adding,in series with the positive input,a compen
sation resistor the value of which is equal to the parallel combination of R
i
and
R
f
.There still is an error,but it is now determined by the difference in bias
current.
DRIFT
The input bias currents are transistor base or gate currents,so they are
temperature sensitive,because transistor gain varies with temperature.How
ever,the changes in gain of the two transistors tend to track together,so the
additional compensation resistor that we have described minimizes the
problem.
NOISE
Figure 3.14 shows how variations in bias current contribute to overall noise.
The noise currents ﬂowthrough the external equivalent resistances so that the
total rms noise voltage is
v ﬃf½v
2
n
þði
n
R
1
Þ
2
þði
n
R
2
Þ
2
þ4kTR
1
þ4kTR
2
BWg
1=2
(3.18)
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where
R
1
andR
2
¼ equivalent source resistances
v
n
¼ meanvalue of the rms noise voltage;inVHz
1=2
;
across the frequency range of interest
i
n
¼ meanvalue of the rms noise current;inAHz
1=2
;
across the frequency range of interest
k ¼ Boltzmann’s constant ðAppendixÞ
T ¼ temperature;K
BW¼ noise bandwidth;Hz
The speciﬁcation sheet provides values of v
n
and i
n
(sometimes v
2
n
and i
2
n
),
thus making it possible tocompare different opamps.If the source resistances are
10 kV,bipolartransistor op amps yield the lowest noise.For larger source
resistances,lowinputcurrent ampliﬁers such as the ﬁeldeffect transistor (FET)
input stage are best because of their lower current noise.Ary (1977) presents
design factors and performance speciﬁcations for a lownoise ampliﬁer.
For ac ampliﬁers,the lowest noise is obtained by calculating the charac
teristic noise resistance R
n
¼ v
n
=i
n
and setting it equal to the equivalent source
resistance R
2
(for the noninverting ampliﬁer).This is accomplished by inserting
a transformer with turns ratio 1:N,where N ¼ ðR
n
=R
2
Þ
1=2
,between the source
and the op amp (Jung,1986).
3.14 INPUT AND OUTPUT RESISTANCE
INPUT RESISTANCE
The opamp differentialinput resistance R
d
is shown in Figures 3.1 and 3.15.
For the FETinput 411,it is 1 TV,whereas for BJTinput op amps,it is about
2 MV,which is comparable to the value of some feedback resistors used.
However,we shall see that its value is usually not important because of the
beneﬁts of feedback.Consider the follower shown in Figure 3.15.In order to
calculate the ampliﬁercircuit input resistance R
ai
,assume a change in input
voltage v
i
.Because this is a follower,
Dv
o
¼ ADv
d
¼ AðDv
i
Dv
o
Þ
¼
ADv
i
Aþ1
Di
i
¼
Dv
d
R
d
¼
Dv
i
Dv
o
R
d
¼
Dv
i
ðAþ1ÞR
d
R
ai
¼
Dv
i
Di
i
¼ ðAþ1ÞR
d
ﬃAR
d
(3.19)
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Thus theampliﬁercircuit input resistanceR
ai
is about ð10
5
Þ ð2 MVÞ ¼200 GV.
This value cannot be achievedinpractice,because surface leakage paths inthe op
amp socket lower it considerably.In general,all noninverting ampliﬁers have a
very high input resistance,which is equal to R
d
times the loop gain.This is not to
say that very large source resistances can be used,because the bias current usually
causes muchlarger problems thanthe ampliﬁercircuit input impedance.For large
source resistances,FET op amps such as the 411 are helpful.
The input resistance of an inverting ampliﬁer is easy to determine.Because
the negative input of the op amp is a virtual ground,
R
ai
¼
Dv
i
Di
i
¼ R
i
(3.20)
Thus the ampliﬁercircuit input resistance R
ai
is equal to R
i
,the input resistor.
Because R
i
is usually a small value,the inverting ampliﬁer has small input
resistance.
OUTPUT RESISTANCE
The opamp output resistance R
o
is shown in Figures 3.1 and 3.15.It is about
40 V for the typical op amp,which may seem large for some applications.
However,its value is usually not important because of the beneﬁts of feedback.
Consider the follower shown in Figure 3.15.In order to calculate the ampliﬁer
circuit output resistance R
ao
,assume that load resistor R
L
is attached to the
output,causing a change in output current Di
o
.Because i
o
ﬂows through R
o
,
there is an additional voltage drop Di
o
R
o
.
Dv
d
¼ Dv
o
¼ ADv
d
þDi
o
R
o
¼ ADv
o
þDi
o
R
o
ðAþ1ÞDv
o
¼ Di
o
R
o
R
ao
¼
Dv
o
Di
o
¼
R
o
Aþ1
ﬃR
o
=A
(3.21)
Figure 3.15
The ampliﬁer input impedance is much higher than the opamp
input impedance R
d
.The ampliﬁer output impedance is much smaller than the
opamp output impedance R
o
.
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Thus the ampliﬁercircuit output resistance R
ao
is about 40=10
5
¼0:0004 V,
a value negligible in most circuits.In general,all noninverting and inver
ting ampliﬁers have an output resistance that is equal to R
o
divided by
the loop gain.This is not to say that very small load resistances can be
driven by the output.If R
L
shown in Figure 3.15 is smaller than 500 V,the
op amp saturates internally,because the maximal current output for a
typical op amp is 20 mA.This maximal current output must also be
considered when driving large capacitances C
L
at a high slew rate.Then
the output current
i
o
¼ C
L
dv
o
dt
(3.22)
The R
o
C
L
combination also acts as a lowpass ﬁlter,which introduces
additional phase shift around the loop and can cause oscillation.The cure
is to add a small resistor between v
o
and C
L
,thus isolating C
L
from the
feedback loop.
To achieve larger current outputs,the current booster is used.An ordinary
op amp drives highpower transistors (on heat sinks if required).Then we can
use the entire circuit as an op amp by connecting terminals v
1
,v
2
,and v
o
to
external feedback networks.This places the booster section within the feed
back loop and keeps distortion low.
3.15 PHASESENSITIVE DEMODULATORS
Figure 2.7 shows that a linear variable differential transformer requires a
phasesensitive demodulator to yield a useful output signal.Aphasesensitive
demodulator does not measure phase but yields a fullwaverectiﬁed output of
the inphase component of a sine wave.Its output is proportional to the
amplitude of the input,but it changes sign when the phase shifts by 1808.
Figure 3.16 shows the functional operation of a phasesensitive de
modulator.Figure 3.16(a) shows a switching function that is derived from a
carrier oscillator and causes the doublepole doublethrow switch in Figure
3.16(b) to be in the upper position for þ1 and in the lower position for 1.In
effect,this multiplies the input signal v
i
by the switching function shown in
Figure 3.16(a).The inphase sine wave in Figure 3.16(c) is demodulated by this
switch to yield the fullwaverectiﬁed positive signal in Figure 3.16(d).The sine
wave in Figure 3.16(e) is 1808 out of phase,so it yields the negative signal in
Figure 3.16(f).
Ampliﬁer stray capacitance may cause an undesirable quadrature voltage
that is shifted 908,as shown in Figure 3.16(g).The demodulated signal in Figure
3.16(h) averages to zero when passed through a lowpass ﬁlter and is rejected.
The dc signal shown in Figure 3.16(i) is demodulated to the wave shown in
Figure 3.16(j) and is rejected.Any frequency component not locked to the
3.1 5 P H A S E  S E N S I T I V E D E M O D U L A T O R S
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carrier frequency is similarly rejected.Because the phasesensitive de
modulator has excellent noiserejection capabilities,it is frequently used to
demodulate the suppressedcarrier waveforms obtained from linear variable
differential transformers (LVDTs) and the acexcited straingage Wheatstone
bridge (Section 2.3).A carrier system and phasesensitive demodulator are
also essential for operation of the electromagnetic blood ﬂowmeter (Section
8.3).The noiserejection capability may be improved by placing a tuned
ampliﬁer before the phasesensitive demodulator,thus forming a lockin
ampliﬁer (Aronson,1977).
Figure 3.16 Functional operationof aphasesensitivedemodulator
(a) Switch
ing function.(b) Switchswitch.(c),(e),(g),(i) Several several input voltages.(d),
(f),(h),(j) Corresponding corresponding output voltages.
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Apractical phasesensitive demodulator is shown in Figure 3.17.This ring
demodulator operates with the following action,provided that v
c
is more than
twice v
i
If the carrier waveformv
i
is positive at the black dot,diodes D
1
and D
2
are forwardbiased and D
3
and D
4
are reversebiased.By symmetry,points A
and Bare at the same voltage.If the input waveformv
i
,is positive at the black
dot,this transforms to a voltage v
DB
that appears at v
o
,as shown in the ﬁrst half
of Figure 3.16(d).
During the second half of the cycle,diodes D
3
and D
4
are forwardbiased
and D
1
and D
2
are reversebiased.By symmetry,points A and C are at the
same potential.The reversed polarity of v
i
yields a positive v
DC
,which appears
at v
o
.Thus v
o
is a fullwaverectiﬁed waveform.If v
i,
changes phase by 1808,as
shown in Figure 3.16(e),v
o
changes polarity.To eliminate ripple,the output is
usually lowpass ﬁltered by a ﬁlter the corner frequency of which is about one
tenth of the carrier frequency.
The ring demodulator has the advantage of having no moving parts.Also,
because transformer coupling is used,v
i
,v
c
,and v
c
can all be referenced to
different dc levels.The availability of type 1495 solidstate doublebalanced
demodulators on a single chip (Jung,1986) makes it possible to eliminate the
bulky transformers but requires more care in biasing v
i
,v
c
,and v
o
at different
dc levels.
EXAMPLE 3.4 (a) For Figure 3.17,assume that the carrier frequency is
3 kHz.Design the RC output lowpass ﬁlter to have a corner frequency of
20 Hz and a reasonable value capacitor (100 nF).Use (b) a onesection active
ﬁlter.
Figure 3.17 A ring demodulator
This phasesensitive detector produces a
fullwaverectiﬁed output v
o
that is positive when the input voltage v
i
is in phase
with the carrier voltage v
c
and negative when v
i
is 1808 out of phase with v
c
.
3.1 5 P H A S E  S E N S I T I V E D E M O D U L A T O R S
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ANSWER (a) See Figure 1.6(a)
2pfRC ¼ 1
R ¼ 1ð2pfCÞ ¼ 1=ð2p20 0:0000001Þ ¼ 80kV
(b) See Figure 3.12(a)
C
f
¼ 0:1mF;R
i
¼ 80kV;R
f
¼ 80kV
3.16 TIMERS
In electronic design,there is often a need to generate signals that repeat
at regular intervals.One type of signal is the square wave,shown in Figure
3.18.
The voltage of a square wave is high for a ﬁxed amount of time,T
h
,then it
drops to a lower voltage for a length of time T
l
.This pattern of alternating high
and low cycles continuously repeats.The total period of the square wave,the
time it takes to repeat,is thus
T ¼ T
h
þT
l
(3.23)
The duty cycle of a square wave is deﬁned as the percentage of the time
that the square wave is at its higher output voltage.Thus
Duty cycle ¼
T
h
T
ð100%Þ (3.24)
For example,a square wave in which T
h
¼ T
l
is said to have a 50%duty
cycle.
There are many ways to generate square waves.Digital systems use square
waves with 50%duty cycles as clocks to synchronize digital logic;thus,there
Figure 3.18
A square wave of period T oscillates between two values.
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are many commercially available clock generator chips that yield square waves
with 50% duty cycles.
Many times,however,we want to generate square waves with duty cycles
other than 50%.A popular means of doing this is with a 555 timer.The 555
timer is an 8pin integrated circuit,as shown in Figure 3.19(a).The 555 timers
formthe core of many different kinds of timing circuits.One popular conﬁgu
ration is shown in Figure 3.19(b).When powered,this circuit oscillates
internally,alternately charging and discharging capacitor C.Figure 3.19(c)
shows the output of the circuit.Note that the duty cycle of this circuit is always
greater than 50%because R
a
must be nonzero.To get square waves with duty
cycles less than 50%,the output of this circuit may be fed into an inverting
ampliﬁer or logic inverter.
Figure 3.19 The 555 timer
(a) Pinout for the 555 timer IC.(b) A popular
circuit that utilizes a 555 timer and four external components creates a square
wave with duty cycle > 50%.(c) The output fromthe 555 timer circuit shown
in (b).
3.1 6 T I M E R S
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This method of generating square waves is simple and requires only a small
integrated circuit (IC) and four external components.The circuit of Figure
3.19(b),however,is not very useful for precision timing applications,because
of the difﬁculty of creating precision capacitors.Using typical offtheshelf
components,the period may vary by as much as 25%fromthe nominal values.
Using variable resistances for R
a
and R
b
,which allows ﬁnetuning of the time
constants,can minimize this.
EXAMPLE 3.5 Design a timer for a nerve stimulator that stimulates for 200
ms every 50 ms.
ANSWER Use the circuit shown in Figure 3.19(b).From Figure 3.19(c)
T
l
¼ lnð0:5ÞR
b
C
R
b
¼ T
1
=ðlnð0:5ÞC ¼ 200 ms=ð0:693 0:1 mFÞ ¼ 2886 V
T
h
¼ lnð0:5ÞðR
l
þR
h
ÞC:
R
h
¼ R
l
þT
h
=ðlnð0:5ÞC ¼ 2886 þ50 ms=ð0:693 0:1mFÞ ¼ 717 kV:
Use 7404 TTL chip or 4049 CMOS chip logic gate inverter to yield þ5 V for
200 ms.
3.17 MICROCOMPUTERS IN MEDICAL INSTRUMENTATION
The electronic devices that we have described so far in this chapter are useful
for acquiring a medical signal and performing some initial processing,such as
ﬁltering or demodulation.Microcomputers can frequently replace analog
circuits by performing the signalprocessing functions of comparator,limiter,
rectiﬁer,logarithmic ampliﬁer,integrator,differentiator,active ﬁlter,and
phasesensitive demodulator in software (Tompkins,1993;Ritter et al.,
2005).The generalized instrumentation system shown in Figure 1.1 also
indicates additional signal processing,data storage,and control and/or feed
back capability.Traditionally,this additional processing was handled either by
using relatively simple digitalelectronic circuits or,if a signiﬁcant amount of
processing was required,by connecting the instrument to a computer.
The development of microcomputers has led to the combining of a medical
instrument with a signalprocessing capability sufﬁcient to perform functions
normally done by an operator or a computer.This computing function can
certainly be implemented.But from the point of view of medical instrumen
tation,it is more instructive to view the microcomputer as a microcontroller.
The use of a microcomputer generally results in fewer IC packages.This
reduced complexity,together with the capability for selfcalibration and
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detection of errors,enhances the reliability of the instrument.The most useful
applications of microcomputers for medical instrumentation involve this
controller function.Microcomputers can provide selfcalibration for measure
ment systems,automatic sequencing of events,and an easy way to enter such
patient data as height,weight,and sex for calculating expected or normal
performance.All these functions are made possible by the basic structure of
the microcomputer system.Further development has resulted in chipbased
systems.For instance digital ﬁlters are nowdirectly hard coded onto dedicated
chips which result in signiﬁcant computational savings The LabVIEW PC
based system provides modular softwarebased instruments for data acquisi
tion.It permits graphical system design of embedded applications for micro
processor and microcontroller devices.Thus the LabVIEW developed
software can be used in many new medical instruments after the purchase
of one LabVIEW system that includes the Microprocessor SDK toolkit.
(http://www.ni.com/labview/;Tompkins and Webster,1981;Tompkins and
Webster,1988;Carr and Brown,2001).
PROBLEMS
3.1 (a) Design an inverting ampliﬁer with an input resistance of 20 kV and a
gain of 10.(b) Include a resistor to compensate for bias current.(c) Design a
summing ampliﬁer such that v
o
¼ ð10v
1
þ2v
2
þ0:5v
3
Þ.
3.2 The axon action potential (AAP) is shown in Figure 4.1.Design a dc
coupled oneopamp circuit that will amplify the 100 mV to 50 mV input range
to have the maximal gain possible without exceeding the typical guaranteed
linear output range.
3.3 Use the circuit shown in Figure E3.1 to design a dccoupled oneopamp
circuit that will amplify the 100 mV EOG to have the maximal gain possible
without exceeding the typical guaranteed linear output range.Include a control
that can balance (remove) series electrode offset potentials up to 300 mV.Give
all numerical values.
3.4 Design a noninverting ampliﬁer having a gain of 10 and R
i
of Figure 3.4(b)
equal to 20kV.Include a resistor to compensate for bias current.
3.5 An opamp differential ampliﬁer is built using four identical resistors,each
having a tolerance of 5%.Calculate the worst possible CMRR.
3.6 Design a threeopamp differential ampliﬁer having a differential gain of 5
in the ﬁrst stage and 6 in the second stage.
3.7 Design a comparator with hysteresis in which the hysteresis width extends
from 0 to 2 V.
3.8 For an inverting halfwave perfect rectiﬁer,sketch the circuit.Plot the
input–output characteristics for both the circuit output and the opamp output,
which are not the same point as in most opamp circuits.
P R O B L E M S
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3.9 Using the principle shown in Figure 3.8,design a signal compressor
for which an inputvoltage range of 10 V yields an outputvoltage range of
4 V.
3.10 Design an integrator with an input resistance of 1MV.Select the
capacitor such that when v
i
¼ þ10 V;v
o
travels from 0 to 10 V in 0.1 s.
3.11 In Problem 3.10,if v
i
¼ 0 and offset voltage equals 5 mV,what is the
current through R?Howlong will it take for v
o
to drift from0 Vto saturation?
Explain how to cure this drift problem.
3.12 In Problem 3.10,if bias current is 200 pA,how long will it take for v
o
to
drift from 0 V to saturation?Explain how to cure this drift problem.
3.13 Design a differentiator for which v
o
¼ 10V when dv
i
=dt ¼ 100 V/s.
3.14 Design a onesection highpass ﬁlter with a gain of 20 and a corner
frequency of 0.05 Hz.Calculate its response to a step input of 1 mV.
3.15 Design a oneopamp highpass active ﬁlter with a highfrequency gain of
10 (not –10),a highfrequency input impedance of 10 MV,and a corner
frequency of 10 Hz.
3.16 Find V
o
ð jvÞ=V
i
ð jvÞ for the bandpass ﬁlter shown in Figure 3.12(c).
3.17 Figure 6.16 shows that the frequency range of the AAP is 1
˜
10 to 10kHz.
Design a oneopamp active bandpass ﬁlter that has a midband input impedance
of approximately 10 kV,a midband gain of approximately 1,and a frequency
response from 1 to 10 kHz (corner frequencies).
3.18 Figure 6.16 shows the maximal singlepeak signal and frequency
range of the EMG.Design a oneopamp bandpass ﬁlter circuit that will
amplify the EMG to have the maximal gain possible without exceeding the
typical guaranteed linear output range and will pass the range of frequencies
shown.
3.19 Using 411 op amps,explain how an ampliﬁer with a gain of 100 and a
bandwidth of 100 kHz can be designed.
3.20 Refer to Figure 3.13.If the ampliﬁer gain is 1000,what is the loop gain at
100 Hz?
3.21 For the differentiator shown in Figure 3.11,ground the input,break the
feedback loop at any point,and determine the phase shift in each section.
Explain why the circuit tends to oscillate.
3.22 For Problem3.21,calculate the ampliﬁer input and output resistances at
100 Hz,for inverting and noninverting ampliﬁers.
3.23 For Figure 3.15,what is the maximal capacitive load C
L
that can be
connected to a 411 without degrading the normal slew rate ð15V/msÞ at the
maximal current output (20 mA)?
3.24 For Figure 3.17,if the forward drop of D
1
is 10%higher than that of the
other diodes,what change occurs in v
o
?
3.25 Given an oscillator block,design (show the circuit diagram for)
an LVDT,phasesensitive demodulator and a ﬁrstorder lowpass ﬁlter
with a corner frequency of 100 Hz.Sketch waveforms at each signiﬁcant
location.
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REFERENCES
Aronson,M.H.,‘‘Lockin and carrier ampliﬁers.’’ Med.Electron.Data,8(3),1977,C1–C16.
Ary,J.P.,‘‘A headmounted 24channel evoked potential preampliﬁer employing lownoise
operational ampliﬁers.’’ IEEE Trans.Biomed.Eng.,BME24,1977,293–297.
Carr,J.J.,and J.M.Brown,Introduction to Biomedical Equipment Technology,4th ed.,Upper
Saddle River,NJ:PrenticeHall,2001.
Franco,S.,Design with Operational Ampliﬁers and Analog Integrated Circuits.3rd ed.,NewYork:
McGrawHill,2002.
Graeme,J.G.,‘‘Rectifying widerange signals with precision,variable gain.’’ Electron.,Dec.12,
1974,45(25),107–109.
Horowitz,P.,and W.Hill,The Art of Electronics,2nd ed.Cambridge,England:Cambridge
University Press,1989.
Jung,W.G.,1C OpAmp Cookbook,3rd ed.Indianapolis:Howard W.Sams,1986.
Ritter,A.B.,S.Reisman,and B.B.Michniak,Biomedical Engineering Principles.Boca Raton:
CRC Press,2005.
Shepard,R.R.,‘‘Active ﬁlters:Part 12,Short cuts to network design.’’ Electron.,Aug.18,1969,
42(17),82–92.
Tobey,G.E.,J.G.Graeme,and L.P.Huelsman,Operational Ampliﬁers:Design and Application.
New York:McGrawHill,1971.
Tompkins,W.J.(ed.),Biomedical Digital Signal Processing:CLanguage Examples and Labora
tory Experiments for the IBM PC.Englewood Cliffs,NJ:Prentice Hall,1993.
Tompkins,W.J.,and J.G.Webster (eds.),Design of MicrocomputerBased Medical Instrumen
tation.Englewood Cliffs,NJ:PrenticeHall,1981.
Tompkins,W.J.,and J.G.Webster (eds.),Interfacing Sensors to the IBMPC.Englewood Cliffs,
NJ:PrenticeHall,1988.
Wait,J.V.,L.P.Huelsman,and G.A.Korn,Introduction to Operational Ampliﬁer Theory and
Applications.New York:McGrawHill,1975.
R E F E R E N C E S
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