LIC UNIT II.pptx

UNIT – II
APPLICATIONS OF OPERATIONAL
AMPLIFIERS
LINEAR INTEGRATED CIRCUITS
1
KONGUNADU COLLEGE OF ENGINEERING
AND TECHNOLOGY (AUTONOMOUS)

APPLICATIONS OF OPERATIONAL
AMPLIFIERS
Syllabus
Basic Op-amp Applications - Scale Changer -
Summing Amplifier – Subtractor - Instrumentation
amplifier - V-to-I and I-to-V converters – Precision
Rectifier – Peak detector - Clipper and Clamper -
Sample and Hold circuit - Log amplifier - Antilog
amplifier - Differentiator -- Integrator - Comparators –
Schmitt trigger – Low pass – high pass and band
pass butterworth filters.
2

BASIC OP-AMP APPLICATIONS
3
 Operational amplifiers have more number of applications
and they forms the basic building block of many linear and
non-linear analog systems.
 Sign changer, adder, Subtractor, voltage follower and
instrumentation amplifier are some of the linear
applications of the operational amplifiers.
 Rectifier, peak detector, clipper and clamper, logarithmic
amplifier and anti-logarithmic amplifier are some of the
non-linear analog applications of the operational amplifier.
 Operational amplifiers (OPAMP) are high gain direct-
coupled amplifiers and used to perform many
mathematical operations. Ideal OPAMP's are having
infinite voltage gain and infinite input impedance with zero
output impedance.

VIRTUAL GROUND
4

5
 The above result indicates that, input voltage is zero. If
there is a short circuit between the two input terminal, then
the input voltage Vi becomes zero. But there exist no real
short circuit between the two input terminals. That is, there
exist a virtual short circuit across the input terminals of an
OPAMP.
 Since one of the input terminal is grounded, the
virtual short becomes virtual ground and voltages at
the point X is zero (VX = 0). That is, the point X is at
virtual ground.

SCALE CHANGER
6
 Scale changer is a circuit in which the output voltage is
equal to the input voltage multiplied by a constant scale. If
Vi is the input voltage and ‘K’ is the Scale factor, then the
output voltage Vo is equal to KVi

7

SUMMING AMPLIFIER
8
 It is an inverting amplifier with more than one input at the
inverting terminal of the OPAMP. In figure, V1, V2 and V3
are the three input voltages to be added.
 Due to these voltages, the currents i1, i2 and i3 flows
through the corresponding input resistors R1, R2 and R3.
 In this circuit, Rf is the feedback resistor through which
the feedback current If flows.

9
 Since the OPAMP has high input impedance, all the
input currents flow through the feedback path.
Therefore,

10

SUBTRACTOR OR DIFFERENTIAL AMPLIFIER
11
 Subtractor is used to subtract the two input voltages
given to the OPAMP circuit. The circuit diagram of a
Subtractor is shown in the figure. The Subtractor is
also called as difference amplifier or differential
amplifier.

12
 V1 and V2 are the inputs given to the Subtractor and
Vo is its output. R1 and R2 are the input resistor
connected to the inverting and non-inverting
terminals of the OPAMP.
 To find the output, superposition principle has to be
applied by considering one input at a time.

13
Case (i) :
 First consider that, only the input V1 is given to the
circuit and V2 is made as zero. Now, the circuit
diagram is changed as shown in figure.

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Case (ii) :
 Next, the input V2 is given to the circuit and V1 is
made as zero. The circuit diagram for this case is
shown in figure

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INSTRUMENTATION AMPLIFIER
19
 The necessary demand of the industry is to measure
and control many physical quantities. Some typical
examples are measurement and control of
temperature, humidity, flow etc. Transducers are
usually used to measure all these, low output
voltages. This low voltage has to be amplified so as
to drive the indicator or display. An instrumentation
amplifier does this. Such an amplifier is an integral
part of modern testing and measurement
instrumentation.

INSTRUMENTATION AMPLIFIER
20
 A good instrumentation amplifier has to meet the following
requirements.
1. Finite, accurate and stable gain
2. Easy gain adjustment
3. High input and low input impedances
4. High CMRR
5. Low power consumption
6. High slew rate
7. Low thermal and time drifts
8. Low dc offset
 There are specially designed OPAMP’s such as A 725 that
satisfies the above requirements.
 AD 521, AD524, AD620, LM 363, are some of the commercially
available chips.

21

ANALYSIS OF THREE OPAMP INSTRUMENTATION
AMPLIFIER
22
 The instrumentation amplifier has a basic difference
amplifier at the output stage. If the output of the op-
amp A1 is Vo1 and that of A2 is Vo2 then,

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ADVANTAGES
26
 The three op-amp instrumentation amplifier has
certain advantages. They are :
1. With variable resistance RG the gain can be easily
controlled.
2. The input impedance depends on the input
impedance of the non-inverting amplifiers
(Non-inverting amplifiers have very high input
impedance)
3. The output impedance depends on the output
impedance of A3 which is very low.
4. CMRR is also very high.

V-TO-I CONVERTERS
27
 In voltage to current converters, the output or load
current is proportional to the input voltage. The output
load may or may not be connected to ground.
 According to this, the following two types of V–to–I
converters are available.
 (i) Floating Load converter and
 (ii) Grounded Load converter
 The load resistor RL is not connected to the ground in
floated type V–to–I converters.

V–TO–I CONVERTER WITH FLOATING LOAD.
28
 The load resistor RL is not connected to the ground
and it acts as a feedback resistor.

29

V–TO–I CONVERTER WITH GROUNDED LOAD
30
 The input voltage is given to the non-inverting terminal of the
OPAMP through the resistor divider network.
 The resistor divider network consist of the resistors R1, R2 and
the load resistor RL. The load resistor RL is grounded.

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CURRENT TO VOLTAGE (I–TO–V) CONVERTER
33
 In current to voltage converter, the output voltage is
proportional to the input current.
 In this circuit, the input current is given to the inverting
terminal of the OPAMP and in the feedback, a resistor
R is connected.

34

PRECISION RECTIFIER
35
 Rectifiers are circuits used to convert ac signal into dc
signal. Normally diodes are used for rectification purpose.
 The major demerit of using diode is that it cannot rectify
voltages below 0.7V, the cut in voltage for a diode. Due to
this, the conventional rectifiers produce distortion at the
output.
 To achieve precision rectification, a circuit is needed which
keeps output equal to input for input greater than zero. This
is done by using OPAMP along with diodes and these
circuits are called as Precision rectifiers.
 They rectify voltages below 0.7V also, hence called small
signal precision rectifiers. The precision rectifiers are
classified as Half wave and Full wave precision rectifiers.

HALF WAVE RECTIFIER
36

HALF WAVE RECTIFIER
37
 An ideal inverting amplifier can be converted into an
ideal half wave rectifier by adding two diodes. When
the input is positive, diode D1 conducts causing VOA to
go to negative by one diode drop. Hence diode D2
gets reverse biased and the output becomes zero. For
all practical purposes no current flows through R1 and
input current flows through D1 only.

HALF WAVE RECTIFIER
38
 For negative input, D2 conducts and D1 is in OFF
state. The negative input forces the op-amp output
positive and causes D2 to conduct. The circuit now
acts like an inverter if Rf = R1 and the output becomes
positive.

FULL WAVE RECTIFIER
39
 For the positive input, diode D1 is in ON condition and
diode D2 is in OFF condition. Both the op-amps act as
inverter and V0 = Vi.

40
 For the negative input, diode D1 is in OFF condition
and diode D2 is ON condition. Let the output of the
first op-amp be V. As the differential input to the
second op-amp is zero, the inverting input terminal is
also at V.

41

PEAK DETECTORS
42
 Square, triangular, saw tooth and pulse waves are non
sinusoidal waveforms. To measure the true rms value
of the signal, the peak values of those signals should
be known. This is achieved by using a peak detector.
 It is a circuit that measures the positive peaks of the
input. The circuit follows the voltage peaks of a signal
and stores the highest value on a capacitor.
 If a higher peak signal value comes along, then this
new value is stored. The highest value is stored until
the capacitor is discharged.

43
 When the input voltage exceeds the voltage across the capacitor, the
diode is ON and the circuit acts like a voltage follower. Hence, the
output follows the input until the condition exists.
 If the input is less than the voltage across the capacitor, the diode
becomes reverse biased. The capacitor holds the stored charge till the
input voltage again exceeds the voltage
 across the capacitor.

44
 At time ‘t1’ the signal peak is
recognized as V1. The peak at ‘t2’
cannot be recognized as the
voltage at the point V2 is less than
the previously occurred peak (V1)
in the input.
 At time ‘t3’, the input signal has a
peak voltage of V3 and it is greater
than the voltage V1.
 Therefore, V3 is recognized as
new peak of the input signal
 The circuit can be reset or the
capacitor voltage can be made
zero by connecting a low leakage
MOSFET switch across the
capacitor.
 Reversing the diode position
detects the lowest or most
negative voltage of the input
signal. Peak detectors are used in
test and measurement instruments
and in AM communication circuits..

CLIPPERS
45
 The circuits used to clip off unwanted portions of the input
above or below certain level so as to give required output
are called as limiting circuits or clipper circuits.
 They may be classified as Positive clipper and Negative
clippers. Positive Clippers remove some positive part of
the input whereas negative clippers remove some negative
part of the input.

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47
 The clipping level is
determined by the internal
setting of Vset obtained from
VCC or –VEE.
 During the positive half cycle,
diode D conducts till the input
is equal to the reference
voltage.
 When it becomes less than
the reference voltage, the op-
amp acts like a voltage
follower and V0 =Vin. As it
becomes greater, D get
reversed biased and the op-
amp goes to open loop.
Hence the op-amp goes into
positive saturation (+Vsat).
 Now the output remains at
Vset and the entire waveform
above Vset gets clipped off.

CLAMPERS
48
 Circuits that are used to add a d.c. level as per the
requirement to the a.c. output are called clamper circuits.
 They are also called as dc restorer circuits. Clampers are
classified as positive clamper and negative clampers.

49
 In this circuit, OPAMP is in
inverting mode. When the input
is negative, output goes
positive. This makes diode D to
ON state.
 Now the capacitor charges to
the peak value of the negative
cycle of the input.
 Beyond the negative input
peaks, the diode comes to OFF
state and stops conducting.
Thus, the output is the sum of
the input and capacitor voltage.
 Let Vp be the voltage up to
which the capacitor gets
charged. Now the input
voltage is equal to (Vi + Vp).
 Thus the d.c. voltage equal
to Vp gets added in the ac
output. The output is same

LOGARITHMIC AMPLIFIER
50
 There are several applications of log and antilog amplifiers.
Antilog computations may require functions such as ln x,
log x, sinh x, etc. These are performed with log-antilog
amplifiers.
 Log amplifiers can perform direct dB display on digital
voltmeter and spectrum analyzer.
 Dynamic range of a signal can also be compressed by
using log-amplifiers.

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ANTILOGARITHMIC AMPLIFIER
53
 The positions of diode and resistor are interchanged
with respect to log amplifier.

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 The basic antilog amplifier can also be constructed by
replacing the diode with a transistor. The output is
proportional to the antilogarithm of the input and given
by the equation (5).
 This equation is temperature dependent and the
antilog amplifier also suffers from the same problem
as that of log amplifier. Hence, some sort of
temperature compensation is required.
 This antilog amplifier is subjected to noise, bias
currents, offset voltages, drifts and frequency stability
problems.
 Transistorized circuits give accuracy, reduced bulk
resistance and high operating ranges.

INTEGRATOR
56
 Integrator is a circuit in which the output voltage is
equal to the integration of input voltage.
 It consists of a resistor R at the input path and a
capacitor C at the feedback path.

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58

PRACTICAL INTEGRATOR
59
 The input impedance of an ideal OPAMP is
infinity and no current flows through the
amplifier through its input terminal.
 In practical amplifier, it is not the case and
some amount of current flows into the
amplifier. That is, the input transistor of an
OPAMP draws a small bias current.
 This bias current charges the capacitor to
the maximum value and the output
becomes saturated. To prevent the
saturation of the capacitor, a large
resistance Rs in the order of 1MΩ is
connected across the capacitor.
 This high valued resistance RS provides a
discharge path for the charges in C. This
resistance prevents the output from going
into full saturation condition. Since the
value of RS is very high, it will not affect the
normal performance of the integrator.

PRACTICAL INTEGRATOR (LOSSY INTEGRATOR)
60
 At DC (ie.ω=0) the gain of Integrator can be
controlled.
 To avoid saturation problem if Cf is shunted by Rf.
 Rf and Cf parallel combination behaves like practical
capacitor, not like ideal capacitor. Hence this circuit is
called as Lossy Integrator.
 Rf provides DC stabilization.

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PRACTICAL INTEGRATOR
64

DIFFERENTIATOR
65
 The circuit which produces the differentiation of the
input at the output is called as differentiator circuit.

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PRACTICAL DIFFERENTIATOR
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PRACTICAL DIFFERENTIATOR
70

COMPARATORS
71
 Operational amplifiers in the open loop configuration
can be used as a comparator. Based on the input,
comparator produces the output that is either positive
saturation voltage or negative saturation voltage.
 A comparator is a circuit that compares the signal
applied at one input with a known reference signal at
the other input and produce an output of +Vsat or –
Vsat. The output is non linear with respect to the input.

TRANSFER CHARACTERISTICS OF COMPARATOR
72

NON-INVERTING COMPARATOR
73
 In this type of comparator, the input signal is applied to
the non-inverting terminal and no reference signal is
applied. The inverting input is at zero potential.

SCHMITT TRIGGER
74
 Open loop is used in basic comparator circuits. As the gain
is large, even small noise voltages can cause false
triggering in comparators to change its state.
 The comparator used to avoid such false triggering is
called as Schmitt trigger or Regenerative comparator. They
use positive feedback. Such positive feedback when added
to a comparator increases the gain. Hence the transfer
curve becomes closer to the ideal one.
 In practical circuits it is not possible to maintain loop gain
to unity for a long time because of supply voltage and
temperature variations. Therefore, a gain value greater
than unity is chosen. This also gives an output virtually
discontinuous at the comparison voltage. This circuit
however now exhibits a phenomenon called Hysteresis or
Backlash.

SCHMITT TRIGGER
75

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 The input is applied at the
inverting terminal of the OPAMP. It
produces an opposite polarity
output.
 The output is feedback to the non-
inverting input terminal that is of
same polarity as that of output.
This ensures positive feedback.
 When Vin is slightly positive than
Vref the output goes into positive
saturation +Vsat.
 When Vin is more negative than –
Vref, the output goes into negative
saturation -Vsat. Hence the output
is saturated between ±Vsat. The
voltage at which it changes its
state can be controlled by the
resistance R1 and R2.

TRANSFER CHARACTERISTIC OF SCHMITT TRIGGER
77
 +Vref is called as upper threshold voltage VUT and –Vref is called as lower
threshold voltage VLT. Output remains in a given state until the input exceeds
the threshold level either positive or negative.
 Once the output changes its state, it remains there indefinitely until the input
crosses any of the threshold voltage levels. This is called hysteresis or dead
band or dead zone. The difference between VUT and VLT is called as
hysteresis width (H).

SCHMITT TRIGGER - INPUT AND OUTPUT
WAVEFORMS
78

LOW PASS BUTTERWORTH FILTER
79
 The first order filter is realized by a RC network
connected to the non-inverting terminals of the op-
amp.

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Design Steps
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HIGH PASS BUTTERWORTH FILTER
84
 A high pass circuit attenuates all the frequencies
lesser than the cut off frequency fL. This filter is
complementary to the LPF. This cut off frequency is
called as the lower cut off frequency.

85
 All the frequencies greater than the lower cut off
frequency are passed. The simple RC network
attached to then on-inverting terminal is similar to that
of LPF but the positions of R
 and C are interchanged. The circuit can be analyzed
for its cut off frequency. Voltage at the point A is given
by,

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BAND PASS FILTERS
89
 A band pass filter has pass band between two cut off
frequencies fH and fL such that fH is greater than fL. Any
frequency outside this pass band is rejected or
attenuated. This is a frequency selective filter.
 The pass band which is between these two cut off
frequencies is called bandwidth of the filter (BW).
 The frequency at the centre of the pass band is called
Centre frequency (fC) where the gain is maximum.
Practically it is not exactly at the centre and also called
as resonant frequency.

WIDE BAND PASS FILTER
90

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WIDE BAND PASS FILTER
92

Narrow Band Pass Filter
93
A narrow band pass filter uses two feedback paths with
only one op-amp as against the two op-amp in wide band
filter. This filter is unique in the following respects :
1. It has two feedback paths, hence named multiple-
feedback filter.
2. The op-amp is used in inverting mode.

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SAMPLE AND HOLD CIRCUITS
96
 Analog voltage given as input to the analog to digital
converter should be held constant during the
conversion time period. It is necessary to have an
accurate A/D conversion. Otherwise, some variation
may be noticed in the output digital data.
 If the analog input voltage is changed by more than an
amount of half-LSB value, then an error can be
occurred in the digital output code.
 To avoid or minimize these errors, it is necessary to
hold the analog input value as constant during the
conversion period. To serve this purpose, sample and
hold circuits are used.

Principle of operation
97
 The sample and hold circuit samples the analog input signal in response
to the sampling command and hold it at the output until the arrival of the
next command.
 The sampling command is used to operate the sampling switch ‘S’. When
the switch ‘S’ is in the closed position, it connects the analog input signal
to the hold capacitor (CH) and the charging of capacitor begins. The
capacitor charges quickly to the sampled analog value.
 When the switch ‘S’ is in the open position, the capacitor holds the stored
value for an extended period. This stored or hold value of the analog
signal is used for the A/D conversion.

Open loop architecture
98

Closed loop architecture
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LIC UNIT II.pptx

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LIC UNIT II.pptx