LINEAR INTEGRATED CIRCUITS

UNIT – I
BASICS OF OPERATIONAL
AMPLIFIERS

1
 BASICS OF OPERATIONAL
AMPLIFIERS
Syllabus
Current mirror and current sources, Current
sources as active loads, Voltage sources,
Voltage References, BJT Differential amplifier
with active loads, Basic information about
op-amps – Ideal Operational Amplifier -
General operational amplifier stages -and
internal circuit diagrams of IC 741, DC and AC
performance characteristics, slew rate, Open
and closed loop configurations, JFET
operational amplifiers-LF 155 and TL082.

2
 Integrated Circuit

⚫ The integrated circuit or IC is a miniature, low

cost electronic circuit consisting of active and
passive components that are irreparably joined
together on a single crystal chip of
silicon.

⚫ Most of the components used in ICs are not

similar to conventional components in
appearance although they perform similar
electrical functions.

3
 Advantages of Integrated Circuit
1. Miniaturization and hence increased equipment
density
2. Cost reduction due to batch processing
3. Increased system reliability due to elimination of
soldered joints
4. Improved functional performance
5. Matched devices
6. Increased operating speeds (due to the absence
of parasitic
capacitance effect)
7. Reduction in power consumption.

4
 Classifications of ICs

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 Monolithic integrated circuits
⚫ In monolithic integrated circuits, all circuit
components, both active and passive elements and
their interconnections are manufactured into or on
top of a single chip of silicon.
⚫ The monolithic circuit is ideal for applications
where identical circuits are required in very large
quantities and hence provides lowest per-unit cost
and highest order of reliability.

Hybrid integrated circuits

⚫ In hybrid circuits, separate component parts are
attached to a ceramic substrate and interconnected
by means of either metallization pattern or wire
bonds. This technology is more adaptable to small
6 quantity custom circuits.
 IC Chip Size

7
 Amplifier

⚫ An amplifier is an electronic device that can

increase the power of a signal (a
time-varying voltage or current).

⚫ It is a two-port electronic circuit that uses

electric power from a power supply to increase
the amplitude of a signal applied to its input
terminals, producing a proportionally greater
amplitude signal at its output.

⚫ The amount of amplification provided by an

amplifier is measured by its gain: the ratio of
output voltage, current, or power to input.
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⚫ An amplifier is a circuit that has a power
 Operational amplifiers (op-amps)

⚫ An operational amplifier is an amplifier circuit

which typically has very high open loop gain
and differential inputs.

⚫ Op amps have become very widely used as

standardized "gain blocks" in circuits due to
their versatility; their gain, bandwidth and
other characteristics can be controlled
by feedback through an external circuit.

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 Introduction to OPAMP
⚫ Linear Integrated Circuits are being used in a
number of applications such as in audio and radio
communication, medical electronics,
instrumentation and control etc.

⚫ An important linear IC is the Operational

Amplifier (OPAMP) introduced in 1940s.

⚫ Robert J Widlar at Fairchild brought out the

popular OPAMP IC 741 between 1964 and 1968.

⚫ It uses BJTs and FETs fabricated along with other

components on a single chip of silicon.
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 Introduction to OPAMP
⚫ The OPAMP is a multi terminal device that has
complex internal circuitry.

⚫ OPAMP's performance can be described by its

terminal characteristics and those external
components that are connected to it.

⚫ ICs have now become an integral part of all

electronic circuits and work at even low voltages. Its
cost is also low due to bulk production.

⚫ Due to the low cost, small size, versatility, flexibility

and dependability of OPAMPs they are used in the
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fields of process control, communication,
computers and measuring devices.
 OPAMP-Symbols and terminals

⚫ The input and output are in antiphase having

180 degree phase difference.

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 Power Supply Connection
⚫ The V+ and V- power supply terminals are connected to two
dc voltage sources. The V+ pin is connected to the positive
terminal of one source and the V- pin is connected to the
negative terminal of the other source as illustrated in figure
where the two sources are 15 V batteries each.
⚫ These are typical values, but in general, the power supply
voltage may range from about + 5 V to + 22 V. The common
terminal of the V+ and V- sources is connected to a reference
point or ground.

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14
 Packages
The three popular packages available for OPAMP's.
1. Metal Can (TO) package
2. Dual in line package (DIP) and
3. Flat package

⚫ Typical packages may have 8 terminals (TO or DIP),

10 terminals (Flat pack) and 14 terminals (DIP and
Flat pack).
⚫ The widely used op-amp uA 741 consist of a single
OPAMP and available as 8 pin DIP/Can or 14 pin DIP
or 10 pin Metal Can package.
⚫ The OPAMP works with a dual power supply. Both of
them are dc and generally balanced with +Vcc and
15
-VEE Commercially used supply is ± 15V or ± 12V
16
17
 Manufacturer

⚫ Some linear ICs are available in different versions such as A, C, E, S

and SC. For example the 741, 741A, 741C, 741E, 741S and 741SC are
different versions of the same OPAMP.

⚫ The 741S and 741SC are military grade OPAMPs whose operating
range is -55°C to 125°C
and have better slew rate compared to 741 and 741C.

⚫ The 741C is commercial grade OPAMP whose operating range is 0°

C to 75°C. 741E and 741C are improved versions having better
electrical specifications.
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 Block Diagram

Input Stage
⚫ The input stage requires high input impedance to avoid
the loading of sources. It is a dual input, balanced output
differential amplifier. This stage provides most of the
voltage gain of the amplifier. It also requires low output
impedance.

Intermediate stage
⚫ This is also a differential amplifier stage driven by the
output of the first stage. It has dual input, unbalanced
(single ended) output. As direct coupling is used, the dc
voltage at the output of this stage is well above ground
potential. This stage also provides additional gain.
⚫ Practically the intermediate stage is a cascade of
19 amplifiers called Multistage Amplifier.
 Block Diagram

Buffer and Level Shifting Stage

⚫ All the stages are directly coupled to each other. The dc
quiescent voltage level of previous stage gets directly applied
as the input to the next stage.
⚫ Therefore, stage by stage dc level increases well above the
ground potential. These dc voltages drive the transistor into
saturation and cause distortion in the output due to clipping.
⚫ Hence, before the output stage, it is necessary to reduce such
a high de voltage level to zero volts with respect to ground.
⚫ The buffer is usually an emitter follower whose input
impedance is very high. This prevents loading of the high
gain stages.
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 Block Diagram

Output Stage
⚫ The output stage must have a low output
impedance, large ac output voltage swing and high
current sourcing and sinking capability.
⚫ A push-pull complementary amplifier meets all
these requirements and it is used as the output
stage. This stage also raises the current supplying
capability of the op-amp.

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 Ideal Operational Amplifier

1. An ideal OPAMP draws no current at both the inputs

i.e. I1 =12 = 0. Thus, the input impedance tends to
infinity and no loading effect on the driver stage.
2. The gain of the ideal OPAMP is infinite. Therefore,
the differential input V1-V2, = Vd is essentially zero for
a finite output voltage.
3. The output is independent of the current drawn
from either of the input terminals. Its output
impedance is zero and hence can able drive number
22 of output stages.
The ideal characteristics of OPAMP are :
1. Infinite voltage gain
2. Infinite input impedance
3. Zero output impedance
4. Infinite CMRR
5. Infinite slew rate
6. Zero offset voltage
7. Infinite bandwidth and
8. Zero Power Supply Rejection Ratio (PSRR)

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 Offset Voltage
⚫ The presence of the small output voltage though
V1-V2 = 0 is called as Offset Voltage. It is zero for ideal
op-amp ensuring zero output for zero input voltage.
Infinite Band Width
⚫ The range of frequency over which the amplifier
performance is satisfactory is called as Bandwidth.
⚫ For ideal case, it is infinity ensuring that the gain of
the op-amp will be constant over the frequency
range from dc to infinity. That is, the OPAMP can
amplify both d.c. and ac signals.
Infinite CMRR
⚫ The ability of the OPAMP to reject the common
mode signals is called as Common Mode Rejection
Ratio (CMRR).
⚫ It is the ratio of differential mode gain to common
mode gain.
⚫ Infinite CMRR ensures zero common mode gain.
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Therefore, common mode noise output voltage is
Infinite Slew rate (S = 0)
⚫ This ensures that, the changes in output voltage
occurs simultaneously with the changes in the
input voltage. If the input changes, output must
also change accordingly. If this is not met then
distortion occurs.
⚫ Slew rate is defined as the maximum rate of
change of output voltage with time and expressed
in V/us.

Zero PSRR
⚫ Power Supply Rejection Ratio (PSRR) is defined as
the ratio of input offset voltage due to change in
supply voltage producing it.
⚫ If VEE is constant and Vcc alone changes, then
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PSRR can be defined as,
 VOLTAGE TRANSFER CURVE OF OPAMP
⚫ The graph of output voltage V0 plotted against the
differential input voltage Vd keeping the gain constant is
called as Voltage Transfer Characteristic curve of an
OPAMP.
Ideal voltage transfer curve
⚫ Ideally the open loop gain of an OPAMP is infinity. Also,
V0. = AOL Vd. Thus, for zero input voltage the output
voltage is always at a saturation level of ±Vsat due to
infinite gain.

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 Practical voltage transfer curve

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Inverting Amplifier
⚫ An amplifier which provides a
phase shift of 180° between the
input and the output is called
as inverting amplifier.

⚫ When the input signal Vi is

applied to the inverting
terminal (-ve terminal) of the
OPAMP, an input current Ii
starts to flow in to the OPAMP.

⚫ For an ideal OPAMP, the input

impedance is infinity and the
point X is at at virtual ground
potential.

⚫ Therefore, the input current Ii,

will not flow into the OPAMP
and it will flow through the
28 feedback resister Rf with
respect to the virtual ground
Inverting Amplifier

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30
Non-Inverting Amplifier
⚫ The input signal is amplified without any
phase inversion.

⚫ The input signal Vi is applied to the non-inverting

terminal (+ve terminal) of the OPAMP. Since the point
X is at virtual ground.
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Ii = I f
32
Voltage Follower
⚫ Voltage follower is a unity gain amplifier
and it has very large input impedance.
As the name implies, the output follows
the input.

⚫ In this circuit, the input resistor (Ri) and

feedback resistor (Rf) are removed. The
inverting terminal is connected or
shorted with the output terminal.

⚫ Due to the existence of virtual short

circuit at the input side, the voltage
available at the inverting terminal is
equal to Vi.

⚫ Therefore, the output voltage is equal

to the input voltage (Vo=Vi)

⚫ Whenever there is a change in input

33 voltage Vi, that will be followed in the
output voltage (Vo).
Differential Amplifier
⚫ A circuit that amplifies the difference between two
signals is called a difference or differential
amplifier. This type of the amplifier is very useful
in instrumentation circuits.
⚫ Since, the differential voltage at the input terminals
of the op-amp is zero, nodes 'a' and b' are at the
same potential, designated as V3.

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 Subtracting Eq.(4) from Such a circuit is very useful in detecting
(3) very small difference in signals, since
the gain R2/ R1 can be chosen to be very
large. For example, if R2= 100 R1, then a
small difference V1-V2 is amplified 100
times.

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Difference-mode and common-mode gains

⚫ If, V1= V2, then V0 = 0. That is, the signal common to

both inputs gets cancelled and produces no output
voltage. This is true for an ideal op-amp, however, a
practical op-amp exhibits some small response to the
common mode component of the input voltages too.

⚫ For example, the output V0 will have different value for

case (i) with V1 = 100 µV and V2 = 50 µV and case (ii)
with V1= 1000 µV and V2 = 950 µV, even though the
difference signal V1-V2= 50 µV in both the cases. The
output voltage depends not only upon the difference
signal Vd at the input, but is also affected by the average
voltage of the input signals, called the common-mode
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signal VCM defined as,
 For differential amplifier, the gain at the output with respect
to the positive terminal is slightly different in magnitude to
that of the negative terminal. So, even with the same voltage
applied to both inputs, the output is not zero. The output,
therefore, must be ex-pressed as,

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COMMON-MODE REJECTION RATIO
⚫ The relative sensitivity of an op-amp to a
difference signal as compared to a common-mode
signal is called common-mode rejection ratio
(CMRR) and gives the figure of merit p for the
differential amplifier.
⚫ So, CMRR is given by:

and is usually expressed in decibels (dB).

⚫ For example, the µA741 op-amp has a minimum
CMRR of 70 dB whereas a precision op-amp such
as µA725A has a minimum CMRR of 120 dB.
Clearly, we should have ADM large and Acm should
be zero ideally. So, higher the value of CMRR,
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better is the op-amp.
DC CHARACTERISTICS OF OP-AMP
⚫ An ideal op-amp draws no current from the
source and its response is also independent of
temperature. However, a real op-amp does not
work in this way. Current is taken from the
source into the op-amp inputs. Also the two
inputs respond differently to current and
voltage due to mismatch in transistor.
⚫ A real op-amp also shifts its operation with
temperature. These non-ideal dc
characteristics that add error components to
the dc output voltage are
• Input bias current
• Input offset current
• Input offset voltage
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• Thermal drift
1.Input bias current
⚫ Practically, input terminals do conduct a small value of
dc current to bias the input transistors. The base
currents entering into the inverting and non-inverting
terminals are IB- and IB+ respectively as shown in the
figure

⚫ Even though both the transistors are identical, IB- and

IB+ are not exactly equal due to the internal imbalance
between the two inputs. Manufacturers specify the
input bias current IB as the average value of the base
currents entering into the terminals of an op-amp.
40 Therefore,
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42
43
44
 2.Input offset current

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Input offset voltage
⚫ In spite of the use of the above compensating
techniques, it is found that the output voltage
may still not be zero with zero input voltage.

⚫ This is due to unavoidable imbalances inside

the op-amp and one may have to apply a small
voltage at the input terminal to make output
(V0) = 0. This voltage is called input offset
voltage Vos.

⚫ This is the voltage required to be applied at the

input for making output voltage to zero (V0 =
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0).
Total output offset voltage
⚫ The total output offset voltage VOT could be
either more or less than the offset voltage
produced at the output due to input bias
current (IB) or input offset voltage alone(VOS)
because IB and VOS could be either positive or
negative with respect to ground.

⚫ Therefore, the maximum offset voltage at the

output of an inverting and non-inverting
amplifier without any compensation technique
provide offset compensation pins to nullify the
offset voltage.
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⚫ A 10K potentiometer is placed across offset null
pins 1&5. The wipes connected to the negative
supply at pin 4. The position of the wipes is
adjusted to nullify the offset voltage.
⚫ When the given op-amps does not have these
offset null pins, external balancing techniques
are used as shown in figure.

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49
Thermal drift
⚫ Bias current, offset current and offset voltage
change with temperature. A circuit carefully
nulled at 25°C may not remain so when the
temperature rises to 35°C. This is called drift.

⚫ Offset current drift is expressed in nA/°C. This

indicate the change in offset for each degree
Celsius change in temperature.

⚫ Techniques to avoid drift: Careful printed

circuit board layout must be used to keep
op-amps away from source of heat.
50 Forced air cooling may be used to stabilize the
⚫
AC Characteristics of OPAMP
1. Slew Rate
2. Frequency response
Slew Rate
⚫ It is defined as the maximum rate of change of
output voltage with time. It is expressed in
V/µsec. The slew rate S is given by,

⚫ The slew rate is caused due to the charging rate

of the compensating capacitor, current limiting
capability and saturation of the internal stages
of the OPAMP, when a high frequency large
51 amplitude signal is applied.
Slew Rate
⚫ The internal capacitor voltage cannot change
instantaneously.
⚫ For large charging rate, the capacitor should be
small or charging current must be large. Hence, the
slew rate of an OPAMP whose maximum internal
capacitor charging current is known can be found
using the formula,

⚫ For IC741, the charging current is 15 µA and the

internal capacitance is 30 pF. Therefore the slew
rate is 0.5V/µsec. Ideally, it should be infinite.

⚫
52 Higher the value of S, better is the OPAMP
performance.
 Slew Rate Equation

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54
Frequency Response of OPAMP
⚫ Ideally, an OPAMP should have an infinite bandwidth. If
the open loop gain is 90dB with dc signal then, its gain
should remain the same 90dB through audio and on to
high radio frequencies. In practical, the gain decreases at
high frequencies.

⚫ There must be some capacitive component present due

to the physical characteristics of the device and this
component is responsible for the reduction in the gain.
Such a reduction in the gain with respect to frequency is
called as roll off. The gain depends on frequency and is
complex.

⚫ Its magnitude and phase angle changes with respect to

frequency.

⚫
55 The plot showing the variation of gain with the variation
in frequency is termed as frequency response.
 ⚫ In such plots, magnitude and phase angle
variation for variation in frequency can be drawn
on a logarithmic scale.
⚫ It is easy to represent gain in dB than on a linear
scale. Such a plot containing magnitude and phase
are called as Bode Plots.
⚫ To obtain the frequency response of an OPAMP,
consider the high frequency model of the OPAMP
with a capacitor C at the output.

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 ⚫ The open loop voltage gain of an OPAMP with
only one corner frequency is obtained as

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58
 Magnitude Response

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 Phase Response

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61
 Frequency Compensation

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 External Compensation
Technique

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 Dominant Pole Compensation

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65
66
 Pole Zero Compensation

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68
69
70
 Feed Forward Compensation

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72
 Internal Compensation

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74
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Stability of OPAMP

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77
78
 Stability Specification from frequency
response

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80
 OPEN LOOP CONFIGURATION OF
OPAMP

INVERTING
AMPLIFIER

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82
 NONINVERTING
AMPLIFIER

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 Closed – loop op-amp configuration:
Inverting Closed Loop Configuration:
The inverting close-loop configuration
∙ External components R1 and R2 form a close loop
∙ Output is fed back to the inverting input terminal
∙ Input signal is applied from the inverting terminal

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 Non inverting closed loop Configuration

o The required conditions to apply virtual short for op- amp

circuit:
o Negative feedback configuration
o Infinite open loop gain
o Closed loop gain: G = VO /VI = 1 + R2 /R1
o Infinite differential gain: V+ - V- = VO /A = 0
o Infinite input impedance: i2 = i1 = V- /R1
o Zero output impedance: VO = V- + i1R2 = VI (1 + R2 /R1)
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JFET Operational Amplifiers– LF155
⚫ FEATURES
⚫ Guaranteed Offset
Voltage Drift on All
Grades.
⚫ Guaranteed Slew Rate
on All Grades.
⚫
⚫ Guaranteed Low Input
Offset Current 10pA
Max.
⚫ Guaranteed Low Input
Bias Current 50pA Max.
⚫ Guaranteed High Slew
Rate (156A/356A) 10V/µs
Min.
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⚫ Fast Settling to 0.01% 1.5
 Applications:
∙ Precision high speed integrators
∙ Fast D/A and A/D converters
∙ High impedance buffers Wideband, low noise, low drift
amplifiers
∙ Logarithmic amplifiers

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 TL082 JFET OP-AMP

⚫ FEATURES

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 APPLICATIONS

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