LIC UNIT I.pptx

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

BASICS OF OPERATIONAL
AMPLIFIERS
Syllabus
Basic information about Op-amps: Symbol, Power
Supply Connection – Ideal Operational Amplifier –
Inverting Amplifier – Non-Inverting Amplifier - Voltage
Follower – Differential Amplifier - Op-amp: Block
Diagram, DC characteristics, AC characteristics:
Frequency response, Frequency Compensation,
Slew rate - open and closed loop configurations.
2

Integrated Circuit
3
 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.

Advantages of Integrated Circuit
4
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.

Classifications of ICs
5

6
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 quantity custom circuits.

IC Chip Size
7

8
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.

Operational amplifiers (op-amps)
9
 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.

Introduction to OPAMP
10
 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.

Introduction to OPAMP
11
 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 fields of
process control, communication, computers and measuring
devices.

OPAMP-Symbols and terminals
12
 The input and output are in antiphase having 180
degree phase difference.

Power Supply Connection
13
 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.

14

Packages
15
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 -VEE
Commercially used supply is ± 15V or ± 12V

16

17

Manufacturer
18
 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.

Block Diagram
19
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 amplifiers called
Multistage Amplifier.

Block Diagram
20
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.

Block Diagram
21
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.

Ideal Operational Amplifier
22
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 of output stages.

23
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)

24
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. Therefore,
common mode noise output voltage is zero for an ideal
OPAMP.

25
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 PSRR can
be defined as,

VOLTAGE TRANSFER CURVE OF OPAMP
26
 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.

27
Practical voltage transfer curve

Inverting Amplifier
28
 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 feedback resister Rf
with respect to the virtual ground
point X.

Inverting Amplifier
29

30

Non-Inverting Amplifier
31
 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.
Ii = If

32

Voltage Follower
33
 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 voltage Vi,
that will be followed in the output voltage (Vo).

Differential Amplifier
34
 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.

35
Subtracting Eq.(4) from (3) Such a circuit is very useful in detecting
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.

Difference-mode and common-mode gains
36
 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
signal VCM defined as,

37
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,

38
 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, better is the op-amp.
COMMON-MODE REJECTION RATIO

DC CHARACTERISTICS OF OP-AMP
39
 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
• Thermal drift

1.Input bias current
40
 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. Therefore,

41

42

43

44

2.Input offset current
45

Input offset voltage
46
 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 = 0).

Total output offset voltage
47
 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.

48
 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.

49

Thermal drift
50
 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.
 Forced air cooling may be used to stabilize the

AC Characteristics of OPAMP
51
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 amplitude signal is applied.

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

Slew Rate Equation
53

54

Frequency Response of OPAMP
55
 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.
 The plot showing the variation of gain with the variation in
frequency is termed as frequency response.

56
 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.

57
 The open loop voltage gain of an OPAMP with only
one corner frequency is obtained as

58

Magnitude Response
59

Phase Response
60

61

Frequency Compensation
62

63

Dominant Pole Compensation
64

65

66

Pole Zero Compensation
67

68

69

70

Internal Compensation
71

72

73

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