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Engineered for Tomorrow
Subject Name: LINEAR INTEGRATED CIRCUITS
Subject Code: 10EC46
Prepared By: M.P.PRIYADARSHINI
Department: ECE
Date: 2/3/2015
Engineered for Tomorrow
TABLE OF CONTENTS
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OP AMP INTRODUCTION
SYMBOL AND TERMINALS
BLOCK DIAGRAM OF OP AMP
OP AMP PARAMETERS
I. COMMON MODE SIGNAL
II. CMRR
III. INPUT OFFSET VOLTAGE
IV. INPUT BIAS CURRENT
VI. INPUT IMPEDANCE
VII.SLEW RATE
VIII.OUTPUT IMPEDANCE
NON INVERTING AMPLIFIER
INVERTING AMPLIFIER
VOLTAGE FOLLOWER
SUMMING AMPLIFIER
DIFFERENTIAL AMPLIFIER
OFFSET NULLING
Engineered for Tomorrow
Op-Amp – The Surface
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An Operational Amplifier (Op-Amp) is an integrated circuit that
uses external voltage to amplify the input through a very high
gain.
We recognize an Op-Amp as a mass-produced component
found in countless electronics.
Fig (1) OPAMP IC and Pin Configuration
What is an Op-Amp? – The Inside
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The actual count varies, but an Op-Amp contains several Transistors, Resistors, and a few
Capacitors and Diodes.
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For simplicity, an Op-Amp is often depicted as follows which has mimimum of five
terminals such as inverting input, non inverting input ,power supply and an output.
Positive Power Supply
Inverting Input
Output
Non-Inverting Input
Negative Power Supply
Basic Block Diagram of Op-Amp
An Op-Amp can be conveniently divided in to four main blocks
1.
2.
3.
4.
I/P
V1
V2
An Input Stage or Input Diff. Amp.
The Gain Stage
The Level Translator
An Out put Stage
Input Stage
(Diff. Amp.)
Gain Stage (C
E Amp.)
Level
Shifter
Out put
Stage
(Buffer)
Op-Amp
IC
VO
Basic Block Diagram of Op-Amp(contn.,)
• A Differential Amplifier i.e, it is the input stage for the op-amp.
It provides the amplification of the difference voltage b/w the
two inputs.
• A Voltage Amplifier(s) i.e, it is the second stage of op-amp and
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is usually a classA amplifier that provides additional gain.
A Push-Pull Class B Amplifier i.e, it is typically used for output
stage.
The Ideal Op-Amp
•It has infinite gain and infinite bandwidth.
•It has infinite input impedance (open) so that it does not load the driving
source.
•It has zero output impedance.
•These characteristics are illustrated in figure. The input voltage ,Vin , appears
b/w the two terminals, and the output voltage is AvVin , as indicated by the
internal voltage source symbol.
OPAMP PARAMETERS
Common Mode
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Two signal voltages of the same phase, frequency and amplitude are applied to the two inputs as shown is
figure 12-6.
This results in a zero output voltage (as difference is 0V).
This action is called common-mode rejection.
OPAMP PARAMETERS
CMRR(COMMON MODE REJECTION RATIO):
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Desired signals can appear on only one input or with opposite polarities on both input lines. These desired
signals are amplified and appear on the output.
Unwanted signals (noise) appearing with the same polarity on both input lines are essentially cancelled by
the op-amp and don’t appear on the output. The measure of an amplifier’s ability to reject common-mode
signals is a parameter called the CMRR (Common-Mode Rejection Ratio).
•The CMRR is often expresses in decibels (dB) as
OPAMP PARAMETERS(CONTN)
INPUT OFFSETVOLTAGE
•Ideal op-amp produces zero volts out for zero volts in.
•However, in op-amp a small dc voltage,VOUT(error) ,
appears at the output when no
differential input voltage is applied. Its primary cause is a slight mismatch of the
baseemitter voltages of the differential amplifier input stage of an op-amp.
•Input offset voltage,VOS ,is the differential dc voltage required between the inputs to force
the output to zero volts.
•Typical values of input offset voltage are in the range of 2mV or less. It is 0V in ideal case.
•Input offset voltage drift is a parameter related to,VOS ,that specifies how much change
occurs in the input offset voltage for each degree change in temperature.
OPAMP PARAMETERS(CONTN)
INPUT BIAS CURRENT
•Input
terminals of a differential amplifier are the transistor bases and
therefore , the input currents are the base currents as shown in the figure.
OPAMP PARAMETERS(CONTN)
INPUT IMPEDANCE
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The Differential input impedance is the total resistance between the inverting and non-inverting terminals
as illustrated in figure (a) next slide. Differential impedance is measured by determining the change in bias
current for a given change in differential input voltage.
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The common-mode input impedance is the resistance between each input and ground and is measured by
determining the change in bias current for a given change in common-mode input voltage shown in figure
next slide .
OPAMP PARAMETERS(CONTN)
INPUT OFFSET CURRENT
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Ideally, the two input bias currents are equal and thus their difference is zero.
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However, in a practical op-amp, the bias currents are not exactly equal.
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The input offset current , IOS is the difference of the input bias currents expressed as an absolute value.
OPAMP PARAMETERS(CONTN)
INPUT OFFSET VOLTAGE
OPAMP PARAMETERS(CONTN)
OUTPUT IMPEDANCE
•The output impedance is the resistance viewed from the output terminal
of the op-amp as shown in figure below
OPAMP PARAMETERS(CONTN)
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Slew Rate
The slew rate (SR) of an op-amp is the maximum rate at which the output voltage can change in response
to input voltage.
When the SR is too slow for the input, distortion results.
For example when a input sine wave is applied to a voltage follower it produces a triangular output
waveform.
The triangular waveform results because the op-amp simply cannot move fast enough to follow the sine
wave input.
This happens because voltage change in the second stage ( Voltage Amplifier(s) ) is limited by the charging
and discharging of capacitors.
The slew rate is expressed as : SR = ∆VO/∆t .The unit of SR is volts per microseconds.
Non inverting amplifier configuration
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An op-amp in a closed-loop configuration as a
non-inverting amplifier with a controlled
amount of voltage gain is shown in figure 12-16
next slide.
The input signal is applied to the non-inverting
(+) input.
The output is applied back to the inverting(-)
input through the feedback circuit (closed loop)
formed by the input resistor Ri and
the feedback resistor Rf.
Resistors Ri and Rf form a voltage-divider
circuit
which reduces Vout and connects the reduced
voltage Vf to the inverting input.
The feedback voltage is expressed as
VOLTAGE FOLLOWER CONFIGURATION
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Voltage Follower
It is a special case of the non-inverting amplifier
where all of the output voltage is fed back to
the inverting (-) input by a straight connection
as shown in figure 12-19.
The straight feedback connection has a voltage
gain of 1.
The closed-loop voltage gain of a non-inverting
amplifier is 1/B.
Since B= 1 for a voltage follower case, the
closed-loop voltage gain of the voltage follower
is Acl(VF) = 1/1= 1
INVERTING AMPLIFIER CONFIGURATION
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InvertingAmplifier
An op-amp connected as an inverting amplifier
with a controlled amount of voltage gain as
shown in figure.
The input signal is applied through a series input
resistor Ri to the inverting (-) input.
The output is fed back through Rf to the same
input. The noninverting input is grounded.
At this point, the ideal op-amp parameters
mentioned earlier are useful in simplifying the
analysis of this circuit.
Particularly the concept of infinite input
impedance is of great value.
An infinite input impedance implies zero current
at the inverting input.
If there is zero current through the input
impedance, then there must be no voltage drop
between the inverting and noninverting
terminals.
This means that the voltage at the inverting (-)
input is zero. This zero voltage at the inverting
input terminal is referred to as virtual ground.
INVERTING AMPLIFIER CONFIGURATION
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Since there is no current at the inverting input, the
current through Ri and the current through Rf are
equal as shown in figure i.e, Iin = If.
Also the voltage across Rf equals -Vout because of
virtual ground and therefore
IF = -VOUT/RF
since If = IIN+
-VOUT/RF = VIN/Ri
Rearranging the terms
-VOUT/VIN = RF/Ri
Therefore the overall gain is given by
ACL= -RF/Ri.
The –ve sign indicates out of phase relation
between the input and output.
Op-Amp Summing Amplifier
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The summing amplifier is a circuit in which the
ouput is the summation of all the inputs
amplified by the gain.
If equal value resistors are used output equals
the input.
If input resistor equals twice the feedback
resistor then the circuit can be used as a
average circuit.
It can be configured either to be a inverting
summer /non invertring summer circuit.
The output is given as
VOUT= -RF/RIN (V1+V2+…VN)
Differential Amplifier Using Op Amp
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The differential amplifier is a circuit in which the output is the difference of the inputs. Its also called as
subtractor circuit.
From the figure since the inverting terminal is at virtual ground therefore
R2
v  v
v1  v
i1 
R1
v  v0
i1 
R2
R2
v 
v2
R1  R2
v1  v v  v0

R1
R2
v1
v2
i1
R1
v
v
+
R1
R2
i1
vo
OFFSET NULLING
• The op amp offset null capability is one that is available on many op-amp chips.
• The offset null capability is used to reduce small DC offsets that can be amplified. These can be important in DC
amplifiers where these small voltages can then become significant where large gains are required.
Op amp input offsets :
• An op amp is a differential amplifier. This means that when there is no difference between the two inputs, e.g.
when the inputs are shorted together, there should be no voltage on the output.
• Unfortunately under these circumstances there is always a small offset because no op amp is never perfect and
completely balanced. There is always a small input offset voltage.
• This input offset voltage is small and arises from mismatches in the differential input stage of the op amp chip.
These small offsets are caused by a variety of unavoidable issues within the manufacture of the op amp. They
include aspects including mismatched transistor pairs, collector currents, current-gain betas (β), collector or
emitter resistors, etc..
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OFFSET NULLING(CONTN.,)
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The op amp offset null connections enable the input circuit balance to be obtained by
applying external circuitry.
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For circuits where it is necessary to remove or null the offset, many op-amp chips provide
two pins that enable this to be done. Using the offset null adjustment requires a
potentiometer with its wiper connected to the negative supply with some op amps or to
0 V with others so it is necessary to check the data sheet. The value for the
potentiometer may typically be around 10 KΩ to 100 KΩ but again check the data sheet
for the most suitable value.
op amp offset null adjustment
OFFSET NULLING (CONTN.,)
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On op amps with an offset null capability two pins are
provided as shown in the diagram.
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The offset null capability for op amps is often used in instrumentation
applications. For example where the small DC voltages produced by
thermocouples or other sensors need to be amplified. The offset null may be
used in other applications where DC amplification is required and the offsets
need to be removed.
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Although the most common method is to use a preset potentiometer, many
instruments may use self-calibration where the DC offset is measured under
zero input conditions and then a digital value used to counter the offset that is
measured. This could be achieved by applying the required voltage to the
offset null, applying an equal and opposite DC offset, or applying the
measured offset in any digital processing that may be undertaken.