Lecture: 7-8 (Part-II)

Lecture II: Linear Applications of Opamp
Engr. Tayab
Din Memon,
Dept of
To introduce the differential amplifier And its typical
To introduce the CMRR.
Reasons due to which CMRR is affected
Instrumentation Amplifier (IA)
Applications of Instrumentation Amplifier
Instrumentation Monolithic IC Packages
Different Aspects of In-Amplifier
AD 620 In-Amplifier
Summing Amplifier
Lab-Session II
Differential Amplifier?
A differential amplifier circuit is commonly
used to amplify or buffer differential signals
whilst rejecting common mode signals.
A differential signal is presented across two
terminals; the voltage on one terminal rises
as the voltage on the other terminal falls
(relative to earth).
A common mode signal is one where the
voltages on both terminals rise and fall
Typical Differential Amplifier
amplifiers also
allow one signal to
be subtracted
from another as
shown in figure.
It can be simply
called a subtractor
Figure 1:
Vo 
(V  V )
R1 2 1
How Common Mode signals are
According to the ideal performance equation of the
circuit in Figure 1, the output is zero if the two input
signals v1 and v2 are equal. The ideal circuit has an
infinite CMRR – not the case with practical circuits.
In a practical circuit any mismatch in the resistor
ratio values connected to the op-amp input terminals
causes a common mode signal (e1 e2 ecm) to inject
a differential signal to the amplifier. This differential
signal is amplified to produce a non-zero output
CMRR is thus degraded unless the resistor values are
exactly matched.
Common Mode Rejection Ratio (CMRR)
Common-mode Rejection (CMR), the
property of canceling out any signals
that are common (the same potential on
both inputs), while amplifying any
signals that are differential (a potential
difference between the inputs), is the
most important function of a differential
and instrumentation amplifier.
Common Mode Gain (ACM)
Common-mode gain (ACM), the ratio of change in output
voltage to change in common-mode input voltage, is related to
common-mode rejection. It is the net gain (or attenuation) from
input to output for voltage common to both inputs. For
example, differential amplifier with a common-mode gain of
1/1000 and a 10V common-mode voltage at its inputs will
exhibit a 10mV output change.
The differential gain is the gain between input and output for
voltages applied differentially (or across) the two inputs. The
common-mode rejection ratio (CMRR) is simply the ratio of the
differential gain AD, to the common-mode gain.
Resistor Mismatch effect upon CMRR
Ideally the CMRR of the differential amplifier should be infinite.
But practically it is not possible because resistance change at
the input terminals of Opamp.
In the circuit of Figure 1, CMRR depends upon both resistor
matching and upon the CMRR of the op-amp.
This resistance change is undesirable for CMRR.
In order to overcome the resistance change due to different
reasons like temperature variation etc, input impedance should
be increased.
But there are side effects of increasing the Differential and
Common mode Input Impedance.
At unity gain, typical dc values of CMR are 70dB to more than
100dB, with CMR usually improving at higher gains.
Side Effects of Input Impedance Change
These side affects are:
One effect is stray capacitance that causes
degradation in CMRR at the higher frequencies.
Another effect is to give an increased offset error
because of op-amp bias current.
The single differential amplifier has limitation
in its performance.
To improve its performance two (2) or more
opamps configuration should be used.
Instrumentation Amplifier
An Instrumentation Amplifier is a closed-loop
gain block that has a differential input and an
output that is single ended with respect to a
reference terminal.
Most common, the impedance of the two input
terminals are balanced and have high values,
typically 109Ω, or greater.
The input bias currents should also be low,
typically 1nA to 50nA. As with op amps, output
impedance is very low, nominally only a few
milliohms, at low frequency.
Typical configuration of In-Amplifier
Different Monolithic IC Packages of InAmplifier.
Selection of Differential and In-Amp
A difference amplifier is basically an op amp subtractor,
typically using large value input resistors. In general,
difference amplifiers should be used in applications where
signal to noise ratio, CMRR and Accuracy are not so
In contrast, an instrumentation amplifier is most
commonly an op amp subtractor with two input buffer
amplifiers. In-amps are needed in applications where the
highest accuracy, best signal-to-noise ratio, and lowest
input bias current are essential.
Applications of In-Amplifier
Data Acquisition
Medical Instrumentation
From Pressure, Temperature transducers in noisy
Strain Gauges and RTD with Wheat stone bridge.
ECG, EEG monitors, blood pressure monitors and
Audio applications
As microphone pre-amplifier to extract weak signal
from a noisy environment and to minimize offset
and noise due to ground loops.
Applications of In-Amplifier Cont….
High Speed Signal Conditioning
Video Applications
Because the speed and accuracy of modern video data
acquisition systems have improved, there is now a growing
need for high bandwidth instrumentation amplifiers,
particularly in the field of CCD imaging equipment.
High speed in-amps may be used in many video and cable
RF systems to amplify or process high frequency signals.
Power Control Applications
In-amplifier can also be used for motor monitoring (to
monitor and control motor speed, torque, etc), by measuring
voltages, currents, and phase relationships of a 3-phase acphasor motor.
Applications of Instrumentation Amplifier
Instrumentation Amplifier Applications
Figure: 4mA to 20mA Receiver Circuit
External View of In-Amplifier
An Instrumentation Amplifier with unity gain.
3-Opamp Instrumentation Amplifier with
variable gain.
Gain of Instrumentation Amplifier
If R1=R2=R3=R4=R5=R6=R then gain
of Instrumentation Amplifier is known
Av= (1+2R/RG) (V2-V1)
Monolithic In-Amplifier
For many years, the AD620 has been the
industry-standard, high performance, low
cost in-amp. The AD620 is a complete
monolithic instrumentation amplifier
offered in both 8-lead DIP SOIC packages.
The user can program any desired gain
from 1 to 1000 using a single external
resistor. By design, the required resistor
values fo gain of 10 and 100 are standard
1% metal film resistor values.
Typical Diagram of AD620
Gain Equation in AD620
Datasheet of AD620
Summing Amplifier
Summing Amplifier is an amplifier that
adds different inputs provided at
inverting or non-inverting input
It is highly used when we need to add
different inputs.
Its applications are in Analog to digital
Typical Circuit of Summing Amplifier
Analysis of Differential Amplifier.
Analysis of Instrumentation Amplifier
Analysis of Adder Circuit.
Output Observation of Instrumentation
Amplifier when both inputs are same.
Instrumentation Amplifier Output when there
are different inputs i.e. 2V=V2 and 1V=V1
Differential and Instrumentation amplifier are used
in different applications.
Instrumentation Amplifiers are widely used in those
applications when low level signals are needed to
amplify where as differential amplifiers are used in
those applications when common voltage signals may
exceed supply voltage.
CMRR is higher in Instrumentation amplifier as
compare to the DA.
CMRR is increased with Bandwidth.
Typical CMRR of 741 is 70dB where as of TL084 is
Summing amplifiers are used for the summation of
different inverting inputs.

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