1

Applications of operational amplifier

as Inverting amplifier, Summing amplifier or adder and

Differential amplifier or subtractor

Lecture in

Online WorkshopOn

“lectures and virtual practical demonstrations”

Conducted by

Uttarakhand open University,Haldwanifor

M.Sc.I year studentsfrom

06 July to 19 July 2020

By

Charu Chandra DhondiyalDepartment of Physics

M.B.Govt. P.G.College HaldwaniUttarakhand 263139

charu_dhondiyal@yahoo.co.in

Object

• To study the following applications of

operational amplifier.

➢Inverting amplifier

➢Adder

➢Subtractor

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Apparatus required

• Experimental setup (may be complete or

required additional components)

• (Here I use OMEGA TYPE ETB-159 )

• Other apparatus required with OMEGA TYPE

ETB-159

• Digital multimeter

• Cathode ray oscilloscope (optional)

• Audio frequency generator (optional)

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OMEGA type ETB (159)

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Description of Apparatus

• The board Consists of the following built in parts

• ± 15 V D.C. at 50 mA, IC regulated power supply

• Three 0-2 V D.C. at 100 mA, Continuously variable regulated power supplies.

• OP –AMP IC-741

• Other components (Input resistances, Output resistances, capacitance)

• Mains On/Off switch , Fuse

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Description of Apparatus (cont.)

• The Unit is operative on 230 V ± 10 % at 50 Hz A.C. Mains

• Adequate no. of patch cords

• Terminal/Sockets at appropriate places on panel for connections /observation of waveforms.

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Basic operational Amplifier

• Operational Amplifiers are represented both

schematically and realistically below:

– Active component!

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Terminals on an Op Amp

Non-inverting Input terminal

Inverting inputterminal

Output terminal

Positive power supply (Positive rail)

Negative power supply (Negative rail)

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Symbols for Ideal and Real Op Amps

Op-Amp uA741

LM111 LM324

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Ideal Op-Amp Characteristics

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An ideal op-amp is usually considered to have the following characteristics

➢Infinite open-loop gain G = vout / vin

➢Infinite input impedance Rin, and so zero input current

➢Zero input offset voltage

➢Infinite output voltage range

➢Infinite bandwidth with zero phase shift and infinite slew rate

➢Zero output impedance Rout

➢Zero noise

➢Infinite common-mode rejection ratio (CMRR)

➢Infinite power supply rejection ratio.

Practical Op-Amp Characteristics

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An ideal op-amp is usually considered to have the following characteristics

➢Very high open-loop gain G = vout / vin

➢Very high input impedance Rin, and so zero input current

➢low input offset voltage

➢High output voltage range

➢High bandwidth with small phase shift and high slew rate

➢Low output impedance Rout

➢Low noise

➢High common-mode rejection ratio (CMRR)

➢High power supply rejection ratio.

Comparison Between Ideal Op Amp And Practical Op Amp

Ideal Op-amp Typical Op-amp

Input Resistance Infinity106 (Bipolar)

109 - 1012 (Fet)

Input Current 0 10-12 – 10-8 A

Output Resistance 0 100 – 1000

Operational Gain Infinity 105 - 109

Common Mode Gain 0 10-5

Bandwidth Infinity

Attenuates And Phases At High

Frequencies (Depends On Slew

Rate)

Temperature Independent Bandwidth And Gain

Ideal Op-Amp Analysis

To analyze an op-amp feedback circuit:

•Assume no current flows into either input terminal

•Assume no current flows out of the output terminal

• Constrain: V+ = V-

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741 Op-Amp Schematic

differential amplifier high-gain amplifier

voltage level

shifter

current mirror

current mirror current mirror

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Op Amp Equivalent Circuit

vd = v2 – v1

A is the open-loop voltage gainv2

v1Voltage controlled voltage source

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Mathematics of the Op-Amp

• The gain of the Op-Amp itself is calculated as:

G = Vout/(V+ – V-)

• The maximum output is the power supply voltage

• When used in a circuit, the gain of the circuit (as opposed to

the op-amp component) is:

Av = Vout/Vin

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Op-Amp Saturation

• As mentioned

earlier, the

maximum output

value is the supply

voltage, positive and

negative.

• The gain (G) is the

slope between

saturation points.

Vout

Vin

Vs-

Vs+

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Inverting Amplifier Analysis

virtual ground

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Non-Inverting Amplifier Analysis

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Op-Amp Buffer

Vout = Vin

Isolates loading effects

A

High output impedance

B

Low input impedance

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Op-Amp Differentiator

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Op-Amp Integrator

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Op-Amp Summing Amplifier

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Op-Amp Differential Amplifier

If R1 = R2 and Rf = Rg:7/8/2020 24

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Op-Amp Specifications – DC Offset Parameters

• Even though the input voltage is 0, there will

be an output. This is called offset. The

following can cause this offset:

– Input Offset Voltage

– Output Offset Voltage due to Input Offset Current

– Total Offset Voltage Due to Input Offset Voltage

and Input Offset Current

– Input Bias Current

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General Op-Amp Specifications VIO

• Input Offset Voltage VIO

– The voltage that must be applied to the input terminals of an op amp to

null the output voltage

– Typical value is 2mV with a max of 6mV

– When operated open loop, must be nulled or device may saturate

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General Op-Amp Specifications IIO

• Input Offset Current

– The algebraic difference between the two input currents

– These are base currents and are usually nulled

– Typical value IIO 20 nA with a max of 200nA

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General Op-Amp Specifications CMRR

• Common Mode Rejection Ratio– The ratio of the differential voltage gain (AD) to the common mode gain

(ACM)

– ACM is the ratio between the differential input voltage (VINCM) applied common mode, and the common mode output voltage (VOCM)

– it can exceed minimum is 70db with a typical value of 90 db

– in properly designed circuit, it may exceed 110db

OD

IN

OCMCM

CM

D

CM

VA =

V

V A =

V

A CMRR = 20 log

A

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General Op-Amp Specifications

• Input Bias Current– The average of the currents that flow into the inverting and

noninverting terminals

– Typical values rage from 7nA to 80 nA

• Differential Input Resistance– Also know as the input resistance

– Resistance seen looking into the input terminals of the device

– Runs from a low of 2M for an LM741 to a high of 1012for FET input devices

• Output resistance– Resistance between the output terminal ad ground

– Typical values are 75 or less

• Input Capacitance– The equivalent capacitance measured at either the inverting or

noninverting terminal with the other terminal connected to ground

– May not be on all spec sheets

– Typical value for LM741 is 1.4pF

B+ B-B

I + I I =

2

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General Op-Amp Specifications

• Power Supply Range

– May be differential or single ended

– Max is ± 22V

• Output Voltage Swing

– Range of output voltage

– Depends on power supply voltage used (typically about 85% to 90%)

– Usually about ±13.5V for a power supply voltage of ±15V

• Slew Rate

– The maximum rate of change in the output voltage in response to an input change

– Depends greatly on device, higher is better (output resonds faster to input changes)

– For LM741 it is .5V/s while for the LM318 it is 70V /s

• Gain Bandwidth Product

– The bandwidth of the device when the open loop voltage gain is 1

Inverting Amplifier

Experimental set up & Procedure

• Make the circuit diagram asshown in fig. with the help ofpatch cords.

• Apply known D.C. voltage tothe inverting input of OP Amp(pin 2) from any of threevariable power supply and notecorresponding output usingdigital multimeter or C.R.O.

• Calculate the voltage gain ofinverting amplifier by dividingoutput voltage by inputvoltage.

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Inverting Amplifier

Experimental set up & Procedure

• You can observe theinput and output signalsfrom C.R.O. Note thatoutput will be inverted.

• Calculate the gain of theinverting amplifier forparticular value of Rf andR1. compare it withobserved gain.

• Now change the value offeedback resistance Rf

and repeat the step.

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Observation table

Sr.No.

Observed value of voltage gain Calculated value of voltage gain

Input

Ein(V)

Output

E0(V)

Voltage gain

=E0/Ein

Feedback

resistance used

( Rf in ohm)

Input Resistance

used

( R1 in ohm)

Voltage gain= -

Rf/R1

1 .1 1 10 10 K 1 K 10

2 .1 5.1 51 51K 1 K 51

3 .2 2 10 10 K 1 K 10

4

5

Summing amplifier or Adder(Inverting Type)

Experimental set up & Procedure

• Make the circuit diagramas shown in fig. with thehelp of patch cords.

• Apply known D.C.voltages to all the threeinverting input or twoinverting input (asrequired) of OP Amp (pin2) from three variablepower supply and notecorresponding output usingdigital multimeter orC.R.O.

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Summing amplifier or Adder(Inverting Type)

Experimental set up & Procedure

• Calculate the output of Opamp using formula

V0=-Rf/R1(E1(V)+ E2(V)+ E3(V)

• Generally using OP amp as anadder we take Rf = R1 so that

V0=-(E1(V)+E2(V)+ E3(V)

• Note that output will beinverted in this case as theinputs are given to invertinginput.

• Apply different D.C. voltagesand repeat the step.

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Observation table

Sr.No.

Observed value of voltage gain Output

First

Input

E1(V)

Second

Input

E2(V)

Third Input

E3(V)

Output

Voltage

Observed

Feedback

resistance

used

( Rf in ohm)

Input Resistance used

( R1 in ohm)

Output voltage calculated

V0=Rf/R1(E1(V)+ E2(V)+ E3(V)

1

2

3

4

5

Differential amplifier

Experimental set up & Procedure

• Make the circuit diagramas shown in fig. with thehelp of patch cords.

• Apply known D.C.voltages to inverting andnoninverting input of OPAmp (pin 2 and pin 3)from three variablepower supply and notecorresponding outputusing digital multimeteror C.R.O.

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Differential amplifier

Experimental set up & Procedure

• Calculate the output of Opamp using formula

V0 = Rf/R1[Ein2(V)- Ein1(V)]

• Generally using OP ampas an subtractor we takeRf = R1 so that

V0=[Ein2(V)-Ein1(V) ]

• Apply different D.C.voltages and repeat thestep.

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Observation table

Sr.No.

Observed value of difference Calculated value of Output

First Input

E1(V)

Second Input

E2(V)

Output Voltage

Observed

Input Resistance

used

( R1 in ohm)

Feedback

resistance used

( Rf in ohm)

Output voltage

calculated

V0=Rf/R1(E2(V) - E1(V)

1

2

3

4

5

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We have studied use of operational amplifier as

inverting amplifier ,summing amplifier or adder and

differential amplifier and subtractor.The observed

value and calculated value in different cases are

approximately same.

Results

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• Circuit connections should be made carefully.

• Ratio of feedback resistance and input resistance

should be such that the output voltage is less than the

bias voltage to operational amplifier.

• Measuring instrument should be checked and set into

the range of measurement.

Sources of error and precautions

Thanks