Module 4 Operational Amplifier. Contents Introduction Introduction OPAMP Symbol OPAMP Symbol Internal block diagram Internal block diagram Open-loop configuration

Module 4Module 4

Operational AmplifierOperational Amplifier

ContentsContents

IntroductionIntroduction OPAMP SymbolOPAMP Symbol Internal block diagramInternal block diagram Open-loop configurationOpen-loop configuration Common mode rejection ratioCommon mode rejection ratio OPAMP equivalent circuitOPAMP equivalent circuit OPAMP characteristicsOPAMP characteristics Closed-loop configurationClosed-loop configuration

ContentsContents

Inverting AmplifierInverting Amplifier Non-inverting AmplifierNon-inverting Amplifier Summing Amplifier Difference AmplifierSumming Amplifier Difference Amplifier DifferentiatorDifferentiator IntegratorIntegrator TutorialsTutorials

Reference bookReference book

OPAMPS and Linear Integrated OPAMPS and Linear Integrated CircuitsCircuits

by Ramakanth Gayakwadby Ramakanth Gayakwad

IntroductionIntroduction

Operational Amplifier (OPAMP) is a very Operational Amplifier (OPAMP) is a very high gain amplifier fabricated on high gain amplifier fabricated on Integrated Circuit (IC)Integrated Circuit (IC)

Finds application in Finds application in Audio amplifierAudio amplifierSignal generatorSignal generatorSignal filtersSignal filtersBiomedical InstrumentationBiomedical Instrumentation

And numerous other applicationsAnd numerous other applications

IntroductionIntroduction

Advantages of OPAMP over transistor Advantages of OPAMP over transistor amplifieramplifierLess power consumptionLess power consumptionCosts lessCosts lessMore compactMore compactMore reliableMore reliableHigher gain can be obtainedHigher gain can be obtainedEasy designEasy design

OPAMP terminalsOPAMP terminals

+VC

C

+

–

Non inverting input

Inverting input

Output

–VEE



If input is applied to non inverting input If input is applied to non inverting input terminal, then output will be in-phase with terminal, then output will be in-phase with inputinput

If input is applied to inverting input If input is applied to inverting input terminal, then output will be 180 degrees terminal, then output will be 180 degrees out of phase with inputout of phase with input

If inputs are applied to both terminals, If inputs are applied to both terminals, then output will be proportional to then output will be proportional to difference between the two inputsdifference between the two inputs


Two DC power supplies (dual) are required Two DC power supplies (dual) are required Magnitudes of both may be sameMagnitudes of both may be same The other terminal of both power supplies The other terminal of both power supplies

are connected to common groundare connected to common ground All input and output voltages are All input and output voltages are

measured with reference to the common measured with reference to the common groundground


Integrated Circuit

Internal Block DiagramInternal Block Diagram

Four stages can be identified – Four stages can be identified – Input stage or differential amplifier stage Input stage or differential amplifier stage

can amplify difference between two input can amplify difference between two input signals; Input resistance is very high; signals; Input resistance is very high; Draws zero current from the input sourcesDraws zero current from the input sources

IntermediateStage

LevelShifterStage

OutputStage

Differential Amplifier

Stage

Internal Block DiagramInternal Block Diagram

Intermediate stage (or stages) use direct Intermediate stage (or stages) use direct coupling; provide very high gaincoupling; provide very high gain

Level shifter stage shifts the dc level of Level shifter stage shifts the dc level of output voltage to zero (can be adjusted output voltage to zero (can be adjusted manually using two additional terminals)manually using two additional terminals)

Output stage is a power amplifier stage; Output stage is a power amplifier stage; has very small output resistance; so has very small output resistance; so output voltage is the same, no matter output voltage is the same, no matter what is the value of load resistance what is the value of load resistance connected to the output terminalconnected to the output terminal

Open-loop configurationOpen-loop configuration

If v1 = 0, then vo = –AOLv2 Inverting amplifier

If v2 = 0, then vo = AOLv1 Non inverting amp


AAOLOL is the open-loop voltage gain of OPAMP is the open-loop voltage gain of OPAMP

Its value is very highIts value is very high

Typical value is 0.5 millionTypical value is 0.5 million So, even if input is in micro volts, output So, even if input is in micro volts, output

will be in voltswill be in volts But output voltage cannot cross the value But output voltage cannot cross the value

of power supply of power supply VVCCCC

So, if input is in milli volts, output reaches So, if input is in milli volts, output reaches saturation value saturation value VVsatsat = = VVCCCC (or (or VVEEEE))


If vIf v11 = v = v22, then ideally output should be , then ideally output should be zerozero

But in practical Op-Amp, output is But in practical Op-Amp, output is

Where, Where, AACMCM is the common-mode gain of is the common-mode gain of Op-AmpOp-Amp

So, final gain equation is:So, final gain equation is:

2

vvAv 21

cmo

2

vvAvvAv 21

cm21do

icmcmiddo vAvAv


Common-mode rejection ratioCommon-mode rejection ratioIt is a measure of the ability of Op-Amp It is a measure of the ability of Op-Amp

to reject the signals common to both to reject the signals common to both input terminals (noise)input terminals (noise)

Defined asDefined as

cm

d

AA

CMRR

cm

d10dB A

Alog 20CMRR

ProblemsProblems

An OPAMP has differential voltage gain of 100,000 An OPAMP has differential voltage gain of 100,000 and CMRR of 60 dB. If non inverting input voltage and CMRR of 60 dB. If non inverting input voltage is 150 is 150 μμV and inverting input voltage is 140 V and inverting input voltage is 140 μμV, V, calculate the output voltage of OPAMPcalculate the output voltage of OPAMP

Ans: 1.01 VAns: 1.01 V

For an OPAMP, when v1 is 0.5 mV and v2 is –0.5 For an OPAMP, when v1 is 0.5 mV and v2 is –0.5 mV, output voltage is 8 V. For the same OPAMP, mV, output voltage is 8 V. For the same OPAMP, when v1 = v2 = 1 mV, output voltage is 12 mV. when v1 = v2 = 1 mV, output voltage is 12 mV. Calculate the CMRR of the OPAMPCalculate the CMRR of the OPAMP

Ans: 56.48 dBAns: 56.48 dB

OPAMP equivalent circuitOPAMP equivalent circuit

+–

vovi2vi1

Ri

AOLvidRo

Practical OPAMP

OPAMP equivalent circuitOPAMP equivalent circuitIdeal OPAMP

+–

vovi2vi1

AOLvid

OPAMP CharacteristicsOPAMP Characteristics

Ideal OPAMPIdeal OPAMP Infinite differential mode gainInfinite differential mode gain Zero common mode gainZero common mode gain Infinite CMRRInfinite CMRR Infinite input resistanceInfinite input resistance Zero output resistanceZero output resistance Infinite bandwidthInfinite bandwidth Infinite slew rate Infinite slew rate Zero input offset voltageZero input offset voltage Zero input offset currentZero input offset current Zero output offset voltageZero output offset voltage


Differential mode gain Differential mode gain AAdd

It is the factor by which the difference between It is the factor by which the difference between the two input signals is amplified by the OPAMPthe two input signals is amplified by the OPAMP

Common mode gain Common mode gain AAcmcm

It is the factor by which the common mode It is the factor by which the common mode input voltage is amplified by the OPAMPinput voltage is amplified by the OPAMP

Common mode rejection ratio Common mode rejection ratio CMRRCMRR Is the ratio of Is the ratio of AAdd to to AAcmcm expressed in decibels expressed in decibels


Input resistance Input resistance RRii It is the equivalent resistance measured It is the equivalent resistance measured

between the two input terminals of OPAMPbetween the two input terminals of OPAMP

Output resistance Output resistance RRoo It is equivalent resistance measured between It is equivalent resistance measured between

output terminal and groundoutput terminal and ground

BandwidthBandwidth It is the range of frequency over which the gain It is the range of frequency over which the gain

of OPAMP is almost constantof OPAMP is almost constant


Output offset voltage Output offset voltage VVoooo

It is the output voltage when both input It is the output voltage when both input voltages are zerovoltages are zero

Denoted as Denoted as VVoooo

Input offset voltage Input offset voltage VVioio

It is the differential input voltage that must be It is the differential input voltage that must be applied at the input terminals in order to make applied at the input terminals in order to make output voltage equal to zerooutput voltage equal to zero

VVioio = | = |vv1 1 – v– v22| for | for vvoo = 0 = 0


Input offset current Input offset current IIioio

It is the difference between the currents in the It is the difference between the currents in the input terminals when both input voltages are input terminals when both input voltages are zerozero

IIioio = | I = | I11 – I – I22 | when | when vv11 = v = v2 2 = 0= 0

Input bias current Input bias current IIibib

It is the average of the currents in the input It is the average of the currents in the input terminals when both input voltages are zeroterminals when both input voltages are zero

IIibib = (I = (I11 + I + I22) / 2) / 2 when when vv11 = v = v22 = 0 = 0


Slew rate Slew rate SRSR It is the maximum rate of change of output It is the maximum rate of change of output

voltage with respect to timevoltage with respect to time Slew rate has to be very high if OPAMP has to Slew rate has to be very high if OPAMP has to

operate efficiently at high frequenciesoperate efficiently at high frequencies

Supply voltage rejection ratio Supply voltage rejection ratio SVRRSVRR It is the maximum rate at which input offset It is the maximum rate at which input offset

voltage of OPAMP changes with change in voltage of OPAMP changes with change in supply voltagesupply voltage


Practical characteristics of 741C OPAMPPractical characteristics of 741C OPAMP Differential mode gain is 200,000Differential mode gain is 200,000 CMRR is 90 dBCMRR is 90 dB Input resistance is 2 MInput resistance is 2 MΩΩ Output resistance is 75 Output resistance is 75 ΩΩ Unity-gain Bandwidth is 1 MHzUnity-gain Bandwidth is 1 MHz Slew rate is 0.5 V / Slew rate is 0.5 V / μμss Output offset voltage is 1 mVOutput offset voltage is 1 mV Input offset current is 20 nAInput offset current is 20 nA Input bias current is 80 nAInput bias current is 80 nA

Closed-loop configurationsClosed-loop configurations

Open-loop voltage gain of OPAMP is very Open-loop voltage gain of OPAMP is very high; such high gain is not required in high; such high gain is not required in most applicationsmost applications

In order to reduce gain, a part of output In order to reduce gain, a part of output signal is fed back to the inverting input signal is fed back to the inverting input terminal (called negative feedback)terminal (called negative feedback)

Many other OPAMP characteristics are Many other OPAMP characteristics are improvised with thisimprovised with this

Inverting AmplifierInverting Amplifier

RF


Input is applied to inverting terminalInput is applied to inverting terminal Non inverting is groundedNon inverting is grounded Feedback is given to inverting terminal Feedback is given to inverting terminal

through resistor through resistor RRFF

Assuming Assuming vvoo is less than is less than VVCCCC

since since AAdd is very high, is very high, vvidid should be very should be very small; small; vvidid taken as almost zero taken as almost zero

Current entering OPAMP input terminal is Current entering OPAMP input terminal is almost zeroalmost zero


RF

Virtual groun

d

+

0 V

–

0i1

i2


111

0

R

v

R

vi inin

F

o

F

o

R

v

R

vi

02

21 ii

F

oin

R

v

R

v

1

1R

Rvv Fino

1R

R

v

vA F

in

oV

Non Inverting AmplifierNon Inverting Amplifier

RF


Input is applied to non inverting terminalInput is applied to non inverting terminal Feedback is given to inverting terminalFeedback is given to inverting terminal Output voltage will be in-phase with input Output voltage will be in-phase with input

voltagevoltage Here again, the following assumptions are Here again, the following assumptions are

mademade Since Since AAdd is very high, is very high, vvidid should be very small; should be very small;

vvidid taken as almost zero taken as almost zero Current entering OPAMP input terminal is Current entering OPAMP input terminal is

almost zeroalmost zero


RF

v1

v2

i2i1


0idv invvv 21

11

21 R

v

R

vi in

F

ino

F

o

R

vv

R

vvi

2

2

21 ii

F

inoin

R

vv

R

v

1

1

1R

Rvv Fino

ProblemsProblems

For an inverting amplifier using OPAMP, RFor an inverting amplifier using OPAMP, R11=1K, =1K, RRFF=100K, v=100K, vinin=0.1sin(=0.1sin(ωωt). Find vt). Find voo..

Ans: –10sin(Ans: –10sin(ωωt)t)

For a non inverting amplifier, RFor a non inverting amplifier, R11=10K, R=10K, RFF=100K. =100K. Calculate vCalculate voo if v if vii = 25 mV dc. = 25 mV dc.

Ans: 275 mV dcAns: 275 mV dc

An ac signal of rms value 2 mV needs to be An ac signal of rms value 2 mV needs to be amplified to 1.024 V rms, 180 degree phase amplified to 1.024 V rms, 180 degree phase shifted. Design a suitable amplifier choosing shifted. Design a suitable amplifier choosing RR11=1.2K=1.2K

Ans: Inv amplifier with RAns: Inv amplifier with RFF=614.4K=614.4K

Voltage FollowerVoltage Follower

Special case of non inverting amplifier where RSpecial case of non inverting amplifier where RFF=0=0 Voltage gain is unity.Voltage gain is unity. vvoo = v = vinin

Has very high input resistance and very low output Has very high input resistance and very low output resistance; Used as buffer for impedance matchingresistance; Used as buffer for impedance matching

Summing Amplifier (Adder)Summing Amplifier (Adder)

iA

iB

iF

Summing Amplifier (Adder)Summing Amplifier (Adder)

A

AA R

vi

B

BB R

vi

FBA iii

F

oF R

vi

F

o

B

B

A

A

R

v

R

v

R

v

B

FB

A

FAo R

Rv

R

Rvv

Difference Amplifier Difference Amplifier (Subtractor)(Subtractor)


The circuit is analyzed using superposition The circuit is analyzed using superposition theoremtheorem

Consider only vConsider only v11 to be present; v to be present; v22=0=0

Now derive expression for output voltage Now derive expression for output voltage vvo1o1

Next consider only vNext consider only v22 to be present; v to be present; v11=0=0

Derive expression for output voltage vDerive expression for output voltage v0202

Actual output voltage vActual output voltage voo = v = vo1o1+v+vo2o2


vx

21 1

R

Rvv Fxo

231

311 1

R

R

RR

Rvv Fo


222 R

Rvv F

o

22

231

3121 1

R

Rv

R

R

RR

Rvvvv FFooo

FR3R2R1R if21 vv

ProblemsProblems

Design an OPAMP circuit such that output Design an OPAMP circuit such that output is given by is given by vvoo=–(0.5=–(0.5vv11+0.75+0.75vv22) where ) where vv11 and and vv22 are input voltages. Choose are input voltages. Choose RRFF=10K=10K

Design an OPAMP subtractor to have Design an OPAMP subtractor to have output given by output given by Choose RChoose RFF=R=R22=1K=1K

Design an OPAMP adder/subtractor to get Design an OPAMP adder/subtractor to get output voltage output voltage

2132

o vvv

3232

121

o vvvv

IntegratorIntegrator


Integrator is a circuit whose output is Integrator is a circuit whose output is proportional to (negative) integral of the proportional to (negative) integral of the input signal with respect to timeinput signal with respect to time

Feedback is given through capacitor to Feedback is given through capacitor to inverting terminalinverting terminal

Since same current flows through R and C,Since same current flows through R and C,

dtdv

CRv oin

t

0ino dtvv

RC

1


DifferentiatorDifferentiator

DifferentiatorDifferentiator

Differentiator is circuit whose output is Differentiator is circuit whose output is proportional to (negative) differential of proportional to (negative) differential of input voltage with respect to timeinput voltage with respect to time

Input is given through capacitor, feedback Input is given through capacitor, feedback given through resistor to inv terminalgiven through resistor to inv terminal

Since current through R and C are same,Since current through R and C are same,

Rv

dtdv

C oin dt

dvv in

o RC

Documents

Module 4 Operational Amplifier. Contents Introduction Introduction OPAMP Symbol OPAMP Symbol Internal block diagram Internal block diagram Open-loop configuration