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Chapter 11Op-Amp Applications

OpOp--Amp ApplicationsAmp Applications

ConstantConstant--gain multipliergain multiplierVoltage summingVoltage summing

Voltage bufferVoltage bufferControlled sourcesControlled sources

Instrumentation circuitsInstrumentation circuitsActive filtersActive filters

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

Active filtersActive filters

22

ConstantConstant--Gain AmplifierGain Amplifier

Inverting VersionInverting Version



more…more…

33

ConstantConstant--Gain AmplifierGain Amplifier

Noninverting VersionNoninverting Version



44

MultipleMultiple--Stage GainsStage Gains

= AAAA 321

The total gain (3-stages) is given by:

or



55

−

−

+=

R3R

R2R

RR

1A ff

1

f

or

Voltage SummingVoltage Summing

++++++++−−−−==== fff VR

VR

VR

V

The output is the sum of individual signals times the gain:



[Formula 14.3]

++++++++−−−−==== 3

3

f2

2

f1

1

fo V

RR

VRR

VRR

V

66

Voltage BufferVoltage Buffer

Any amplifier with no gain or loss is called a unity gain unity gain amplifieramplifier. The advantages of using a unity gain amplifier:

• Very high input impedance • Very low output impedance



Realistically these circuits are designed using equal resistors (R1 = Rf) to avoid problems with offset voltages.

77

Controlled SourcesControlled Sources

VoltageVoltage--controlled voltage sourcecontrolled voltage sourceVoltageVoltage--controlled current sourcecontrolled current sourceCurrentCurrent--controlled voltage sourcecontrolled voltage sourceCurrentCurrent--controlled current sourcecontrolled current source



88

VoltageVoltage--Controlled Voltage SourceControlled Voltage Source

The output voltage is the gain times the input voltage. What makes an op-amp different from other amplifiers is its impedance

Noninverting Amplifier VersionNoninverting Amplifier Version



impedance characteristics and gain calculations that depend solely on external resistors.

more…more…

99

VoltageVoltage--Controlled Voltage SourceControlled Voltage Source

The output voltage is the gain times the input voltage. What makes an op-amp different from other amplifiers is its impedance

Inverting Amplifier VersionInverting Amplifier Version



impedance characteristics and gain calculations that depend solely on external resistors.

1010

VoltageVoltage--Controlled Current SourceControlled Current Source

The output current is:

11

o kVRV

I ========



11

o kVR

I ========

1111

CurrentCurrent--Controlled Voltage SourceControlled Voltage Source

This is simply another way of applying the op-amp operation. Whether the input is a current determined by Vin/R1 or as I 1:

fR−−−−==



or

in1

fout V

RR

V−−−−

====

1212

L1out RIV −=

CurrentCurrent--Controlled Current SourceControlled Current Source

This circuit may appear more complicated than the others but it is really the same thing.

R V



in

in

f

out

21

in

f

out

inin

fout

R

V

R

V

R||RV

R

V

VR

RV

−−−−====

−−−−====

−−−−====

kIRR

1II

RRR

RV

I

RRRR

VI

R||RV

I

2

1o

2

21

1

ino

21

21ino

21

ino

====

++++−−−−====

++++−−−−====

××××

++++−−−−====

−−−−====

1313

Instrumentation CircuitsInstrumentation Circuits

Some examples of instrumentation circuits using op-amps:

• Display driver• Instrumentation amplifier



• Instrumentation amplifier

1414

Display DriverDisplay Driver



1515

Instrumentation AmplifierInstrumentation Amplifier



For all Rs at the same value (except Rp):

(((( )))) (((( ))))2121P

o VVkVVR2R

1V −−−−====−−−−

++++====

1616

Active FiltersActive Filters

Adding capacitors to op-amp circuits provides external control of the cutoff frequencies. The op-amp active filter provides controllable cutoff frequencies and controllable gain.

• Low-pass filter• High-pass filter• Bandpass filter



1717

LowLow--Pass FilterPass Filter——FirstFirst--OrderOrder



11OH CRπ2

1f ====

1

fv R

R1A ++++====

The upper cutoff frequency and voltage gain are given by:

1818

LowLow--Pass FilterPass Filter——SecondSecond--OrderOrder



The roll-off can be made steeper by adding more RC networks.

1919

HighHigh--Pass FilterPass Filter



11OL CRπ2

1f ====

The cutoff frequency is determined by:

2020

Bandpass Bandpass FilterFilter

There are two cutoff frequencies: upper and lower. They can be calculated using the same low-pass cutoff and high-pass cutoff frequency formulas in the



formulas in the appropriate sections.

2121

chapter 11 op-amp applications - web page for staffwebstaff.kmutt.ac.th/~suwat.pat/material/ene212...

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