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Chapter 20Special-Purpose

Op-Amp Circuits

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Objectives

• Analyze and explain the operation o an

instrumentation ampliier

• Analyze and explain the operation o an isolationampliier

• Analyze and explain the operation o an O!A

• Analyze and explain the operation o active diode

circuits

• Analyze and explain several special types o op-amp

circuits

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"nstrumentation Ampliiers

• A basic instrumentation

ampliier is sho#n

$ Op-amps A% and A2 arenoninvertin&' providin&

hi&h input impedance

and volta&e &ain

$ Op-amp A( is a unity&ain dierential

ampliier

$ )esistor ) * sets the &ain

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"nstrumentation Ampliiers

• "nstrumentation ampliiers are normally used to

measure small dierential si&nal volta&es that are

superimposed on a common-mode volta&e otenlar&er than the si&nal volta&e

$ +evices such as remote pressure or temperature

transducers may have lon& lead len&ths that #ill pic, up

common-mode electrical noise $ !he instrumentation ampliier at the end o the line must

ampliy the small si&nal rom the remote sensor and

reject the lar&e common-mode volta&e

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"solation Ampliiers

• !he dierence bet#een an isolation amplifier and

an instrumentation ampliier is that the isolation

ampliier has an input sta&e' an output sta&e andand po#er supply section that are all electrically

isolated rom each other

$ !ransormer coupled isolation is commonly used

$ An isolation ampliier is capable o operatin& #ith threeindependent &rounds' or the input si&nal' the output

si&nal' and the po#er supply

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"solation Ampliiers

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"solation Ampliiers

• !he isolation ampliier is used in applications that

reuire no common &round bet#een a transducer

and the process circuits #here interacin& tosensitive euipment is reuired

$ "n chemical' nuclear and metal-processin& industries'

millivolt si&nals many exist in the presence o lar&e

common-mode volta&es in the ,. ran&e $ !he isolation ampliier can ampliy small si&nals rom

very noisy euipment and provide a sae output to

sensitive euipment such as computers

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Operational !ransconductance

Ampliiers /O!AS• !he O!A is primarily a volta&e-to-current

ampliier in #hich the output current euals the

&ain times the input volta&e• 1i,e the conventional op-amp' the O!A has

$ !#o dierential input terminals

$ 3i&h input impedance

$ 3i&h C4))

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Ampliiers /O!AS• !he double circle

symbol at the output

represents an outputcurrent source that is

dependent on a bias

current

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Ampliiers /O!AS• 5nli,e the conventional op-amp' the O!A has

$ A bias-current input terminal

$ A hi&h output impedance $ 6o ixed open-loop volta&e &ain

• 7y deinition' the transconductance o anelectronic device is the ratio o the output current to

the input volta&e $ .olta&e is the input variable $ Current is the output variable

$ .olta&e-to-current &ain is the transconductance' gm

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Ampliiers /O!AS• !he transconductance

o the ampliier is

determined by theamount o bias

current' #hich is set

by the dc supply

volta&es and the biasresistor ) 7"AS

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Ampliiers /O!AS• One eature o an O!A is that the volta&e &ain can

be controlled by the amount o bias current

$ 7y chan&in& the resistance' you can produce a chan&ein "7"AS' #hich chan&es the transconductance

$ A chan&e in the transconductance chan&es the volta&e

&ain

$ .ariations in the bias volta&e' applied to ) 7"AS' #illcause a chan&e in the bias current

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Ampliiers /O!AS• Amplitude 4odulator

$ !he volta&e &ain is varied by applyin& a modulation

volta&e to the bias input

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Active +iode Circuits

• A clampin& circuit or clamper is used to add a dc level to a

si&nal volta&e

$ Clampers are reerred to as dc restorers because they are used to

restore a dc level to a si&nal that has been processed throu&hcapacitively coupled ampliiers

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• +iode limiters cut o or limit volta&e above or belo#

speciied volta&e levels

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• 8hen the input volta&e is less than the reerencevolta&e' the op-amp dierential input volta&e is positive

• !he positive output o the op-amp or#ard biases thediode

• !he op-amp operates as a volta&e-ollo#er

•8hen the input is &reater than the reerence volta&e'the op-amp output is ne&ative

• !he ne&ative output o the op-amp reverse biases thediode' so the diode is eectively open

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• Pea, +etector

$ 5sed to detect the

pea, input volta&e andstore that pea, volta&e

on a capacitor

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• !he basic operation o the pea, detector is $ "nput volta&e is applied to the noninvertin& input

throu&h ) i

$ !he hi&h-level output volta&e o the op-amp or#ard- biases the diode and char&es the capacitor

$ !he capacitor char&es until its volta&e reaches a valueeual to the input volta&e and thus both inputs are at the

same volta&e $ 8hen the capacitor volta&e is &reater than or eual to

the input volta&e' the diode is reverse biased' and thecapacitor holds the pea, volta&e

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Other Op-Amp Circuits

• Constant-Current

Source

$ +elivers a load currentthat remains constant

#hen the load resistance

chan&es

$ As lon& as ."6 and ) i

remain constant' "1 #ill

be constant

"1 9 ."6:) i

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• Current-to-.olta&e

Converter

$ Converts a variableinput current to a

proportional output

volta&e

.out 9 "i) f

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• .olta&e-to-Current

Converter

$ "nput volta&e controlsoutput current /load

"1 9 .in:) %

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Summary

• A basic instrumentation ampliier is ormed by

three op-amps and seven resistors' includin& the

&ain-settin& resistor' ) *• An instrumentation ampliier has hi&h input

impedance' hi&h C4))' lo# output oset' and

lo# output impedance

• !he volta&e &ain o a basic instrumentation

ampliier is set by a sin&le external resistor

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Summary

• An instrumentation ampliier is useul in

applications #here small si&nals are embedded in

lar&e common-mode noise• A basic isolation ampliier has three electrically

isolated parts input' output' and po#er

• 4ost isolation ampliiers use transormer couplin&

or isolation

• "solation ampliiers are used to interace sensitive

euipment #ith hi&h-volta&e environments

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Summary

• !he operational transconductance ampliier /O!A isa volta&e-to-current ampliier

• !he output current o an O!A is the input volta&etimes the transconductance

• "n an O!A' transconductance varies #ith the biascurrent; thereore' the &ain o an O!A can be varied#ith a bias volta&e or a variable resistor

• +iode clampers add a dc level to an ac si&nal

• +iode limiters cut o volta&e above and belo#speciied levels< 1imiters are also called clippers

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