Op Amp-technical report

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2010

SANJIB KUMAR SAHA

DEPT:-ECE, 2nd year

ROLL NO-08102003055

TECHNICAL REPORT

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INTRODUCTION

An

An operational amplifieroperational amplifier, which is often called an, which is often called an op-ampop-amp, is a DC-coupled high-gain, is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output. An electronic voltage amplifier with a differential input and, usually, a single-ended output. An op-amp produces an output voltage that is typically millions of times larger than the voltage op-amp produces an output voltage that is typically millions of times larger than the voltage difference

difference between its input terminals.between its input terminals.

Typically the op-amp's very large gain is controlled by negative feedback, which largely Typically the op-amp's very large gain is controlled by negative feedback, which largely determi

determines the nes the magnitmagnitude ude of its of its output ("closed-loooutput ("closed-loop") voltage p") voltage gain in gain in amplifiamplifier applications,er applications, or the transfer function required (in analog computers). Without negative feedback, and or the transfer function required (in analog computers). Without negative feedback, and perhaps with positive feedback for regeneration, an op-amp essentially acts as

perhaps with positive feedback for regeneration, an op-amp essentially acts as a comparator.a comparator. High input impedance at the input terminals (ideally infinite) and low output impedance at High input impedance at the input terminals (ideally infinite) and low output impedance at the output terminal(s) (ideally zero) are important typical characteristics.

the output terminal(s) (ideally zero) are important typical characteristics.

Op-amps are among the most widely used electronic devices today, being used in a vast array Op-amps are among the most widely used electronic devices today, being used in a vast array of consumer, industrial, and scientific devices. Many standard IC op-amps cost only a few of consumer, industrial, and scientific devices. Many standard IC op-amps cost only a few ce

cents nts in in momoderderatate e proproducductition on volvolumume; e; howhoweveever r somsome e intintegregratated ed or or hyhybrbrid id opeoperaratitionaonall am

amplplififieriers s wiwith th spespeciacial l peperforformrmancance e spspeciecifificatcationions s mamay y coscost t oveover r $10$100 0 US US in in smsmallall quantiti

quantities. Op-amps sometimes come in es. Op-amps sometimes come in the form of the form of macroscomacroscopic components, (see photo) or pic components, (see photo) or as integrated circui

as integrated circuit cells; patterns that can be reprinted several times on one chip as part of t cells; patterns that can be reprinted several times on one chip as part of aa more complex device.

more complex device. The op-amp is

The op-amp is one type of one type of differentdifferential amplifier. Other types of ial amplifier. Other types of differendifferential amplifier includetial amplifier include th

the e fufulllly y didiffffererenentitial al amamplplififieier r (s(simimililar ar to to ththe e opop-a-ampmp, , bubut t wiwith th twtwo o ououtptpututs)s), , ththee instrumentation amplifier (usually built from three op-amps), the

instrumentation amplifier (usually built from three op-amps), the isolation amplifier (similar to the instrumentation amplifier, but isolation amplifier (similar to the instrumentation amplifier, but which works fine

which works fine with common-mode voltages that would destroywith common-mode voltages that would destroy an

an ordiordinary nary op-aop-amp), mp), and and neganegativtive e feedfeedback back ampamplifilifier er (usu(usuallallyy built from one or more op-amps and a resistive feedback network). built from one or more op-amps and a resistive feedback network).

Circuit notation

The circuit symbol for an op-amp is shown to the right, where: The circuit symbol for an op-amp is shown to the right, where:

•

• : non-inverting input: non-inverting input •

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•

• : positive power supply: positive power supply

•

• : negative power supply: negative power supply Th

The e popowewer r susupppply ly pipins ns ( ( anand d ) ) cacan n be be lalabebeleled d in in didiffffererenentt ways. Despite different labeling, the function remains the same ways. Despite different labeling, the function remains the same — to provide additional power for amplification of signal. Often — to provide additional power for amplification of signal. Often these pins are left out of the diagram for clarity, and the power these pins are left out of the diagram for clarity, and the power configuration is described or assumed from the circuit.

configuration is described or assumed from the circuit.

HISTORY

1941: First (vacuum tube)

1941: First (vacuum tube) op-ampop-amp An

An op-op-amamp, p, defdefineined d as as a a gengeneraeral-pl-pururpospose, e, DCDC-co-couplupled, ed, highigh h gaigain, n, invinvertertining g feefeedbadback ck amplifier, is first found in US Patent 2,401,779 "Summing Amplifier" filed by Karl D. amplifier, is first found in US Patent 2,401,779 "Summing Amplifier" filed by Karl D. Swartzel Jr. of Bell labs in 1941. This design used three vacuum tubes to achieve a gain of Swartzel Jr. of Bell labs in 1941. This design used three vacuum tubes to achieve a gain of 90dB and operated on voltage rails of ±350V. It had a single inverting input rather than 90dB and operated on voltage rails of ±350V. It had a single inverting input rather than differential inverting and non-inverting inputs, as are common in today's

differential inverting and non-inverting inputs, as are common in today's op-amps. Throughout World War II, swartzel’s design proved its value by being amps. Throughout World War II, swartzel’s design proved its value by being liberally used in the M9 artillery director designed at Bell Labs. This artillery liberally used in the M9 artillery director designed at Bell Labs. This artillery director worked with the SCR584 radar system to achieve extraordinary hit rates director worked with the SCR584 radar system to achieve extraordinary hit rates (near 90%) that would not have been possible otherwise.

(near 90%) that would not have been possible otherwise.

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➢ 1947: First op-amp with an explicit non-inverting input1947: First op-amp with an explicit non-inverting input In 1947, the operational amplifier was first formally defined and In 1947, the operational amplifier was first formally defined and named in a paper by Professor John R. Ragazzini of Columbia named in a paper by Professor John R. Ragazzini of Columbia University. In this same paper a footnote mentioned an op-amp University. In this same paper a footnote mentioned an op-amp design by a student that would turn out to be quite significant. design by a student that would turn out to be quite significant.

This op-amp, designed by Loebe Julie, was superior in a variety of ways. It had two major This op-amp, designed by Loebe Julie, was superior in a variety of ways. It had two major innovations. Its input stage used a long-tailed triode pair with loads matched

innovations. Its input stage used a long-tailed triode pair with loads matched to reduce drift in

to reduce drift in the output and, far the output and, far more importantmore importantly, it was ly, it was the first op-ampthe first op-amp des

design ign to to hahave ve twtwo o ininputputs s (on(one e invinvertertining, g, ththe e othother er nonnon-in-inververtiting)ng). . ThThee differen

differential input made a tial input made a whole range of new whole range of new functionfunctionality possible, but it would not ality possible, but it would not be usedbe used for a long time due to the rise of the chopper-stabilized amplifier.

for a long time due to the rise of the chopper-stabilized amplifier.

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➢ 1949: First chopper-stabilized op-amp1949: First chopper-stabilized op-amp

In 1949, Edwin A. Goldberg designed a chopper-stabilized op-amp. This In 1949, Edwin A. Goldberg designed a chopper-stabilized op-amp. This set-up uses a normal op-amp with an additional AC amplifier that goes set-up uses a normal op-amp with an additional AC amplifier that goes al

alonongsgsidide e ththe e opop-a-ampmp. . ThThe e chchopoppeper r gegets ts an an AC AC sisigngnal al frfrom om DC DC byby switchi

switching between the DC ng between the DC voltage and ground at a voltage and ground at a fast rate (60 Hz or fast rate (60 Hz or 400 Hz). This signal is400 Hz). This signal is then amplified, rectified, filtered and fed into the op-amp's non-inverting input. This vastly then amplified, rectified, filtered and fed into the op-amp's non-inverting input. This vastly improve

improved the d the gain of the gain of the op-amp while significop-amp while significantly reducing the output drift and DC antly reducing the output drift and DC offset.offset. Un

Unforfortuntunateatelyly, , any any dedesigsign n thathat t useused d a a chchoppopper er coucouldnldn't 't use use thetheir ir non non--inverting input for any other purpose. Nevertheless, the much improved inverting input for any other purpose. Nevertheless, the much improved characteristics of the chopper-stabilized op-amp made it the dominant way characteristics of the chopper-stabilized op-amp made it the dominant way

to use op-amps. Techniques that used the non-inverting input regularly would not be very to use op-amps. Techniques that used the non-inverting input regularly would not be very popular until the 1960s when op-amp ICs started to show up in the field.

popular until the 1960s when op-amp ICs started to show up in the field. In 1953, vacuum

In 1953, vacuum tube op-amps became commercialtube op-amps became commercially available with the ly available with the release of the release of the modelmodel K2-W from George A. Philbrick Researches, Incorporated. The designation on the devices K2-W from George A. Philbrick Researches, Incorporated. The designation on the devices

GAP/R's K2-W: a GAP/R's K2-W: a vacuum-tube op-amp vacuum-tube op-amp (1953) (1953) An op-amp in An op-amp in a modern DIP a modern DIP a solid-state, a solid-state, discrete op-amp discrete op-amp (1961) (1961)

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shown, GAP/R, is a contraction for the complete company name. Two nine-pin 12AX7 shown, GAP/R, is a contraction for the complete company name. Two nine-pin 12AX7 vacuum tubes were mounted in an octal package and had a model K2-P chopper add-on vacuum tubes were mounted in an octal package and had a model K2-P chopper add-on available that would effectively "use up" the non-inverting input. This op-amp was based on a available that would effectively "use up" the non-inverting input. This op-amp was based on a descendant of Loebe Julie's 1947 design and, along with its successors, would start the descendant of Loebe Julie's 1947 design and, along with its successors, would start the widespread use of op-amps in industry.

widespread use of op-amps in industry.

1961: First discrete IC op-amps 1961: First discrete IC op-amps

with the birth of the transistor in 1947, and the silicon transistor in 1954, with the birth of the transistor in 1947, and the silicon transistor in 1954, the concept of ICs became a reality. The introduction of the planar the concept of ICs became a reality. The introduction of the planar p

prorocecess ss in in 191959 59 mmadade e trtranansisiststorors s anand d ICICs s ststabable le enenouough gh to to bebe commer

commercially useful. By cially useful. By 1961, solid-state, discrete op-amps were 1961, solid-state, discrete op-amps were beingbeing pro

produceduced. d. TheThese se op-aop-amps mps werwere e effeeffectictively vely smalsmall l circircuit cuit boarboards ds witwithh pac

packagekages s such as such as edgeedge-con-connectnectors. They ors. They usuausually lly had had handhand-sel-selectected ed resiresistorstors s in in ordeorder r toto improve things such as voltage offset and drift. The P45 (1961) had a

improve things such as voltage offset and drift. The P45 (1961) had a gain of 94 dB and ran on ±15 V rails. It was intended to deal with gain of 94 dB and ran on ±15 V rails. It was intended to deal with signals in the range of ±10 V.

signals in the range of ±10 V.

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➢ 1962: First op-amps in potted module1962: First op-amps in potted module

By 1962, several companies were producing modular potted packages that could be plugged By 1962, several companies were producing modular potted packages that could be plugged in

into to priprintnted ed cicircrcuit uit boaboardsrds.T.Theshese e papackackages ges wewere re crucruciciallally y imimporportatant nt as as ththey ey mamade de thethe operatio

operational amplifier into a single black box nal amplifier into a single black box which could be easily treated as a component inwhich could be easily treated as a component in a larger circuit.

a larger circuit.

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➢ 1963: First monolithic IC op-amp1963: First monolithic IC op-amp

In 1963, the first monolithic IC op-amp, the μA702 designed by Bob Widlar at Fairchild In 1963, the first monolithic IC op-amp, the μA702 designed by Bob Widlar at Fairchild Semiconductor, was released. Monolithic ICs consist of a single chip as

Semiconductor, was released. Monolithic ICs consist of a single chip as opposed to a chip and discrete parts (a discrete IC) or multiple chips opposed to a chip and discrete parts (a discrete IC) or multiple chips b

bondonded ed and and conconnenectcted ed on on a a cicircurcuit it boaboard rd (a (a hyhybribrid d ICIC). ). AlAlmomost st alalll modern op-amps are monolithic ICs; however, this first IC did not meet modern op-amps are monolithic ICs; however, this first IC did not meet with much success. Issues such as an uneven supply voltage, low gain with much success. Issues such as an uneven supply voltage, low gain and a small dynamic range held off the dominance of monolithic and a small dynamic range held off the dominance of monolithic op-amps until 1965 when the μA709(also designed by Bob Widlar) was amps until 1965 when the μA709(also designed by Bob Widlar) was released.

released. 19

196868: : RReleleaeasse e oof f tthhe e μAμA74741 1 – – wowouulld d be be seseen en aas s a a nneaearlrlyy ubiquitous chip

ubiquitous chip

The popularity of monolithic op-amps was further improved upon the release of the LM101 The popularity of monolithic op-amps was further improved upon the release of the LM101 in 1967, which solved a variety of issues, and the subsequent release of the μA741 in 1968. in 1967, which solved a variety of issues, and the subsequent release of the μA741 in 1968. The μA741 was extremely similar to the LM101 except that Fairchild's facilities allowed The μA741 was extremely similar to the LM101 except that Fairchild's facilities allowed them to include a 30 pF compensation capacitor inside the chip instead of requiring external them to include a 30 pF compensation capacitor inside the chip instead of requiring external compensation. This simple difference has made the 741

compensation. This simple difference has made the 741 thethe canonical op-amp and manycanonical op-amp and many modern amps base their pinout on the 741s.The μA741 is still in

modern amps base their pinout on the 741s.The μA741 is still in productiproduction, and has on, and has becomebecome ubiq

ubiquitouitous us in in elecelectrontronics—ics—manmany y manmanufacufacturturers ers prodproduce uce a a versversion ion of of thithis s clasclassic sic chipchip,, recognizable by part numbers containing

recognizable by part numbers containing741741.. 1966: First varactor bridge

1966: First varactor bridge op-ampsop-amps

Since the 741, there have been many different directions taken in op-amp design. Varactor Since the 741, there have been many different directions taken in op-amp design. Varactor bridge op-amps started to be produced in the late 1960s; they were designed to have bridge op-amps started to be produced in the late 1960s; they were designed to have extremely small input current and are still amongst the best op-amps available in terms of extremely small input current and are still amongst the best op-amps available in terms of

A solid-state A solid-state op-amp amp a solid-state, a solid-state, discrete op-amp discrete op-amp (1961) (1961)

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common-mode rejection with the ability to correctly deal with hundreds common-mode rejection with the ability to correctly deal with hundreds of volts at their inputs

of volts at their inputs

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➢ 1970: First high-speed, low-input current FET design1970: First high-speed, low-input current FET design

In the 1970s high speed, low-input current designs started to In the 1970s high speed, low-input current designs started to be made

be made

by using FETs. These would be largely replaced by op-amps made with by using FETs. These would be largely replaced by op-amps made with

MOSFE

MOSFETs in Ts in the 1980s. During the the 1980s. During the 1970s single sided supply op-amps1970s single sided supply op-amps also became available.

also became available.

1972: Single sided supply op-amps being produced 1972: Single sided supply op-amps being produced

A single sided supply op-amp is one where the input and output voltages can be as low as the A single sided supply op-amp is one where the input and output voltages can be as low as the negative power supply volt

negative power supply voltage instead of needing to be age instead of needing to be at least two volts above it. The resultat least two volts above it. The result is that it can operate in many applications with the negative supply pin on the op-amp being is that it can operate in many applications with the negative supply pin on the op-amp being connected to the signal ground, thus eliminating the need for a separate negative power connected to the signal ground, thus eliminating the need for a separate negative power supply.

supply.

The LM324 (released in 1972) was one such op-amp that came in a quad package (four The LM324 (released in 1972) was one such op-amp that came in a quad package (four separate op-amp

separate op-amps in s in one package) and became an industry standard. In addition to packagingone package) and became an industry standard. In addition to packaging multiple op-amps in a single package, the 1970s also saw the birth of op-amps in hybrid multiple op-amps in a single package, the 1970s also saw the birth of op-amps in hybrid package

packages. These s. These op-amps were generally improved versions of op-amps were generally improved versions of existing monolitexisting monolithic op-amps.hic op-amps. As the properties of monolithic op-amps improved, the more complex hybrid ICs were As the properties of monolithic op-amps improved, the more complex hybrid ICs were quickly relegated to systems that are required to have extremely long service lives or other quickly relegated to systems that are required to have extremely long service lives or other specialty systems.

specialty systems.

Recent trends Recent trends Recentl

Recently supply voltages in analog circuits have decreased (as they have y supply voltages in analog circuits have decreased (as they have in digital logic) andin digital logic) and low-voltage opamps have been introduced reflecting this. Supplies of ±5V and increasingly low-voltage opamps have been introduced reflecting this. Supplies of ±5V and increasingly 5V are common. To maximize the signal range modern op-amps commonly have rail-to-rail 5V are common. To maximize the signal range modern op-amps commonly have rail-to-rail inputs (the input signals can range from the lowest supply voltage to the highest) and inputs (the input signals can range from the lowest supply voltage to the highest) and sometimes rail-to-rail outputs

sometimes rail-to-rail outputs

Basic Operation

The

The amplifamplifier's ier's differendifferential tial inputs inputs consist consist of of a a input input and and a a input, input, and and ideally ideally the the op-ampop-amp amplifies only the difference in voltage between the two, which is called the

amplifies only the difference in voltage between the two, which is called thedifferential input differential input voltage

voltage. The output voltage of the op-amp is given by the equation,. The output voltage of the op-amp is given by the equation,

wh

where ere is the is the volvoltatage ge at at ththe ne non-on-ininververtiting ng tertermiminalnal, , is is ththe ve vololtatage age at tt the he invinvertertiningg termin

terminal al andand

G

open-loopopen-loop is the open-loop gain of the amplifier. (The term open-loop refers tois the open-loop gain of the amplifier. (The term open-loop refers to

the absence of a feedback loop from the output to the input.) the absence of a feedback loop from the output to the input.) Th

The e mamagnignitutude de of of

G

open-loopopen-loop is is tytypipicalcally ly ververy y lalargerge—se—seldldom om lesless s thathan n a a mimillllionion—an—andd

ther

thereforefore e even even a a quitquite e smasmall ll difdiffereference nce betwbetween een and and (a (a few few micmicrovorovolts lts or or lessless) ) wilwilll result in amplifier saturation, where the output voltage goes to either the extreme maximum result in amplifier saturation, where the output voltage goes to either the extreme maximum

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or minimum end of its range, which is set approximately by the power supply voltages. or minimum end of its range, which is set approximately by the power supply voltages. Fi

Finlenley's y's lawlawstates that "When the inverting and non-inverting inputs of an op-amp are notstates that "When the inverting and non-inverting inputs of an op-amp are not equal, its output is in saturation." Additionally, the precise magnitude of

equal, its output is in saturation." Additionally, the precise magnitude of G

G

open-loopopen-loopis not wellis not well

cont

controllrolled ed by by the manufathe manufactucturing processring process, , and and so so it it is is impimpractractical to ical to use an use an operoperatiationalonal am

amplplififier ier as as a a ststandand-al-alone one defdefereerentintial al amamplplififierier.If .If lilinenear ar opeoperatratioion n is is desdesirired, ed, negnegatativeive feedback must be used, usually achieved by applying a portion of the output voltage to the feedback must be used, usually achieved by applying a portion of the output voltage to the inverti

inverting input. The ng input. The feedback enables the output of the feedback enables the output of the amplifiamplifier to er to keep the inputs at keep the inputs at or near or near the same voltage so that saturation does not occur. Another benefit is that if much negative the same voltage so that saturation does not occur. Another benefit is that if much negative feedback is used,

feedback is used, the circuit's overall gain and the circuit's overall gain and other parameterother parameters become s become determidetermined more ned more byby th

the e feefeedbadback ck netnetwowork rk ththan an by by the the op-op-amamp p ititseself. lf. If If the the fefeedbedback ack netnetwowork rk is is mamade de of of components with relatively constant, predictable, values such as resistors, capacitors and components with relatively constant, predictable, values such as resistors, capacitors and indu

inductorctors, s, the the unprunprediedictabctabilitility y and and incoinconstanstancy ncy of of the the op-aop-amp'mp's s paraparametmeters ers (ty(typicapical l of of semiconductor devices) do not seriously affect the circuit's performance.

semiconductor devices) do not seriously affect the circuit's performance.

If no negative feedback is used, the op-amp functions as a switch or comparator. If no negative feedback is used, the op-amp functions as a switch or comparator. Positive feedback may be used to introduce hysteresis or oscillation.

Positive feedback may be used to introduce hysteresis or oscillation. Returni

Returning to ng to a consideration of linear (negative feedback) operation, the high a consideration of linear (negative feedback) operation, the high open-loop gainopen-loop gain and low input leakage current of the op-amp imply two "golden rules" that are highly useful and low input leakage current of the op-amp imply two "golden rules" that are highly useful in analysing linear op-amp circuits.

in analysing linear op-amp circuits.

Classification of Operational

Amplifiers

Op-amps may be classified by their construction:

•

• discrete (built from individual transistors or tubes/valves)discrete (built from individual transistors or tubes/valves) •

• IC (fabricated in an Integrated circuit) - most commonIC (fabricated in an Integrated circuit) - most common

•

• hybridhybrid

IC op-amps may be classified in many ways, including:

•

• Military, Industrial, or Commercial grade (for example: the LM301 is the commercialMilitary, Industrial, or Commercial grade (for example: the LM301 is the commercial grade version of the LM101, the LM201 is the industrial version). This may define grade version of the LM101, the LM201 is the industrial version). This may define operating temperature ranges and other environmental or quality factors.

operating temperature ranges and other environmental or quality factors. •

• Classification by package type may also affect environmental hardiness, as well asClassification by package type may also affect environmental hardiness, as well as ma

manufnufacacturturining g optoptioions; ns; DIDIP, P, anand d otother her ththrourough-gh-holhole e papackackages ges arare e tetendinding ng to to bebe replaced by Surface-mount devices.

replaced by Surface-mount devices. •

• Classification by internal compensation: op-amps may suffer from high frequencyClassification by internal compensation: op-amps may suffer from high frequency instability in some negative feedback circuits unless a small compensation capacitor instability in some negative feedback circuits unless a small compensation capacitor modifies the phase- and frequency- responses; op-amps with capacitor built in are modifies the phase- and frequency- responses; op-amps with capacitor built in are termed

termed compensatedcompensated, or , or perhaps compensaperhaps compensated for closed-loop gains down ted for closed-loop gains down to (say) 5,to (say) 5, others: uncompensated.

others: uncompensated. •

• Single, dual and quad versions of many commercial op-amp IC are available, meaningSingle, dual and quad versions of many commercial op-amp IC are available, meaning 1, 2 or 4 operational amplifiers are included in the same package.

1, 2 or 4 operational amplifiers are included in the same package.

•

• Rail-toRail-to-rail input (and/or output) -rail input (and/or output) op-amps can work with op-amps can work with input (and/or output) signalsinput (and/or output) signals

very close to the power supply rails. very close to the power supply rails. •

• CMOS op-amps (such as the CA3140E) provide extremely high input resistances,CMOS op-amps (such as the CA3140E) provide extremely high input resistances, higher than JFET-input op-amps, which are normally higher than

higher than JFET-input op-amps, which are normally higher than bipolar-ibipolar-input op-nput op-amps.

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•

• otheother r varivarietieties es of of op-aop-amp mp inclinclude ude progprogramrammablmable e op-aop-amps mps (sim(simply ply meameaning ning thethe quiescent current, gain, bandwidth and so on can be adjusted slightly by an external quiescent current, gain, bandwidth and so on can be adjusted slightly by an external resistor).

resistor). •

• manufacturers often tabulate their op-amps according to purpose, such as low-noisemanufacturers often tabulate their op-amps according to purpose, such as low-noise pre-amplifiers, wide bandwidth amplifiers, and so on.

pre-amplifiers, wide bandwidth amplifiers, and so on.

Internal circuitry of 741 type op-amp

Though designs vary between products and manufacturers, all op-amps have basically the Though designs vary between products and manufacturers, all op-amps have basically the same internal structure, which consists of three stages:

same internal structure, which consists of three stages:

A component level diagram of the common 741 op-amp A component level diagram of the common 741 op-amp

1.

1. VoltaVoltage amplifge amplifier – providier – provides high voes high voltage gailtage gain, a singlen, a single-pole freq-pole frequency rouency roll- ll-off, usually single-ended output.

off, usually single-ended output. 2.

2. OuOutptput ut amamplplififier ier – – prprovovidides es hihigh gh cucurrrrenent t drdrivivining g cacapapabilbilitity, y, lolow w ououtptputut impedance, current limiting and short circuit protection circuitry.

impedance, current limiting and short circuit protection circuitry.

Input stage

Constant-c

Constant-current urrent stabilizatistabilization on systemsystem The input stage DC

The input stage DC conditiconditions are ons are stabilistabilized by zed by a high-gain negative feedback system whosea high-gain negative feedback system whose main parts are the two current mirrors on the left of the figure, outlined in red. The main main parts are the two current mirrors on the left of the figure, outlined in red. The main purpose of this negative feedback system—to supply the differential input stage with a stable purpose of this negative feedback system—to supply the differential input stage with a stable

constant current—is realized as follows. constant current—is realized as follows.

The current through the 39 kΩ resistor acts as a current reference for the other bias currents The current through the 39 kΩ resistor acts as a current reference for the other bias currents used in the chip. The

used in the chip. The voltage across the resistor is equal to the voltage across the supply railsvoltage across the resistor is equal to the voltage across the supply rails (( ) m) miinnuus ts twwo to trraannssiissttoor r ddiioodde de drroopps (s (ii..ee.., f, frroom m QQ111 a1 annd Qd Q1122)), , aannd sd so to thhe ce cuurrrreenntt

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has

has valvalue ue . . The The WidWidlar lar currcurrent ent mirmirror ror builbuilt t byby Q10, Q11, and the 5 kΩ resistor produces a very small fraction of

Q10, Q11, and the 5 kΩ resistor produces a very small fraction of

I

ref ref at the Q10 collector.at the Q10 collector.

This small constant current through Q10's collecto

This small constant current through Q10's collector supplies the r supplies the base currents for Q3 base currents for Q3 and Q4and Q4 as well as the Q9 collector current. The Q8/Q9 current mirror tries to make Q9's collector as well as the Q9 collector current. The Q8/Q9 current mirror tries to make Q9's collector current the same as the Q3 and Q4

current the same as the Q3 and Q4 collector collector currents. Thus Q3 and Q4's combined basecurrents. Thus Q3 and Q4's combined base currents (which are of the same order as the overall chip's input currents) will be a small currents (which are of the same order as the overall chip's input currents) will be a small fraction of the already small Q10 current.

fraction of the already small Q10 current.

So, if the input stage current increases for any reason, the Q8/Q9 current mirror will draw So, if the input stage current increases for any reason, the Q8/Q9 current mirror will draw current away from the bases of Q3 and Q4, which reduces the input stage current, and vice current away from the bases of Q3 and Q4, which reduces the input stage current, and vice versa. The feedback loop also isolates the rest of the circuit from common-mode signals by versa. The feedback loop also isolates the rest of the circuit from common-mode signals by making the base voltage of Q3/Q4 follow tightly

making the base voltage of Q3/Q4 follow tightly

2 2V

V

bebe below the higher of the two inputbelow the higher of the two input

voltages. voltages.

Differential amplifier Differential amplifier

The blue outlined section is a differential amplifier. Q1 and Q2 are input emitter followers The blue outlined section is a differential amplifier. Q1 and Q2 are input emitter followers and together with the common base pair Q3 and Q4 form the differential input stage. In and together with the common base pair Q3 and Q4 form the differential input stage. In addition, Q3 and Q4 also act as level shifters and provide voltage gain to drive the class A addition, Q3 and Q4 also act as level shifters and provide voltage gain to drive the class A amplifier. They also help to increase the reverse

amplifier. They also help to increase the reverse V V be be rating on the input transistors (therating on the input transistors (the emitter-base junctions of the NPN transistors Q1 and Q2 break down at around 7 V but the emitter-base junctions of the NPN transistors Q1 and Q2 break down at around 7 V but the PNP transistors Q3 and Q4 have breakdown voltages around 50 V)

PNP transistors Q3 and Q4 have breakdown voltages around 50 V)..

The differential amplifier formed by Q1–Q4 drives a current mirror active load formed by The differential amplifier formed by Q1–Q4 drives a current mirror active load formed by transistors Q5–Q7 (actually, Q6 is the very active load). Q7 increases the accuracy of the transistors Q5–Q7 (actually, Q6 is the very active load). Q7 increases the accuracy of the current mirror by decreasing the amount of signal current required from Q3 to drive the bases current mirror by decreasing the amount of signal current required from Q3 to drive the bases of Q5 and Q6. This configuration provides differential to single ended conversion as follows: of Q5 and Q6. This configuration provides differential to single ended conversion as follows: The signal current of Q3 is

The signal current of Q3 is the input to the the input to the current mirror while the output of the mirror (thecurrent mirror while the output of the mirror (the collector of Q6) is connected to the collector of Q4. Here, the signal currents of Q3 and Q4 collector of Q6) is connected to the collector of Q4. Here, the signal currents of Q3 and Q4 are summed. For differential input signals, the signal currents of Q3 and Q4 are equal and are summed. For differential input signals, the signal currents of Q3 and Q4 are equal and opposite. Thus, the sum is twice the individual signal currents. This completes the differential opposite. Thus, the sum is twice the individual signal currents. This completes the differential to single ended conversion.

to single ended conversion.

The open circuit signal voltage appearing at this point

The open circuit signal voltage appearing at this point is given by is given by the product of the the product of the summedsummed signal currents and the paralleled collector resistances of Q4 and Q6. Since the collectors of signal currents and the paralleled collector resistances of Q4 and Q6. Since the collectors of Q4 and Q6 appear as high resistances to the signal current, the open circuit voltage gain of Q4 and Q6 appear as high resistances to the signal current, the open circuit voltage gain of this stage is very high.

this stage is very high.

It should be noted that the base current at the inputs is not zero and the effective (differential) It should be noted that the base current at the inputs is not zero and the effective (differential) input impedance of a 741 is about 2 MΩ. The "offset null" pins may be used to place external input impedance of a 741 is about 2 MΩ. The "offset null" pins may be used to place external resi

resistorstors s in parallin parallel with the el with the two 1 two 1 kΩ resistkΩ resistors (typiors (typicallcally y in the in the form of the form of the two ends of two ends of aa potentiometer) to adjust the balancing of the Q5/Q6 current mirror and thus indirectly control potentiometer) to adjust the balancing of the Q5/Q6 current mirror and thus indirectly control

the output of the op-amp when zero signal is applied between the inputs. the output of the op-amp when zero signal is applied between the inputs.

Cl

Class A

ass A gai

gain

n sta

stage

ge

The section outlined in magenta is the class A gain stage. The top-right current mirror The section outlined in magenta is the class A gain stage. The top-right current mirror Q12/Q13 supplies this stage by a constant current load, via the collector of Q13, that is Q12/Q13 supplies this stage by a constant current load, via the collector of Q13, that is largely independent of the output voltage. The stage consists of two NPN transistors in a largely independent of the output voltage. The stage consists of two NPN transistors in a Darlington configuration and uses the output side of a current mirror as its collector load to Darlington configuration and uses the output side of a current mirror as its collector load to achieve high gain. The 30 pF capacitor provides frequency selective negative feedback achieve high gain. The 30 pF capacitor provides frequency selective negative feedback around the class A gain stage as a means of frequency compensation to stabilise the amplifier around the class A gain stage as a means of frequency compensation to stabilise the amplifier in feedback configurations. This technique is called Miller compensation and functions in a in feedback configurations. This technique is called Miller compensation and functions in a

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si

simimilalar r mamannnner er to to an an opop-a-amp mp inintetegrgratator or cicircrcuiuit. t. It It is is alalso so knknowown n as as 'd'domomininanant t popolele compensation' because it introduces a dominant pole (one which masks the effects of other compensation' because it introduces a dominant pole (one which masks the effects of other poles) into the open loop frequency response. This pole can be as low as 10 Hz in a 741 poles) into the open loop frequency response. This pole can be as low as 10 Hz in a 741 amplifier and it introduces a −3 dB loss into the open loop response at this frequency. This amplifier and it introduces a −3 dB loss into the open loop response at this frequency. This inte

internal compensrnal compensatiation on is is provprovided to ided to achiachieve eve uncounconditnditionaional l stabstabiliility ty of of the the amplamplifieifier r inin negative feedback configurat

negative feedback configurations where the ions where the feedback network is non-reactive and the feedback network is non-reactive and the closedclosed loop gain is unity or

loop gain is unity or higher. Hence, the use of the higher. Hence, the use of the operatiooperational amplifier is simplified becausenal amplifier is simplified because no external compensation is

no external compensation is required for unity gain required for unity gain stabilistability; amplifiers without this internalty; amplifiers without this internal compensation may require external compensation or closed loop gains significantly higher compensation may require external compensation or closed loop gains significantly higher than unity.

than unity.

Output bias circuitry

The green outlined section (based around Q16) is a

The green outlined section (based around Q16) is a voltage level shifter or rubber diode (i.e.,voltage level shifter or rubber diode (i.e., aa

V

BEBE multiplier); a type of voltage source. In the circuit as shown, Q16 provides a constantmultiplier); a type of voltage source. In the circuit as shown, Q16 provides a constant

voltage drop between its collector and emitter regardless of the

voltage drop between its collector and emitter regardless of the current through the circuit. If current through the circuit. If the base current to the transistor is assumed to be zero, and the voltage between base and the base current to the transistor is assumed to be zero, and the voltage between base and emitter (and across the 7.5 kΩ resistor) is 0.625 V (a typical value for a BJT in the active emitter (and across the 7.5 kΩ resistor) is 0.625 V (a typical value for a BJT in the active region), then the current through the 4.5 kΩ resistor will be the same as that through the 7.5 region), then the current through the 4.5 kΩ resistor will be the same as that through the 7.5 kΩ, and will produce a voltage of 0.375 V across it. This keeps the voltage across the kΩ, and will produce a voltage of 0.375 V across it. This keeps the voltage across the transistor, and the two resistors at 0.625 + 0.375 = 1 V. This serves to bias the two output transistor, and the two resistors at 0.625 + 0.375 = 1 V. This serves to bias the two output transistors slightly into conduction reducing crossover distortion. In some discrete component transistors slightly into conduction reducing crossover distortion. In some discrete component amplifiers this function is achieved with (usually two) silicon diodes.

amplifiers this function is achieved with (usually two) silicon diodes.

Output stage

The output stage (outlined in cyan) is a Class AB push-pull emitter follower (Q14, Q20) The output stage (outlined in cyan) is a Class AB push-pull emitter follower (Q14, Q20) amplifier with the bias set by the

amplifier with the bias set by the

V

bebe multiplier voltage source Q16 and its base resistors.multiplier voltage source Q16 and its base resistors.

This stage is effectively driven by the

This stage is effectively driven by the collectcollectors of ors of Q13 and Q19. Variations in the Q13 and Q19. Variations in the bias withbias with te

tempmperaeratuture, re, or or betbetweween en paparts rts wiwith th the the samsame e tytype pe numnumberber, , are are cocommmmon on so so crcrossossoveover r distortion and quiescent current may be subject to significant variation. The output range of distortion and quiescent current may be subject to significant variation. The output range of the amplifie

the amplifier is r is about one volt less than the supply voltage, owing in part toabout one volt less than the supply voltage, owing in part to

V

bebeof the outputof the output

transistors Q14 and Q20. transistors Q14 and Q20.

The 25 Ω resistor in the output stage acts as a current sense to provide the output The 25 Ω resistor in the output stage acts as a current sense to provide the output current-limiti

limiting function which limits the current in the emitter follower Q14 to about 25 ng function which limits the current in the emitter follower Q14 to about 25 mA for themA for the 741. Current limiting for the negative output is done by sensing the voltage across Q19's 741. Current limiting for the negative output is done by sensing the voltage across Q19's emitter resistor and using this to reduce the drive into Q15's base. Later versions of this emitter resistor and using this to reduce the drive into Q15's base. Later versions of this amplifier schematic may show a slightly different method of output current limiting. The amplifier schematic may show a slightly different method of output current limiting. The output resistan

output resistance is not ce is not zero, as it would be zero, as it would be in an ideal op-amp, but with negative feedback itin an ideal op-amp, but with negative feedback it approaches zero at low frequencies.

approaches zero at low frequencies.

BIASING OF OPAMP

Theory

An ideal current source is any device that will pass a constant amount of current no matter An ideal current source is any device that will pass a constant amount of current no matter what the voltage drop across it

what the voltage drop across it is. Naturallis. Naturally, there is y, there is no such thing as no such thing as an ideal current source,an ideal current source, so this article will

so this article will describe several alterndescribe several alternatives of increasing complexity and closeness to atives of increasing complexity and closeness to thethe ideal. Most of these variations can be implemented on the PIMETA circuit board and all of ideal. Most of these variations can be implemented on the PIMETA circuit board and all of

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them can be done on the PPA, so the text will make reference to part positions on those them can be done on the PPA, so the text will make reference to part positions on those circuit boards.

circuit boards.

Buffered op-amp circuit with ideal current source for class A Buffered op-amp circuit with ideal current source for class A

bias bias When you put a

When you put a current source on the op-amp's output inside the feedback loop, it current source on the op-amp's output inside the feedback loop, it causes thecauses the op-amp to continuously "fight" against the current source. The chip must put out at least as op-amp to continuously "fight" against the current source. The chip must put out at least as much current as the current source demands in order to

much current as the current source demands in order to force its output to the voltage the op-force its output to the voltage the op-amp inputs demand. If it did not, the current source would pull the output of the op-op-amp to amp inputs demand. If it did not, the current source would pull the output of the op-amp to V-. This continual fight against the current source keeps the op-amp's output stage turned on V-. This continual fight against the current source keeps the op-amp's output stage turned on all the time, with a constant current. Voilá, class A bias.

all the time, with a constant current. Voilá, class A bias.

The reason we connect the current source to V- is that it forces the NPN transistors in the The reason we connect the current source to V- is that it forces the NPN transistors in the op-amp to remain active instead of the PNP ones. Biasing the output to V+ would also work, but amp to remain active instead of the PNP ones. Biasing the output to V+ would also work, but in general PNP transistor

in general PNP transistors don't behave as s don't behave as well as NPN ones, so we'd rather make the well as NPN ones, so we'd rather make the NPN'NPN'ss do the work.

do the work.

The current level that must be passed by

The current level that must be passed by the current source depends on what load the the current source depends on what load the op-ampop-amp is driving. The simple rule for this is that to keep the op-amp in class A, it has to be passing is driving. The simple rule for this is that to keep the op-amp in class A, it has to be passing more current all the time than the load would take on its own. Imagine that the maximum more current all the time than the load would take on its own. Imagine that the maximum signal level we expect is 3Vrms and we're buffering the op-amp with an Elantec EL2001, signal level we expect is 3Vrms and we're buffering the op-amp with an Elantec EL2001, which has a

which has a minimminimum input impedance of 1 um input impedance of 1 MΩ. The peak voltage is MΩ. The peak voltage is 4.2V (1.414 * 3Vrms)4.2V (1.414 * 3Vrms) so the maximum current level between the op-amp's output and the buffer is 4.2 µA, which is so the maximum current level between the op-amp's output and the buffer is 4.2 µA, which is therefore also the minimum value for the current source.

therefore also the minimum value for the current source.

We don't want to make the current source value minimal, though. Down at their lower limits, We don't want to make the current source value minimal, though. Down at their lower limits, tr

transansisistortors s arare e nonnonlilineanear, r, and and wewe'r're e trytryining g to to geget t ririd d of of nonnonlilineaneariritieties s wiwith th thithis s twtweaeak.k. Therefore, in a buffered op-amp circuit, you usually see biases of at least 0.5 mA, and Therefore, in a buffered op-amp circuit, you usually see biases of at least 0.5 mA, and occasionally as high as 5 mA. The amount you must use depends on the op-amp and the occasionally as high as 5 mA. The amount you must use depends on the op-amp and the signal characteristics; you'll have to experiment if you want to find the optimal value.

signal characteristics; you'll have to experiment if you want to find the optimal value. I should repeat at

I should repeat at this point that these numbers only apply this point that these numbers only apply to buffered op-amp circuits. If theto buffered op-amp circuits. If the op-amp is driving a low-impedance load, you will have to use a higher bias level. Imagine op-amp is driving a low-impedance load, you will have to use a higher bias level. Imagine that the op-amp is driving 32-ohm headphones directly. The loudest signal level will likely be that the op-amp is driving 32-ohm headphones directly. The loudest signal level will likely be about 0.5Vrms for such headphones, which means that the peak voltage is 0.7V so the peak about 0.5Vrms for such headphones, which means that the peak voltage is 0.7V so the peak curr

current draw ent draw is about is about 22 22 mA. TheremA. Thereforefore, , youyour r clasclass s A A bias would have to bias would have to be be highhigher er thathann 22 mA to keep the chip operating in class A at all times.

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It might seem from this discussion that the higher the bias level, the better. Not so. Be sure to It might seem from this discussion that the higher the bias level, the better. Not so. Be sure to take a look

take a look at your op-amp's datasheet to find out at your op-amp's datasheet to find out its output current characteristiits output current characteristics. If the cs. If the opop-amp is only capable of delivering 40 mA maximum, putting a 22 mA current source on the amp is only capable of delivering 40 mA maximum, putting a 22 mA current source on the output of the op-amp will likely make the

output of the op-amp will likely make the chip perform worse than with no chip perform worse than with no biasing at all. Atbiasing at all. At this level, you're likely activating the current limiting circuitry of the op-amp, and you're this level, you're likely activating the current limiting circuitry of the op-amp, and you're raising the operating tempera

raising the operating temperature of the ture of the chip significanchip significantly. The moral of the tly. The moral of the story is that youstory is that you need an op-amp capable of delivering significantly more current to the load than strictly need an op-amp capable of delivering significantly more current to the load than strictly required if you want to add a bias tweak to it.

required if you want to add a bias tweak to it.

Applications

Use in electronics system design

The use of op-amps as circuit blocks is much easier and clearer than specifying all their The use of op-amps as circuit blocks is much easier and clearer than specifying all their indi

individuvidual al circircuit cuit elemelements ents (tra(transisnsistortors, s, resiresistorstors, s, etcetc.), .), whewhether ther the the ampampliflifiers iers used used areare integrat

integrated or discrete. In the first approed or discrete. In the first approximatioximation op-amps can be used as if they were n op-amps can be used as if they were idealideal differential gain blocks; at a later stage limits can be placed on the acceptable range of differential gain blocks; at a later stage limits can be placed on the acceptable range of

parameters for each op-amp. parameters for each op-amp.

Circuit design follows the same lines for all electronic circuits. A specification is drawn up Circuit design follows the same lines for all electronic circuits. A specification is drawn up governing what the circuit is required to do, with allowable limits. For example, the gain may governing what the circuit is required to do, with allowable limits. For example, the gain may be required to be 100 times, with a tolerance of 5% but drift of less than 1% in a specified be required to be 100 times, with a tolerance of 5% but drift of less than 1% in a specified

temperature range; the input impedance not less than one megohm; etc. temperature range; the input impedance not less than one megohm; etc.

A basic circuit is designed, often with the help of circuit modeling (on a computer). Specific A basic circuit is designed, often with the help of circuit modeling (on a computer). Specific commercially available op-amps and other components are then chosen that meet the design commercially available op-amps and other components are then chosen that meet the design criteria within the specified tolerances at acceptable cost. If not all criteria can be met, the criteria within the specified tolerances at acceptable cost. If not all criteria can be met, the specification may need to be modified.

specification may need to be modified.

A prototype is then built and tested; changes to meet or improve the specification, alter A prototype is then built and tested; changes to meet or improve the specification, alter functionality, or reduce the cost, may be made.

functionality, or reduce the cost, may be made.

Positive feedback configurations

Another typical configuration of op-amps is the positive feedback, which takes a fraction of Another typical configuration of op-amps is the positive feedback, which takes a fraction of the output signal back to the non-inverting input. An important application of it is the

the output signal back to the non-inverting input. An important application of it is the comparator with hysteresis (i.e., the Schmitt trigger).

comparator with hysteresis (i.e., the Schmitt trigger).

Basic single stage amplifiers

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Non-inverting amplifier Non-inverting amplifier

An op-amp connected in the non-inverting amplifier configuration An op-amp connected in the non-inverting amplifier configuration The gain equation for the op-amp is:

The gain equation for the op-amp is:

However, in this circuit

V

– – is a function of is a function of V

V

outout because of the negative feedback through thebecause of the negative feedback through the

R

11

R

22 network.network.

R

11 andand

R

22 form a voltage divider with reduction factor form a voltage divider with reduction factor

Since the

V

– – input is a high-impedance input, it does not load the voltage divider appreciably,input is a high-impedance input, it does not load the voltage divider appreciably, so:

so:

Substituting this into the gain equation, we obtain: Substituting this into the gain equation, we obtain:

Solving for Solving for V

V

outout::

If

If G

G

open-loopopen-loopis very large, this simplifies tois very large, this simplifies to

.. Inverting amplifier

Inverting amplifier

Because it does not require a differential input, this negative feedback connection was the Because it does not require a differential input, this negative feedback connection was the most typical use of an op-amp in the days of analog computers. It remains very popular. most typical use of an op-amp in the days of analog computers. It remains very popular.

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An op-amp connected in the inverting amplifier configuration An op-amp connected in the inverting amplifier configuration This circuit is easily analysed with the help of the two "golden rules". This circuit is easily analysed with the help of the two "golden rules".

Since the non-inverting input is grounded, rule 1 tells us that the inverting input will also be Since the non-inverting input is grounded, rule 1 tells us that the inverting input will also be at ground potential (0 volts):

at ground potential (0 volts):

The current through

R

ininis then:is then:

Rule 2 tells us that no current enters the inverting input. Then, by Kirchoff's current law the Rule 2 tells us that no current enters the inverting input. Then, by Kirchoff's current law the current through

current through

R

f f must be the same as the current throughmust be the same as the current through

R

inin. The voltage drop across. The voltage drop across

R

f f isis

then given by Ohm's law: then given by Ohm's law:

Since

V

--is zero volts,is zero volts,

V

outout is justis just

−

− V

V

Rf Rf ::

•

• Some Variations:Some Variations:

○

○ A resistor is often inserted between the non-inverting input andA resistor is often inserted between the non-inverting input and

ground (so both inputs "see" similar resistances), reducing the input ground (so both inputs "see" similar resistances), reducing the input offset voltage due to different voltage drops due to bias current and offset voltage due to different voltage drops due to bias current and may reduce distortion in some op-amps.

may reduce distortion in some op-amps.

○

○ A DC-blocking capacitor may be inserted in series with the inputA DC-blocking capacitor may be inserted in series with the input

resistor when a frequency response down to DC is not needed and resistor when a frequency response down to DC is not needed and any DC voltage on the input is unwanted. That is, the capacitive any DC voltage on the input is unwanted. That is, the capacitive component of the input impedance inserts a DC

component of the input impedance inserts a DC zerozero and a low-and a low-frequency pole that gives the circuit a bandpass or high-pass frequency pole that gives the circuit a bandpass or high-pass characteristic.

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Ot

Othe

he r ap

r appl

plic

icat

atio

ions

ns

•

• audio- and video-frequency pre-amplifiers and buffersaudio- and video-frequency pre-amplifiers and buffers •

• voltage comparatorsvoltage comparators •

• differential amplifiersdifferential amplifiers •

• differentiators and integratorsdifferentiators and integrators •

• filtersfilters •

• precision rectifiersprecision rectifiers •

• precision peak detectorprecision peak detector •

• voltage and current regulatorsvoltage and current regulators •

• analog calculatorsanalog calculators •

• analog-to-digital convertersanalog-to-digital converters •

• digital-to-analog converterdigital-to-analog converter •

• voltage clampsvoltage clamps •

• oscillators and waveform generatorsoscillators and waveform generators

Most single, dual and quad op-amps available have a standardized pin-out which permits one Most single, dual and quad op-amps available have a standardized pin-out which permits one type to be substituted for another without wiring changes. A specific op-amp may be chosen type to be substituted for another without wiring changes. A specific op-amp may be chosen for its open loop gain, bandwidth, noise performance, input impedance, power consumption, for its open loop gain, bandwidth, noise performance, input impedance, power consumption, or a compromise between any of these factors.

or a compromise between any of these factors.

Limitations of real op-amps

Real op-amps differ from the ideal model in various respects. Real op-amps differ from the ideal model in various respects.

IC op-amps as implemented in practice are moderately complex integrated circuits; see the IC op-amps as implemented in practice are moderately complex integrated circuits; see the internal

internal circuitry for the relatively simple 741 op-amp below, for example.circuitry for the relatively simple 741 op-amp below, for example. DC imperfections

DC imperfections

Real operational amplifiers suffer from several non-ideal effects: Real operational amplifiers suffer from several non-ideal effects:

Finite gain

Op

Open-en-looloop p gaigain n is is ininfifinitnite e in in ththe e idideal eal opeoperatratioional nal amampliplififier er but but fifinitnite e in in reareall oper

operatioational nal ampampliflifiersiers. . TypTypical ical devidevices ces exhiexhibit bit openopen-loo-loop p DC DC gaigain n rangranging ing fromfrom 100,000 to over 1

100,000 to over 1 milliomillion. So long n. So long as the as the loop gain (i.e., the product of open-loop andloop gain (i.e., the product of open-loop and feedback gains) is very large, the circuit gain will be determined entirely by the feedback gains) is very large, the circuit gain will be determined entirely by the amount of negative feedback (i.e., it will be

amount of negative feedback (i.e., it will be independenindependent of t of open-loop gain). In casesopen-loop gain). In cases where

where closeclosed d -loo-loop gainp gain must be very high, the feedback gain will be very low, andmust be very high, the feedback gain will be very low, and the low feedback gain causes low loop gain; in these cases, the operational amplifier the low feedback gain causes low loop gain; in these cases, the operational amplifier will cease to behave ideally.

will cease to behave ideally.

Finite

Finite input

input impedance

impedance

The input impedance of the operational amplifier is defined as the impedance

The input impedance of the operational amplifier is defined as the impedancebetweenbetween its two inputs. It is

its two inputs. It is not not the impedance from each input to the impedance from each input to ground. In the typical high-ground. In the typical high-gain negative-feedback applications, the feedback ensures that the two inputs sit at the gain negative-feedback applications, the feedback ensures that the two inputs sit at the same voltage, and so the impedance between them is made artificially very high. same voltage, and so the impedance between them is made artificially very high. Hence, this parameter is rarely an important design parameter. Because

Hence, this parameter is rarely an important design parameter. Because MOSFET MOSFET--input operational amplifiers often have protection circuits that effectively short circuit input operational amplifiers often have protection circuits that effectively short circuit any input differences greater than a small threshold, the input impedance can appear any input differences greater than a small threshold, the input impedance can appear to be very low in some tests. However, as long as these operational amplifiers are to be very low in some tests. However, as long as these operational amplifiers are

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used in a typical high-gain negative feedback application, these protection circuits used in a typical high-gain negative feedback application, these protection circuits will be inactive and the negative feedback will render the input impedance to be will be inactive and the negative feedback will render the input impedance to be practically infinite. The input bias and leakage currents described below are a more practically infinite. The input bias and leakage currents described below are a more

important design parameter for typical operational amplifier applications. important design parameter for typical operational amplifier applications.

Non-zero output impedance

Low output impedance is important for low resistance loads; for these loads, the Low output impedance is important for low resistance loads; for these loads, the voltage drop across the output impedance of the amplifier will be significant. Hence, voltage drop across the output impedance of the amplifier will be significant. Hence, th

the e outoutput put imimpedpedanance ce of of the the amampliplififier er rerefleflectcts s ththe e mamaxiximumum m powpower er ththat at can can bebe provided. If the output voltage is fed back negatively, the output impedance of the provided. If the output voltage is fed back negatively, the output impedance of the amplifier is effectively lowered; thus, in linear applications, op-amps usually exhibit a amplifier is effectively lowered; thus, in linear applications, op-amps usually exhibit a very low output impedance indeed. Negative feedback can not, however, reduce the very low output impedance indeed. Negative feedback can not, however, reduce the limitations that R

limitations that R loadload in conjunction with R in conjunction with R outout place on the maximum and minimumplace on the maximum and minimum possible output voltages; it can only reduce output errors

possible output voltages; it can only reduce output errorswithinwithin that range.that range. Low-impedance outputs typically require high

Low-impedance outputs typically require high quiescent (i.e., idle) currentquiescent (i.e., idle) current in thein the output stage and will dissipate more power, so low-power designs may purposely output stage and will dissipate more power, so low-power designs may purposely sacrifice low output impedance.

sacrifice low output impedance.

Input current

Due

Due to to biasbiasing ing requrequiremirements or ents or leakageleakage, , a a smasmall ll amoamount of unt of currcurrent (typicent (typically ~10ally ~10 nanoamperes for

nanoamperes for bipolar bipolar op-amps, tens of picoamperes for op-amps, tens of picoamperes for JFETJFET input stages, andinput stages, and only a few pA for

only a few pA for MOSFETMOSFET input stages) flows into the inputs. When large resistorsinput stages) flows into the inputs. When large resistors or sources with high output impedances are used in the circuit, these small currents or sources with high output impedances are used in the circuit, these small currents can produce large unmodeled voltage drops. If the input currents are matched,

can produce large unmodeled voltage drops. If the input currents are matched,and and thethe impedance looking

impedance lookingout out of of bothboth inputs are matched, then the voltages produced at inputs are matched, then the voltages produced at eacheach input will be equal. Because the operational amplifier operates on the

input will be equal. Because the operational amplifier operates on the differencedifference betwee

between its n its inputs, these matched voltages will have no inputs, these matched voltages will have no effect (unless the operationaleffect (unless the operational amp

ampliflifier has ier has poor CMRR, whicpoor CMRR, which h is describis described ed belbelow). It ow). It is more is more commcommon on for thefor the in

inpuput t cucurrrrenents ts (o(or r ththe e imimpepedadancnces es lolookokining g ouout t of of eaeach ch ininpuput) t) to to be be slsligighthtlyly mismatched, and so a small offset voltage can be produced. This offset voltage can mismatched, and so a small offset voltage can be produced. This offset voltage can create offsets or drifting in the operational amplifier. It can often be nulled externally; create offsets or drifting in the operational amplifier. It can often be nulled externally; however, many operational amplifiers include

however, many operational amplifiers include offset null offset null or or balancebalance pins and somepins and some procedure for using them to remove this offset. Some operational amplifiers attempt procedure for using them to remove this offset. Some operational amplifiers attempt

to nullify this offset automatically. to nullify this offset automatically.

Input offset voltage

This voltage, which is what is required across the op-amp's input terminals to drive This voltage, which is what is required across the op-amp's input terminals to drive the output voltage to zero is related to the mismatches in input bias current. In the the output voltage to zero is related to the mismatches in input bias current. In the perfect amplifier, there would be no input offset voltage. However, it exists in actual perfect amplifier, there would be no input offset voltage. However, it exists in actual op-amps because of imperfections in the differential amplifier that constitutes the op-amps because of imperfections in the differential amplifier that constitutes the input stage of the vast majority of these devices. Input offset voltage creates two input stage of the vast majority of these devices. Input offset voltage creates two problems: First, due to the amplifier's high voltage gain, it virtually assures that the problems: First, due to the amplifier's high voltage gain, it virtually assures that the amplifier output will go into saturation if it is operated without negative feedback, amplifier output will go into saturation if it is operated without negative feedback, even when the input terminals are wired together. Second, in a closed loop, negative even when the input terminals are wired together. Second, in a closed loop, negative feedback configur

feedback configuration, the input offset voltage is amplified along with the ation, the input offset voltage is amplified along with the signal andsignal and this may pose a

this may pose a problem if high precision DC amplificatioproblem if high precision DC amplification is n is required or if the required or if the inputinput signal is very small.

signal is very small.

Common mode gain

A perfect operational amplifier amplifies only the

A perfect operational amplifier amplifies only the voltage differencvoltage difference e between its twobetween its two inpu

inputs, ts, comcompletpletely ely rejrejectiecting ng all all voltvoltages that ages that are are comcommon mon to to bothboth. . HoweHowever, thever, the differential input stage of an operational amplifier is never perfect, leading to the differential input stage of an operational amplifier is never perfect, leading to the amplification of these identical voltages to some degree. The standard measure of this amplification of these identical voltages to some degree. The standard measure of this defect is