Monolithic Amp-The Universal Linear Component Operational amplifi

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Monolithic Amp-The Universal Linear Component Operational amplifi

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Monolithic Amp-The Universal Linear Component
Monolithic Amp-The Universal Linear Component
Operational amplifiers undoubtedly easiest best performing wide range linear functions from simple amplification complex analog computation. cost monolithic amplifiers less than $2.00, large quantities, which makes attractive design them into circuits where they would otherwise considered. cost only attraction monolithic amplifiers. Since components simultaneously fabricated chip, much higher circuit complexities than used with discrete amplifiers economical. This used give improved performance. Further, there insurmountable technical difficulties temperature stabilizing amplifier chip, giving chopper-stabilized performance with little added cost. Operational amplifiers designed high gain, offset voltage input current. result, biasing considerably simplified most applications; they used with fairly simple design rules because many potential error terms neglected. This article will give examples demonstrating range usefulness operational amplifiers linear circuit design. examples certainly all-inclusive, hoped that they will stimulate even more ideas from others. practical hints preventing oscillations operational amplifiers will also given since this probably largest single problem that many engineers have with these devices. Although designs presented LM101 operational amplifier LM102 voltage follower produced National Semiconductor, most generally applicable monolithic devices manufacturer's recommended frequency compensation used differences maximum ratings taken into account. complete description LM101 given elsewhere;1 but, briefly, differs from most other monolithic amplifiers, such LM709,2 that differential input voltage range, +15V, -12V common mode range with supplies compensated with single capacitor. LM102,3 which also used here, designed specifically voltage follower features maximum input current V/µs slew rate.
National Semiconductor Application Note Robert Widlar April 1968
00735701
*Chosen oscillation
FIGURE Free-Running Multivibrator Another advantage circuit that will always self start cannot hang since there more negative feedback than positive feedback. This problem with many "textbook" multivibrators. Since operational amplifier used open loop, usual frequency compensation components required since they will only slow down. even without capacitor, LM101 does have speed limitations which restrict this circuit frequencies below about kHz. large input voltage range LM101 (both differential single ended) permits large voltage swings input that several time constants timing capacitor, used. With most other amplifiers, must reduced keep from exceeding these ratings, which requires that increased. Nonetheless, even when large values needed smaller polarized capacitors used returning them positive supply voltage instead ground.
Operational-Amplifier Oscillator
free-running multivibrator shown Figure excellent example application where does normally consider using operational amplifier. However, this circuit operates frequencies with relatively small capacitors because longer portion capacitor time constant since threshold point operational amplifier well determined. addition, completely-symmetrical output waveform along with buffered output, although symmetry varied returning some voltage other than ground.
Level Shifting Amplifier
Frequently, design linear equipment, necessary take voltage which referred some level produce amplified output which referred ground. most straight-forward doing this differential amplifier similar that shown Figure This circuit, however, disadvantages that signal source loaded current from input divider, that feedback resistors must very well matched prevent erroneous outputs from common mode input signal.
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2002 National Semiconductor Corporation
AN007357
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Level Shifting Amplifier
(Continued)
circuit which does have these problems shown Figure Here, transistor output operational amplifier produces voltage drop across feedback resistor, which equal input voltage. voltage across will then equal input voltage multiplied ratio, R2/R1; common mode rejection will good basic rejection amplifier, independent resistor tolerances. This voltage buffered LM102 voltage follower give impedance output. advantage LM101 this circuit that will work with input voltages positive supply voltages long supplies less than 15V.
Voltage Comparators
LM101 well suited comparator applications reasons: first, large differential input voltage range and, second, output easily clamped make compatible with various driver logic circuits. true that doesn't have speed LM7104 versus under equivalent conditions); however, many linear applications speed problem lower input currents along with higher voltage capability LM101 tremendous benefit. comparator circuits using LM101 shown Figure Figure shows clamping scheme which makes output signal directly compatible with
integrated circuits. LM103 breakdown diode clamps output high states, respectively. This particular diode chosen because sharp breakdown equivalent capacitance. When working comparator, amplifier operates open loop normally frequency compensation needed. Nonetheless, stray capacitance between Pins amplifier should minimized prevent level oscillations when comparator active region. this becomes problem capacitor normal compensation terminals will eliminate Figure shows connection LM101 comparator lamp driver. switches lamp, with limiting current surge resulting from turning cold lamp. determines base drive while keeps amplifier from putting excessive reverse bias emitter-base junction when turns off.
More Output Current Swing
Because almost monolithic amplifiers class-B output stages, they have good loaded output voltage swings, delivering with supplies. Demanding much more current from integrated circuit would require, one, that output transistors made considerably larger. addition, increased dissipation could give rise troublesome thermal gradients chip well excessive package heating high-temperature applications. therefore advisable external buffer when large output currents needed.
00735703
Level-Isolation Amplifier
00735702
Standard Differential Amplifier FIGURE Level-Shifting Amplifiers
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More Output Current Swing
(Continued)
00735704
Comparator driving integrated circuits
00735705
Comparator Lamp Driver FIGURE Voltage Comparator Circuits available since simple matter diffuse high quality FET's same chip amplifier. Nonetheless, possible make good amplifier using discrete pair conjunction with monolithic circuit. Such circuit illustrated Figure matched pair, connected source followers, front integrated operational amplifier. composite circuit roughly same gain integrated circuit itself compensated unity gain with capacitor shown. Although works well summing amplifier, circuit leaves something desired applications requiring high common mode rejection. This happens both because resistors used current sources because FET's themselves have good common mode rejection.
00735706
FIGURE High Current Output Buffer simple accomplishing this shown Figure pair complementary transistors used output LM101 increased current swing. Although this circuit does have dead zone, neglected frequencies below because high gain amplifier. included eliminate parasitic oscillations from output transistors. addition, adequate bypassing should used collectors output transistors insure that output signal coupled back into amplifier. This circuit does have current limiting, added putting resistors series with collectors
00735707
Amplifier
ambient temperatures less than about 70°C, junction field effect transistors give exceptionally input currents when they used input stage operational amplifier. However, monolithic amplifiers
FIGURE Operational Amplifier
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Storage Circuits
sample-and-hold circuit which combines input current FET's with offset voltage monolithic amplifiers shown Figure circuit unity gain amplifier employing operational amplifier source follower. operation, when sample switch,
turned closes feedback loop make output equal input, differing only offset voltage LM101. When switch opened, charge stored holds output level equal last value input voltage.
00735708
*Polycarbonate-dielectric capacitor
FIGURE Drift Sample Hold Some care must taken selection holding capacitor. Certain types, including paper mylar, exhibit polarization phenomenon which causes sampled voltage drop about then stabilize, when capacitor exercised over range during sample interval. This drop time constant order seconds. effect, however, minimized using capacitors with teflon, polyethylene, glass polycarbonate dielectrics. Although this circuit does have particularly output resistance, fixed loads upset accuracy since loading automatically compensated during sample interval. However, load expected change after sampling, buffer such LM102 must added between output. second pole introduced into loop response amplifier switch resistance holding capacitor, This cause problems with overshoot oscillation compensated adding resistor, series with LM101 compensation capacitor such that breakpoint R1C1 combination roughly equal that switch holding capacitor. possible transistor without worrying about threshold stability. threshold voltage balanced during every sample interval only short-term threshold stability important. When transistors used along with mechanical switches, drift rates less than mV/min realized. Additional features circuit that amplifier acts buffer that circuit does load input signal.
00735709
FIGURE Positive Peak Detector with Buffered Output Further, gain also provided feeding back inverting input LM101 through resistive divider instead directly.
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Storage Circuits
(Continued)
peak detector Figure similar many respects sample-and-hold circuit. diode used place sampling switch. Connected shown, will conduct whenever input greater than output, output will equal peak value input voltage. this case, LM102 used buffer storage capacitor, giving drift along with output resistance. with sample hold, differential input voltage range LM101 permits differences between input output voltages when circuit holding.
Non-Linear Amplifiers
When non-linear transfer function needed from operational amplifier, many methods obtaining present themselves. However, they usually require diodes therefore difficult temperature compensate accurate breakpoints. getting around this make output swing large that diode threshold negligible comparison, this always practical. method producing very sharp, temperature-stable breakpoints transfer function operational amplifier shown Figure small input signals, gain determined Both conducting
some degree, they affect gain because their current gain high they feed appreciable current back into summing mode. When output voltage rises (determined V-), draws enough current saturate, connecting parallel with This cuts gain half. Similarly, when output voltage rises will saturate, again halving gain. Temperature compensation achieved this circuit including compensates emitter-base voltage keep voltage across feedback resistors, very nearly equal output voltage while compensates emitter base voltage these transistors they into saturation, making voltage across equal negative supply voltage. detrimental effect that causes output resistance amplifier increase high output levels. therefore necessary output buffer circuit must drive appreciable load.
Servo Preamplifier
certain servo systems, desirable rate signal required loop stability from some sort electrical, lead network. This can, example, accomplished with reactive elements feedback network servo preamplifier.
00735710
FIGURE Nonlinear Operational Amplifier with temperature-compensated breakpoints Many saturating servo amplifiers operate over extremely wide dynamic range. example, maximum error signal could easily 1000 times signal required saturate system. Cases like this create problems with electrical rate networks because they cannot placed part system which saturates. signal into rate network saturates, rate signal will only developed over narrow range system operation; instability will result when error becomes large. Attempts place rate networks front error amplifier make error amplifier linear over entire range error signals frequently gives rise excessive error from signal attenuation. These problems largely overcome using kind circuit shown Figure This amplifier operates linear mode until output voltage reaches approximately with output current from solar cell sensors. this point breakdown diodes feedback loop begin conduct,
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Servo Preamplifier
(Continued)
drastically reducing gain. However, rate signal will still developed because current being back into rate network (R1, just would amplifier remained linear operating region. fact, amplifier will actually saturate until error current reaches which would same having linear amplifier with 600V output swing.
capacitors feedback circuit amplifier. Many these circuits described reference Multiplication division, however, more difficult. These operations usually performed taking logarithms quantities, adding subtracting required then taking antilog. first glance, might appear that obtaining voltage difficult; been shown6 that emitter-base voltage silicon transistor follows collector current over many nine decades. This means that common transistors used perform antilog operations. circuit which performs both multiplication division this fashion shown Figure gives output which proportional product inputs divided third, about same complexity divider alone. circuit consists three converters antilog generator. converters similar these have been described elsewhere,7 brief description follows. Taking amplifier logging transistor, inserted feedback loop such that collector current equal input voltage divided input resistor, Hence, emitter-base voltage will vary input voltage similar amplifier operating with logging transistor, emitter-base junctions connected series, adding voltages. third converter produces This series-connected with antilog transistor, combination hooked parallel with output other convertors. Therefore, emitter-base will subtracted from logs Since collector current transistor varies exponent emitter-base voltage, collector current will proportional product divided This current summing amplifier, giving desired output.
00735711
FIGURE Saturating Servo Preamplifier with Rate Feedback
Computing Circuits
analog computation relatively simple matter perform such operations addition, subtraction, integration differentiation incorporating proper resistors
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Computing Circuits
(Continued)
00735712
LM394
FIGURE Analog Multiplier/Divider
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Computing Circuits
(Continued)
This circuit give 1-percent accuracy input voltages from 50V. this precision lower input voltages, offset amplifiers handling them must individually balanced out. zener diode, increases collector-base voltage across logging transistors improve high current operation. needed, fact undesirable, when these transistors running currents less than currents above lead resistances transistors become important (0.25 1-percent transistors should installed with short leads sockets. important feature this circuit that operation independent temperature because scale factor change converter with temperature compensated equal change scale factor antilog generator. only required that same temperature. Dual transistors should used arranged shown figure that thermal mismatches between cans appear inaccuracies scale factor (0.3-percent/°C) rather than balance error (8-percent/°C). balance potentiometer which nulls offset voltages logging transistors. adjusted setting input voltages equal adjusting output voltage. logging transistors provide gain which dependent their operating level, which complicates frequency compensation. Resistors (R3, amplifier
output limit maximum loop gain, compensation capacitor chosen correspond with this gain. result, amplifiers especially designed speed, techniques optimizing this parameter given reference Finally, clamp diodes through prevent exceeding maximum reverse emitter-base voltage logging transistors with negative inputs.
Root Extractor*
Taking root number using converters fairly simple matter. that needed take voltage, divide square root, then take antilog. circuit which accomplishes this shown Figure form converter input signal. This feeds which produces level shift give zero voltage into divider input. This divider reduces voltage ratio root desired drives buffer amplifier, second level shifting diode, feedback network which gives output voltage needed output from antilog generator, consisting with unity input. offset voltages transistors nulled imbalancing give output input, since root one.
Note: *The "extraction" used here doubtless origin dental operation most would fear less than having find even square root without tables other aids.
00735713
LM394
FIGURE Root Extractor
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Root Extractor*
(Continued)
00735714
Measuring loop gain Typical response FIGURE Illustrating Loop Gain connected diodes order simplify circuitry. This doesn't introduce problems because both operate over very limited current range, really only required that they match. gain-compensating resistor which keeps currents equal with changes signal level. with multiplier/divider, circuit insensitive temperature long transistors same temperature. Using transistor pairs matching them shown minimizes effects gradients. circuit 1-percent accuracy input voltages between 50V. lower input voltages, must have their offsets balanced individually.
00735715
Frequency Compensation Hints
ease designing with operational amplifiers sometimes obscures some rules which must followed with feedback amplifier keep from oscillating. general, these problems stem from stray capacitance, excessive capacitive loading, inadequate supply bypassing improper frequency compensation. frequency compensating operational amplifier, best follow manufacturer's recommendations. However, operating speed frequency response consideration, greater stability margin usually obtained increasing size compensation capacitors. example, replacing compensation capacitor LM101 with capacitor will make times less susceptible oscillation problems unity-gain connection. Similarly, LM709, using 0.05 2000 components instead 5000 will give more stability margin. Capacitor values less than those specified manufacturer particular gain connection should used since they will make amplifier more sensitive strays capacitive loading, circuit even oscillate with worst-case units. basic requirement frequency compensating feedback amplifier keep frequency roll-off loop gain from exceeding dB/octave when goes through unity gain. Figure shows what meant loop gain.
feedback loop broken output, input sources replaced their equivalent impedance. Then response measured such that feedback network included. Figure gives typical responses both uncompensated compensated amplifiers. uncompensated amplifier generally rolls dB/octave, then dB/octave even dB/octave various frequency-limiting effects within amplifier come into play. loop with this kind response were closed, would oscillate. Frequency compensation causes gain roll uniform dB/octave right down through unity gain. This allows some margin excess rolloff external circuitry. Some external influences which affect stability operational amplifier shown Figure load capacitance which come from wiring, cables actual capacitor output. This capacitance works against output impedance amplifier attenuate high frequencies. this added rolloff occurs before loop gain goes through zero, cause instability. should remembered that this single rolloff point give more than dB/octave rolloff since output impedance amplifier increasing with frequency.
00735716
FIGURE External Capacitances that Affect Stability
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Frequency Compensation Hints
(Continued) second source excess rolloff stray capacitance inverting input. This becomes extremely important with large feedback resistors might used with FET-input amplifier. relatively simple method compensating this stray capacitance shown Figure lead capacitor, across feedback resistor. Ideally, ratio stray capacitance lead capacitor should equal closed-loop gain amplifier. However, lead capacitor made larger long amplifier compensated unity gain. only disadvantage doing this that will reduce bandwidth amplifier. Oscillations also result there large resistance non-inverting input amplifier. differential input impedance amplifier falls high frequencies (especially with bipolar input transistors) this resistor produce troublesome rolloff much greater than 10K, with most amplifiers. This easily corrected bypassing resistor ground.
value important, minimum capacitive reactance should one-tenth resistance unity-gain crossover frequency amplifier.
00735718
FIGURE Compensating Very Large Capacitive Loads When operational amplifier operated open loop, might appear first glance that needs frequency compensation. However, this always case because external compensation sometimes required stabilize internal feedback loops. LM101 will oscillate when operated open loop, although there problems capacitance between balance terminal output held absolute minimum. Feedback between these points regenerative balanced with larger feedback capacitance across compensation terminals. Usually compensation capacitor will completely eliminate problem. LM709 will oscillate when operated open loop unless capacitor connected across input compensation terminals capacitor connected output compensation terminals. Problems encountered with supply bypassing insidious that they will hardly ever show Nyquist plot. This problem really been thoroughly investigated, probably because sure cure known: bypass positive negative supply terminals each amplifier ground with least 0.01 capacitor. example, LM101 take over inductance either supply lead without oscillation. This should suggest that they should without bypass capacitors. been established that LM101's single printed circuit board with common supply busses will oscillate supplies bypassed about every fifth device. This happens even though inputs outputs completely isolated.
00735717
FIGURE Compensating Stray Input Capacitance When capacitive load integrated amplifier much greater than some consideration must given effect stability. Even though amplifier does oscillate readily, there worst-case conditions under which will. However, amplifier stabilized value capacitive loading using circuit shown Figure capacitive load isolated from output amplifier with which value both LM101 LM709. high frequencies, feedback path through lead capacitor, that produced load capacitance does cause instability. this circuit, amplifier must compensated unity gain, regardless closed loop gain.
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Frequency Compensation Hints
(Continued) LM709, other hand, will oscillate under many load conditions with little inches wire between negative supply lead bypass capacitor. Therefore, almost essential have bypass capacitors every device. Operational amplifiers specified power supply rejection frequencies less than first break frequency open loop gain. higher frequencies, rejection reduced depending amplifier frequency compensated. both LM101 LM709, rejection high frequency signals positive supply excellent. However, situation different negative supplies. These amplifiers have compensation capacitors from output down signal point which referred negative supply, causing high frequency rejection negative supply much reduced. therefore important have sufficient bypassing negative supply remove transients they cause trouble appearing output. fairly large tantalum capacitor negative power lead each printed-circuit card usually enough solve potential problems. When high-current buffers used conjunction with operational amplifiers, supply bypassing decoupling even more important since they feed considerable amount signal back into supply lines. reference, bypass capacitors least required buffer. When emitter followers used drive long cables, additional precautions required. emitter follower itself which contained feedback loop will frequently oscillate when connected long length cable. When emitter follower connected output operational amplifier, produce oscillations that will persist matter loop gain compensated. analysis this happens very enlightening, suffice that these oscillations usually eliminated putting ferrite bead8 between emitter follower cable. Considering loop gain amplifier valuable tool understanding influence various factors stability feedback amplifiers. helpful determining amplifier indeed stable. reason that most problems well-designed system caused secondary effects which occur only under certain conditions output voltage, load current, capacitive loading, temperature, etc. Making frequency-phase plots under these conditions would require unreasonable amounts time, invariably done. better check stability small-signal transient response. shown mathematically that transient response network one-for-one correspondence with frequency domain response. advantage transient response tests that they displayed instantaneously oscilloscope, reasonable test circuit under wide range conditions. Exact methods analysis using transient response will presented here. This because these methods
difficult, although they are. Instead, because very easy determine which conditions unfavorable from overshoot ringing step response. stability margin determined much more easily much greater aggravating conditions made before circuit oscillates than analysis response under given conditions. little practice with this technique quickly yield much better results than classical methods even inexperienced engineer.
Summary
number circuits using operational amplifiers have been proposed show their versatility circuit design. These have ranged from frequency oscillators through circuits complex analog computation. Because cost monolithic amplifiers, almost foolish design amplifiers without integrated circuits. Moreover, price makes practical take advantage operational-amplifier performance variety circuits where they normally used. Many potential oscillation problems that encountered both discrete integrated operational amplifiers were described, some conservative solutions these problems were presented. areas discussed included stray capacitance, capacitive loading supply bypassing. Finally, simplified method quickly testing stability amplifier circuits over wide range operating conditions suggested.
Note: frequency-domain characteristics determined from impulse response network this directly relatable step response through convolution integral.
References
Widlar, "Monolithic with Simplified Frequency Compensation", EEE, Vol. 58-63, July, 1967. Widlar, Unique Circuit Design High Performance Operational Amplifier Especially Suited Monolithic Construction", Proc. NEC, Vol. XXI, 85-89, October, 1965. Widlar, Fast Integrated Voltage Follower with Input Current", National Semiconductor AN-5, March, 1968. Widlar, "The Operation Fast Integrated Circuit Comparator", Fairchild Semiconductor APP-116, February, 1966. "Handbook Operational Amplifier Applications", Burr-Brown Research Corporation, Tucson, Arizona. Gibbons Horn, Circuit with Logarithmic Transfer Response over Nine Decades", IEEE Trans. Circuit Theory, Vol. CT-11, 378-384, September, 1964. Widlar Giles, "Avoid Over-Integration", Electronic Design, Vol. 56-62, Feb.1, 1966. Leslie Solomon, "Ferrite Beads", Electronics World, 42-43, October, 1966.
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Monolithic Amp-The Universal Linear Component
Notes
LIFE SUPPORT POLICY NATIONAL'S PRODUCTS AUTHORIZED CRITICAL COMPONENTS LIFE SUPPORT DEVICES SYSTEMS WITHOUT EXPRESS WRITTEN APPROVAL PRESIDENT GENERAL COUNSEL NATIONAL SEMICONDUCTOR CORPORATION. used herein: Life support devices systems devices systems which, intended surgical implant into body, support sustain life, whose failure perform when properly used accordance with instructions provided labeling, reasonably expected result significant injury user.
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AN-4
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National does assume responsibility circuitry described, circuit patent licenses implied National reserves right time without notice change said circuitry specifications.

Monolithic Amp-The Universal Linear Component Operational amplifi

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