Beats FETs Input Current Robert Widlar Apartado Postal Puerto Val
|
|
|
Top Searches for this datasheet
Beats FETs Input Current
Beats FETs Input Current
Robert Widlar Apartado Postal Puerto Vallarta Jalisco Mexico
abstract monolithic operational amplifier having input error currents order over temperature range described Instead FETs circuit used bipolar transistors with current gains 5000 that offset voltage drift degraded power consumption voltage also featured number novel circuits that make current characteristics amplifier given Further special design techniques required take advantage these currents explored Component selection treatment printed circuit boards also covered introduction year loudest complaints heard about amps that their input currents were high This longer case Today provide ultimate performance many applications even surpassing amplifiers input stages have long been considered best input currents Low-picoamp input currents fact obtained room temperature However this current which leakage current gate junction doubles every performance severely degraded high temperatures Another disadvantage that difficult match FETs closely Unless expensive selection trimming techniques used typical offset voltages drifts must tolerated Super gain transistors2 challenging FETs These devices standard bipolar transistors which have been diffused extremely high current gains Typically current gains 5000 obtained collector currents This makes possible input currents which competitive with FETs also possible operate these transistors zero collector base voltage eliminating leakage currents that plague Hence they provide lower error currents elevated temperatures bonus super gain transistors match much better than FETs with typical offset voltages drifts
National Semiconductor Application Note December 1969
6875
Figure Comparing amps with FET-input amplifier switches printed circuit boards rather than itself effects error current operational amplifier input current produces voltage drop across source resistance causing error This effect minimized operating amplifier with equal resistances inputs error then proportional difference input currents offset current Since current gains monolithic transistors tend match well offset current typically factor less than input currents
Figure compares typical input offset currents amps amplifiers Although FETs give superior performance room temperature their advantage rapidly lost temperature increases Still they clearly better than early amplifiers like LM709 Improved devices like LM101A equal performance over temperature range they standard transistors input stage Super gain transistors provide more than order magnitude improvement over LM101A LM108 uses these equal performance over temperature range applications involving operation LM108 about orders magnitude better than FETs fact unless special precautions taken overall circuit performance often limited leakages capacitors diodes anaReprinted from December 1969
C1995 National Semiconductor Corporation 6875
6875
Figure Illustrating effect source resistance typical input error voltage Naturally error current greatest effect high impedance circuitry Figure illustrates this point offset voltage LM709 degraded significantly with source resistances greater than With LM101A this extended source resistances high LM108 other hand works well with source resistances above
AN-29
RRD-B30M115 Printed
High source resistances have even greater effect drift amplifier shown Figure performance LM709 worsened with sources greater than LM101A holds sources while LM108 still works well
Applications that require error currents include amplifiers photodiodes capacitive transducers these usually operate megohm impedance levels Sample-andhold-circuits timers integrators analog memories also benefit from error currents example with LM709 worst case drift rates these kinds circuits order LM108 improves this worst case over temperature range input currents also helpful oscillators active filters frequency operation with reasonable capacitor values LM108 used frequency with capacitors larger than logarithmic amplifiers dynamic range extended nearly going from LM709 LM108 other applications having error currents often permits entirely different design approach which greatly simplify circuitry LM108 Figure shows simplified schematic LM108 kinds transistors used chip super gain (primary) transistors which have current gain 5000 with breakdown voltage conventional (secondary) transistors which have current gain with breakdown These differentiated schematic drawing secondaries with wider base Primary transistors used input stage they operated cascode connection with bases bootstrapped emitters through that input transistors operated zero collector-base voltage Hence circuit performance impaired breakdown primaries secondary transistors stand
6875-3
Figure Degradation typical drift characteristics with high source resistances difficult include amplifiers Figure because their drift initially unless they selected trimmed Even though their drift well controlled over limited temperature range trimmed amplifiers generally exhibit much higher drift over temperature range rate their average drift rate would best like that LM101A where operation involved
6875
Figure Simplified schematic LM108
commom mode voltage This configuration also improves commom mode rejection since input transistors variations commom mode voltage Further because there voltage across their collectorbase junctions leakage currents input transistors effectively eliminated second stage differential amplifier using high gain lateral PNPs Q10) These devices have current gains breakdown voltage collector load resistors input stage diode connected laterals which compensate emitter-base voltage second stage that operating current twice that input stage second stage uses active collector load (Q15 Q16) obtain high gain drives complementary class-B output stage which gives substantial load driving capability dead zone output stage eliminated biasing verge conduction with methods frequency compensation available amplifier capacitor connected from input output second stage (between compensation terminals) This method pin-compatible with LM101 LM101A also compensated connecting capacitor from output second stage ground This technique advantage improving high frequency power supply rejection factor complete schematic LM108 given Appendix along with description circuit This includes such essential features overload protection inputs outputs performance primary design objective LM108 obtain very input currents without sacrificing offset voltage drift secondary objective reduce power consumption Speed little concern long comparable with LM709 This logical quite difficult make high-impedance circuits fast power circuits very resistant being made fast other respects desirable make LM108 much like LM101A possible
There been considerable discussion about using Darlington input stages rather than super gain transistors obtain input currents appropriate make comments about that here Darlington inputs give about same input bias currents super gain transistors room temperature However bias current varies square transistor current gain temperatures super gain devices have decided advantage Additionally offset current super gain transistors considerably lower than Darlingtons when measured percentage bias current Further offset voltage offset voltage drift Darlington transistors both higher more unpredictable Experience seems tell real truth about Darlingtons Quite amps with Darlington input stages have been introduced However none have become industry standards reason that they more sensitive variations manufacturing process Therefore satisfactory performance specifications only obtained sacrificing manufacturing yield
6875
Figure Supply current supply current LM108 plotted function supply voltage Figure operating current about order magnitude lower than devices like LM709 Furthermore does vary radically with supply voltage which means that device performance maintained voltages power consumption held down high voltages
6875
Figure Input currents Figure shows input current characteristics LM108 over temperature range only input currents also they change radically over temperature Hence device lends itself relatively simple temperature compensation schemes that will described later
6875
Figure Output swing output drive capability circuit illustrated Figure output swings within volt supplies
which especially important when operating voltages output falls rapidly current increases above certain level short circuit protection goes into effect useful output drive limited about could have been increased addition Darlington transistors output this would have restricted voltage swing supply voltages amplifier incidentally works with common mode signals within volt supplies used with supply voltages
6875-8
Figure Open loop frequency response open loop frequency response plotted Figure indicates that frequency response about same that LM709 LM101A Curves given compensation circuits shown Figure standard compensation identical that LM101 LM101A alternate compensation scheme gives much better rejection high frequency power supply noise will shown later
With unity gain compensation both methods give 75-degree stability margin However shunt compensation small signal bandwidth opposed other scheme Because compensation capacitor included chip tailored application When amplifier used only frequencies compensation capacitor increased give greater stability margin This makes circuit less sensitive capacitive loading stray capacitances improper supply bypassing Overcompensating also reduces high frequency noise output amplifier With closed-loop gains greater than high frequency performance optimized making compensation capacitor smaller unity-gain compensation used amplifier with gain gain error will exceed 1-percent frequencies above This extended reducing compensation capacitor formula determining minimum capacitor value given Figure should noted that capacitor value does really depend closed-loop gain Instead depends high frequency attenuation feedback networks therefore values When desirable optimize performance high frequencies standard compensation should used With small capacitor values stability margin obtained with shunt compensation inadequate conservative designs frequency response operational amplifier considerably different large output signals than small signals This indicated Figure With unity-gain compensation small signal bandwidth LM108 full output swing cannot obtained above This corresponds slew rate Both fulloutput bandwidth slew rate increased using smaller compensation capacitors indicated figure However this only applicable higher closed loop gains results plotted Figure standard compensations With unity gain compensation same curves obtained shunt compensation scheme Classical theory establishes output resistance important design parameter This true amps output resistance most devices enough that ignored because they class-B output stages frequencies thermal feedback between output
6875-9
standard compensation circuit
6875
Figure Large signal frequency response
6875-10
alternate compensation circuit Figure Compensation circuits
input stages determines effective output resistance this cannot accounted conventional design theories Semiconductor manufacturers take care this specifying gain under full load conditions which combines output resistance with gain affects overall circuit performance This avoids fictitious problem that created amplifier with infinite gain which good that will cause open loop output resistance appear infinite which although none this affects overall performance significantly
6875
Figure Power supply rejection Power supply rejection defined ratio change offset voltage change supply voltage producing Using this definition rejection frequencies unaffected closed loop gain However high frequencies opposite true high frequency rejection increased closed loop gain Hence amplifier with gain will have order magnitude better rejection than that shown Figure vicinity overall performance LM108 summarized Table apparent from table previous discussion that device ideally suited applications that require input currents reduced power consumption speed amplifier spectacular this usually problem high-impedance circuitry Further reduced high frequency performance makes amplifier easier that less attention need paid capacitive loading stray capacitances supply bypassing applications Because input current LM108 opens many design possibilities However extra care must taken component selection assembly printed circuit boards take full advantage performance Further unusual design techniques must often applied around limitations some components sample hold circuits holding accuracy sample hold directly related error currents components used Therefore good circuit start with explaining problems
6875
Figure Closed loop output impedance closed loop output impedance nonetheless important some applications This plotted several operating conditions Figure seen that output impedance rises about 500X high frequencies increase occurs because compensation capacitor rolls open loop gain output resistance reduced intermediate frequencies closed loop gains greater than making capacitor smaller This made apparent figure comparing output resistance with without frequency compensation closed loop gain 1000 output resistance also tends increase frequencies Thermal feedback responsible this phenomenon data Figure taken under large-signal conditions with supplies output zero current swing Hence thermal feedback accentuated more than would case most applications desirable that performance unaffected variations supply voltage amplifiers generally better than discretes this respect because necessary single design cover wide range uses LM108 power supply rejection which typically excess will operate with supply voltages from Therefore well-regulated supplies unnecessary most applications because 20-percent variation little effect performance story different high-frequency noise supplies evident from Figure Above practically noise through output figure also demonstrates that shunt compensation about times better rejecting high frequency noise than standard compensation This difference even more pronounced with larger capacitor values shunt compensation added advantage that makes circuit virtually unaffected lack supply bypassing
6875
Figure Sample hold circuit involved Figure shows configuration sample hold During sample interval turned charging hold capacitor value input signal
Appendix Heading This Application Note
When turned retains this voltage output obtained from that buffers capacitor that discharged loading holding mode error generated capacitor looses charge supply circuit leakages accumulation rate error given where time rate change output voltage input current leakage current holding capacitor board leakages ``off'' current switch When high-temperature operation involved leakage limit circuit performance This minimized using junction indicated because commercial junction FETs have lower leakage than their counterparts However even junction devices problem Mechanical switches such reed relays quite satisfactory from standpoint leakage However they often undesirable because they sensitive vibration they slow they require excessive drive power this case circuit Figure used eliminate leakage
tive diodes gates special arrangements must made drive diode does become forward biased selecting hold capacitor leakage only requirement capacitor must also free dielectric polarization phenomena This rules such types paper mylar electrolytic tantalum high-K ceramic small capacitor values glass silvered-mica capacitors recommended larger values ones with teflon polyethylene polycarbonate dielectrics should used input current LM108 gives drift rate hold only when hold capacitor used this number worst case over military temperature range Even this kind performance needed still beneficial LM108 reduce size hold capacitor High quality capacitors larger sizes bulky expensive Further switches must have ``on'' resistance driven from impedance source charge large capacitors short period time sample interval less than about LM108 fast enough work properly this case advisable substitute LM102A which voltage follower designed both input current high speed slew rate will operate with sample intervals short When hold capacitor larger than isolation resistor should included between capacitor input amplifier Figure This resistor insures that will damaged shorting output abruptly shutting down supplies when capacitor charged This precaution peculiar LM108 should observed integrators Integrators like sample-and-hold circuits have essentially same design problems integrator capacitor used storage element error accumulation rate again proportional input current
6875-15
Teflon polyethylene polycarbonate dielectric capacitor Worst case drift less than
Figure Sample hold that eliminates leakage switches When using P-channel switches substrate must connected voltage which always more positive than input signal source-to-substrate junction becomes forward biased this done troublesome leakage current device occurs across substrate-to-drain junction Figure this current routed output buffer amplifier through that does contribute error current main sample switch while isolates hold capacitor from leakage When sample pulse applied both FETs turn charging input voltage Removing pulse shuts both FETs output leakage goes through output voltage drop across less than substrate bootstrapped output LM108 Therefore voltage across substrate-drain junction equal offset voltage amplifier this voltage leakage reduced about orders magnitude necessary switches when bootstrapping leakages this fashion gate leakage device still negligible high temperatures this case with junction FETs transistors have protec-
Figure shows circuit that compensate bias current amplifier current into summing node through supply bias current potentiometer adjusted that this current exactly equals bias current reducing drift rate zero
6875
Figure Integrator with bias current compensation
diode used reasons First acts regulator making compensation relatively insensitive variations supply voltage Secondly temperature drift diode voltage approximately same temperature drift bias current Therefore compensation more effective temperature changes Over temperature range compensation will give factor reduction input current Even better results achieved temperature change less Normally necessary reset integrator establish initial conditions integration Resetting zero readily accomplished shorting integrating capacitor with suitable switch However with sample hold circuits semiconductor switches cause problems because high-temperature leakage connection that gets switch leakages shown Figure negative-going reset pulse turns
currents integration interval removes compensating error accumulated circuit reset applications involving large temperature changes circuit Figure gives better results than compensation scheme Figure especially under worst case conditions Over temperature range worst case drift reduced from when integrating capacitor used this reduction drift needed circuit simplified eliminating returning non-inverting input amplifier directly ground fabricating drift integrators again necessary high quality components minimize leakage currents wiring comments made capacitors connection with sample-and-hold circuits also apply here additional precaution resistor should used isolate inverting input from integrating capacitor larger than This resistor prevents damage that might occur when supplies abruptly shut down while integrating capacitor charged Some integrator applications require both speed error current output amplifiers photomultiplier tubes solid-state radiation dectectors examples this Although LM108 relatively slow there speed when used inverting amplifier This shown Figure circuit arranged that high-frequency gain characteristics determined while determines low-frequency characteristics non-inverting input connected summing node through operated integrator going through unity gain output drives non-inverting input inverting input also connected summing node through chosen roll below Hence frequencies above feedback path directly around with contributing little Below however direct feedback path rolls gain added that high gain frequency amplifier LM101A connected with feed-forward compensation equivalent small-signal bandwidth slew rate large-signal bandwidth these high-frequency characteristics complete amplifier bias current isolated from summing node Hence does contribute drift integrator inverting input only connection summing junction Therefore error current composite amplifier equal bias current allowed saturate will then start towards saturation output gets zero recovery from saturation will slowed drastically This prevented putting zener clamp diodes across integrating capacitor suitable clamping arrangement shown Figure included clamp circuit along with keep leakage currents zeners from introducing errors addition increasing speed this circuit other advantages increased output drive capability LM101A Further thermal feedback virtually eliminated because LM108 does load variations Lastly open loop gain nearly infinite frequencies product gains amplifiers
6875
should have internal gate-protection diodes
Figure drift integrator with reset shorting integrating capacitor When switches turn leakage current absorbed while isolates output from summing node practically voltage across junctions because substrate grounded hence leakage currents negligible additional circuitry shown Figure makes error accumulation rate proportional offset current rather than bias current Hence drift reduced roughly factor During integration interval bias current non-inverting input accumulates error across just bias current inverting input does across Therefore matched with matched with (within about percent) output will drift rate proportional difference these
6875
Figure Fast integrator
6875
Figure Sine wave oscillator sine wave oscillator Although comparatively easy build oscillator that aproximates sine wave making that delivers highpurity sinusoid with stable frequency amplitude another story Most satisfactory designs relatively complicated require individual trimming temperature compensation make them work addition they generally take long time stabilize final output amplitude unique solution most these problems shown Figure connected two-pole low-pass active filter connected integrator Since ultimate phase introduced amplifiers degrees circuit made oscillate loop gain high enough frequency where degrees gain actually made somewhat higher than required oscillation insure starting Therefore amplitude builds until limited some nonlinearity system
Amplitude stabilization accomplished with zener clamp diodes This does introduce distortion reduced subsequent pass filters have equal breakdown voltages resulting symmetrical clipping will virtually eliminate even-order harmonics dominant harmonic then third this about down output about down output This means that total harmonic distortion outputs percent percent respectively frequency oscillation oscillation threshold determined Therefore precision components with temperature coefficients should used made lower than shown circuit will accept looser component tolerances before dropping oscillation start will also quicker However price paid that distortion increased value critical should made much smaller than that effective resistance does drop when clamp diodes conduct output amplitude determined breakdown voltages Therefore clamp level should temperature compensated stable operation Diode-connected (collector shorted base) transistors with emitter-base breakdown about work well positive temperature coefficient diode reverse breakdown nearly cancels negative temperature coefficient forward-biased diode Added advantages using transistors that they have less shunt capacitance sharper breakdowns than conventional zeners LM108 particularly useful this circuit frequencies since permits small capacitors circuit shown oscillates uses capacitors order This makes much easier find temperature-stable precision capacitors However some judgment must used large value resistors with temperature coefficients exactly easy come LM108s useful this circuit output frequencies Beyond that better performance realized substituting LM102A LM101A with feed-forward compensation improved high-frequency response these devices extend operating frequency capacitance multiplier Large capacitor values eliminated from most systems just raising impedance levels suitable amps available However sometimes possible because impedance levels already fixed some element system like impedance transducer this case capacitance multiplier used increase effective capacitance small capacitor couple into impedance system Previously amps could used effectively capacitance multipliers because equivalent leakages generated offset current were significantly greater than leakages large tantalum capacitors With LM108 this changed circuit shown Figure generates equivalent capacitance with worst case leakage over temperature range
Large-value resistors available from Victoreen Instrument Cleveland Ohio Pyrofilm Resistor Whippany Jersey
6875
Figure Capacitance multiplier performance circuit described equations given Figure where effective output capacitance leakage current this capacitance series resistance multiplied capacitance series resistance relatively high high-Q capacitors cannot realized Hence such applications tuned circuits filters ruled However multiplier still used timing circuits servo compensation networks where some resistance usually connected series with capacitor effect resistance compensated final point that leakage current multiplied capacitance function applied voltage persists even with voltage output Therefore generate offset errors circuit rather than scaling errors caused conventional capacitors instrumentation amplifier many instrumentation applications there frequently need amplifier with high-impedance differential input single ended output Obvious uses this amplifiers bridge-type signal sources such strain gages temperature sensors pressure transducers General purpose amps have satisfactory input characteristics feedback must added determine effective gain addition feedback drastically reduce input resistance degrade common mode rejection
Figure shows classical circuit differential amplifier This circuit three main disadvantages First input resistance inverting input relatively being equal Second there usually large difference input resistance inputs indicated equations schematic Third common mode rejection greatly affected resistor matching balancing source resistances 1-percent deviation resistor values reduces common mode rejection closed loop gain gain gain Clearly only high input impedance very large resistors feedback network must operate from source resistance which orders magnitude larger than resistance signal source Older amps introduced excessive offset drift when operating from higher resistances could used successfully LM108 however relatively unaffected large resistors this approach sometimes employed
With large input resistors feedback resistors quite large higher closed loop gains example must gain difficult accurately match resistors that this high value common mode rejection suffer Nonetheless resistors trimmed take common mode feedthrough caused either resistors mismatches amplifier itself
When bridge goes balance maintains voltage between input terminals zero with current back from output through This circuit does like true differential amplifier large imbalances bridge voltage drops across sensor resistors become unequal bridge goes balance causing some non-linearity transfer function However this usually objectionable small signal swings
6875-21
Figure Feedback connection differential amplifier Another problem caused large feedback resistors that stray capacitance seriously affect high frequency common mode rejection With input resistors mismatch stray capacitance from either input ground drop common mode rejection 1500 high frequency rejection improved expense frequency response shunting with matched capacitors With high impedance bridges feedback resistances become prohibitively large even LM108 circuit Figure cannot used possible alternative shown Figure chosen that their equivalent parallel resistance equal Hence output amplifier will zero when bridge balanced
6875
Figure Differential input instrumentation amplifier
Figure shows true differential connection that problems mentioned previously input resistance greater than 1010X does need large resistors feedback circuitry With component values shown connected non-inverting amplifier with gain feeds into which inverting gain Hence total gain from input output which equal non-inverting gain resistors matched circuit responds only differential input signal common mode voltage This circuit same sensitivity resistor matching previous circuits with percent mismatch between resistors lowering common mode rejection However matching more easily accomplished because lower resistor values Further high frequency common mode rejection less affected stray capacitances high frequency rejection limited though response
logarithmic converter logarithmic amplifier another circuit that take advantage input current increase dynamic range Most practical converters make logarithmic relationship between emitter-base voltage standard double-diffused transistors their collector current This logarithmic characteristic been proven true over decades collector current only problem involved using transistors logging elements that scale factor temperature sensitivity percent However temperature compensating resistors have been developed compensate this characteristic making possible converters that accurate over wide temperature range
R2UR3
6875-22
Figure Amplifier bridge transducers
Sensitivity decade
6875
3500 Available from Vishay Ultronix Grand Junction Series Determines current zero crossing output shown
Figure Temperature compensated one-quadrant logarithmic converter
Figure gives circuit that uses these techniques logging transistor while provides fixed offset temperature compensate emitter-base turn voltage operated fixed collector current emitter-base voltage subtracted from that determining output voltage circuit cola lector current established through collector current proportional input current through therefore proportional input voltage emitter-base voltage varies input voltage fixed emitter-base voltage subtracts from voltage emitter determining voltage temperature-compensating resistor signal will zero when input current equal current through temperature Further this voltage will vary logarithmically changes input current although scale factor will have temperature coefficient output converter essentially multiplied ratio Since positive temperature coefficient percent compensates change scale factor with temperature this circuit LM101A with feedforward compensation used since much faster than LM108 used Since both amplifiers cascaded overall feedback loop reduced phase shift through insures stability Certain things must considered designing this circuit sensitivity changed varying must made considerably larger than resistance effective temperature compensation scale factor should also matched devices same package should same temperature
these transistors Accuracy input currents determined error caused bias current high currents behavior limits accuracy input currents approaching 2N2920 develops logging errors excess percent larger input currents anticipated bigger transistors must used should reduced insure that does saturate transducer amplifiers With certain transducers accuracy depends choice circuit configuration much does quality components amplifier photodiode sensors shown Figure illustrates this point Normally photodiodes operated with reverse voltage across junction high temperatures leakage currents approach signal current However photodiodes deliver short-circuit output current unaffected leakage currents which significantly lower than output current with reverse bias
6875
Figure Amplifier photodiode sensor
6875-26
6875
Figure Amplifier piezoelectric transducers circuit shown Figure responds short-circuit output current photodiode Since voltage across diode only offset voltage amplifier inherent leakage reduced least orders magnitude Neglecting offset current amplifier output current sensor multiplied plus determining output voltage
Figure Inverting amplifier with high input resistance Another disadvantage circuit that four resistors determine gain instead Hence given resistor tolerance worst-case gain deviation greater although this probably more than offset ease getting better tolerances resistor values current sources Although there numerous ways make current sources with amps most have limitations their application concerned Figure however shows current source which fairly flexible restrictions concerned supplies current that proportional input voltage drives load referred ground voltage within output-swing capability amplifier
Figure shows amplifier high-impedance transducers like piezoelectric accelerometer These sensors normally require high-input-resistance amplifier LM108 provide input resistances range using conventional circuitry However conventional designs sometimes ruled either because large resistors cannot used because prohibitively large input resistances needed Using circuit Figure input resistances that orders magnitude greater than values return resistors obtained This accomplished bootstrapping resistors output With this arrangement lower cutoff frequency capacitive transducer determined more product than resistor values equivalent capacitance transducer
resistance multiplication When inverting operational amplifier must have high input resistance resistor values required hand example input resistance needed amplifier with gain feedback resistor called This resistance however reduced using circuit Figure divider with ratio added output amplifier Unitygain feedback applied from output divider giving overall gain using only resistors This circuit does increase offset voltage somewhat output offset voltage given VOUT
IOUT
6875
Figure Bilateral current source With output grounded relatively obvious that output current will determined gain setting yielding IOUT When output zero would seem that current through would reduce accuracy Nonetheless output current will
offset voltage only multiplied conventional inverter Therefore circuit Figure multiplies offset instead This multiplication factor reduced increasing
independent output voltage output resistance circuit given ROUT
where feedback resistors incremental change resistor value from design center Hence circuit Figure percent deviation resistor values will drop output resistance Such errors trimmed adjusting feedback resistors design advisable make feedback resistors large possible Otherwise resistor tolerances become even more critical circuit must driven from source resistance which comparison since this resistance will imbalance circuit affect both gain output resistance shown circuit gives negative output current positive input voltage This reversed grounding input driving ground magnitude scale factor will unchanged long voltage comparators Like most amps possible LM108 voltage comparator Figure shows device used simple zero-crossing detector inputs pro-
Figure shows comparator voltages opposite polarity output changes state when voltage junction equal Mathematically this expressed
6875
Figure Voltage comparator with output buffer LM108 also used differential comparator going through transition when input voltages equal However resistors must inserted series with inputs limit current minimize loading signal sources when input-protection diodes conduct Figure also shows transistor added output increase about with standard power booster LM108 which designed power consumption able drive heavy loads However relatively simple booster added output increase output current This circuit shown Figure added advantage that swings output supplies within fraction volt increased voltage swing particularly helpful voltage circuits
6875
Figure Zero crossing detector tected internally back-to-back diodes connected between them therefore voltages excess cannot impressed directly across inputs This problem taken care which limits current that input voltages excess tolerated absolute accuracy required made much larger than compensating resistor equal value should inserted series with other input Figure output clamped that drive directly This accomplished with clamp diode When output swings positive clamped breakdown voltage zener When swings negative clamped diode drop below ground logic supply used positive supply amplifier zener replaced with ordinary silicon diode maximum that handled device standard under worst case conditions might expected LM108 very fast when used comparator response time tens microseconds LM10311 recommended rather than conventional alloy zener because lower capacitance will slow circuit further sharp breakdown LM103 currents also advantage current through diode clamp only
6875
Figure Power booster
Figure output transistors driven from supply leads important that made enough turned worst case quiescent current amplifier output loaded heavily ground with When output swings about positive increasing positive supply current will turn which pulls load similar situation occurs with negative output swings bootstrapped shunt compensation shown figure only that seems work loading conditions This capacitor made inversely proportional closed loop gain optimize frequency response value given unity-gain follower connection also required loop stability circuit does have dead zone open loop transfer characteristic However frequency gain high enough that neglected Around though dead zone becomes quite noticeable Current limiting incorporated into circuit adding resistors series with emitters because short circuit protection LM108 limits maximum voltage drop across board construction indicated previously certain precautions must observed when building circuits that sensitive very currents proper care taken board leakage currents easily become much larger than error currents prevent this necessary thoroughly clean printed circuit boards Even experimental breadboards must cleaned with trichloroethlene alcohol remove solder fluxes blown with compressed These fluxes insulators impedance levels like electric motors they certainly high impedance circuits addition causing gross errors their presence make circuit behave erratically especially temperature changed
elevated temperatures even leakage clean boards headache leakage resistance between adjacent runs printed circuit board about 1011X 05-inch separation parallel inch) high quality epoxy-glass boards that have been properly cleaned Therefore boards easily produce error currents order much more they become contaminated Conservative practice dictates that boards coated with epoxy silicone rubber after cleaning prevent contamination Silicone rubber easiest However better durability epoxy needed care must taken make sure that gets thoroughly cured Otherwise epoxy will make high temperature leakage much worse Care must also exercised insure that circuit board protected from condensed water vapor when operating vicinity This usually accomplished coating board mentioned above inverting amplifier
6875
follower
6875
non-inverting amplifier
6875-32
Bottom View Figure Printed circuit layout input guarding with TO-5 package
6875
Figure Connection input guards
guarding Even with properly cleaned coated boards leakage currents verge causing trouble standard configuration most amps input pins adjacent pins which supply potentials Therefore advisable employ guarding reduce voltage difference between inputs adjacent metal runs board layout that includes input guarding shown Figure eight lead TO-5 package ten-lead circle used leads formed that holes adjacent inputs vacant when inserted board guard which conductive ring surrounding inputs then connected impedance point that same potential inputs leakage currents from pins supply potentials absorbed guard voltage difference between guard inputs made approximately equal offset voltage reducing effective leakage more than three orders magnitude leads integrated circuit other components connected input through board necessary guard both sides
Guarding non-inverting amplifier little more complicated impedance point must created using relatively value feedback resistors determine gain Figure guard then connected junction feedback resistors resistor connected shown figure compensate large source resistances With dual-in-line flat packages more difficult guard inputs standard configuration LM709 LM101A used because spacings these packages fixed Therefore configuration LM108 changed shown Figure conclusions amps available that equal input current specifications amplifiers most restricted temperature range applications operating temperatures above clearly superior uses bipolar transistors that make possible eliminate leakage currents that plague FETs Additionally bipolar transistors match better than FETs offset voltage drifts obtained without expensive adjustments selection Further bipolar devices lend themselves more readily low-cost monolithic construction These amplifiers open application areas vastly improve performance others example analog memories holding intervals extended minutes even where operation involved Instrumentation amplifiers frequency waveform generators also benefit from error currents
Figure shows guard commited morecommon circuits With integrator inverting amplifier where inputs close ground potential guard simply grounded With voltage follower guard bootstrapped output desirable resistor inverting input compensate source resistance connected shown Figure
6875
6875
NOTE connected bottom package
NOTE connected bottom package
View
View
Figure Comparing connection diagrams LM101A LM108 showing addition guarding
When operating above overall performance frequently limited components other than unless certain precautions observed generally necessary redesign circuits using semiconductor switches reduce effect their leakage currents Further high quality capacitors must used care must exercised selecting large value resistors Printed circuit board leakages also troublesome unless boards properly treated above almost mandatory employ guarding boards protect inputs full potential amplifier realized appendix complete schematic LM108 given Figure description basic circuit presented along with simplified schematic earlier text purpose this Appendix explain some more subtle features design current source supplying input transistors designed supply total input stage current This current drops increases only This temperature characteristic tends
compensate current gain falloff input transistors temperatures without creating stability problems high temperatures biasing circuitry input current source nearly identical that LM101A complete description given Reference However brief explanation follows collector which saturation current about establishes collector current This provides initial turn-on current circuit insures starting under conditions purpose compensate production temperature variations current collector resistor (indicated through made same semiconductor material channel current varies drop across tends compensate changes emitter base voltage collector-emitter voltage equal emitter base voltage plus that This voltage delivered operated substantially higher currents than Hence there
6875
Figure Complete schematic LM108
differential their emitter base voltages that dropped across determine input stage current pinched base resistor indicated slash through This resistor which large positive temperature coefficient operates conjunction with help shape temperature characteristics input stage current source output currents which controlled-gain lateral delivers one-half combined currents output stage also connected with output current approximately Since this type current source makes emitter-base voltage differential between similar transistors operating different collector currents output relatively independent current delivered This current used input stage bootstrapping circuitry also supplies current class-B output stage output current determined ratio current through included that biasing circuit upset when saturates major departure from simplified schematic bootstrapping second stage active loads output This makes second stage gain dependent only well match with variations output voltage Hence second stage gain quite high fact overall gain amplifier typically excess second stage active loads drive high-gain primary transistor used prevent loading second stage collector bootstrapped operate zero collector-base voltage class-B output stage actually driven emitter dead zone output stage prevented biasing verge conduction with used compensate transconductance making output stage quiescent current relatively independent output current drop across this resistor also reduces quiescent current positive-going outputs short circuit protection provided When voltage drop across turns removes base drive from negative-going outputs current limiting initiated when voltage drop across becomes large enough collector base junction become forward biased When this happens base clamped output current cannot increase further Input protection provided which clamp diodes between inputs collectors these transistors bootstrapped emitter through This keeps collector-isolation leakage transistors from showing inputs included that bootstrapping disrupted when saturate with input overload Current-limiting resistors were connected series with inputs since diffused resistors cannot employed such that they work effectively without causing high temperature leakages
TABLE Typical Performance LM108 Operational Amplifier 15V) Input Offset Voltage Input Offset Current Input Bias Current Input Resistance Input Common Mode Range Common Mode Rejection Offset Voltage Drift Offset Current Drift Voltage Gain Small Signal Bandwidth Slew Rate Output Swing Supply Current Power Supply Rejection Operating Voltage Range 300V
references Widlar ``Future Trends Intregrated Operational Amplifiers 24-29 June 1968 Widlar ``Super Gain Transistors IEEE Journal Solid-State Circuits SC-4 August 1969 Widlar Unique Circuit Design High Performance Operational Amplifier Especially Suited Monolithic Construction Proc 85-89 October 1965 Widlar with Improved Input-Current Characteristics 38-41 December 1968 Widlar ``Linear IC's part Compensating Drift Electronics 90-93 February 1968 Widlar ``Design Techniques Monolithic Operational Amplifiers IEEE Journal Solid-State Circuits SC-4 August 1969 Sulllivan Maidique ``Characterization Application High Input Impedance Monolithic Amplifier Transitron Electronic Corporation Application Brief Paul ``An Analysis Certain Errors Electronic Differential Analyzers II-Capacitor Dielectric Absorption Trans Electronic Computers 1722 March 1958 Widlar Fast Integrated Voltage Follower with Input Current Microelectrics June 1968 Dobkin ``Feedforward Compensation Speeds National Semiconductor LB-2 March 1969 Wildlar Voltage Breakdown Diode National Semiconductor TP-5 April 1968 Widlar ``Some Circuit Design Techniques Linear Integrated Circuits IEEE Transactions Circuit Theory CT-12 586-590 December 1965
Beats FETs Input Current
LIFE SUPPORT POLICY NATIONAL'S PRODUCTS AUTHORIZED CRITICAL COMPONENTS LIFE SUPPORT DEVICES SYSTEMS WITHOUT EXPRESS WRITTEN APPROVAL PRESIDENT 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
National Semiconductor Corporation 1111 West Bardin Road Arlington 76017 1(800) 272-9959 1(800) 737-7018
critical component component life support device system whose failure perform reasonably expected cause failure life support device system affect safety effectiveness
AN-29
National Semiconductor Europe (a49) 0-180-530 Email cnjwge tevm2 Deutsch (a49) 0-180-530 English (a49) 0-180-532 Fran (a49) 0-180-532 Italiano (a49) 0-180-534
National Semiconductor Hong Kong 13th Floor Straight Block Ocean Centre Canton Tsimshatsui Kowloon Hong Kong (852) 2737-1600 (852) 2736-9960
National Semiconductor Japan 81-043-299-2309 81-043-299-2408
National does assume responsibility circuitry described circuit patent licenses implied National reserves right time without notice change said circuitry specifications
Other recent searches
TSB12LV01B - TSB12LV01B TSB12LV01B Datasheet
SG186 - SG186 SG186 Datasheet
PC100R-14-AP6 - PC100R-14-AP6 PC100R-14-AP6 Datasheet
MSM6562B - MSM6562B MSM6562B Datasheet
M3D187 - M3D187 M3D187 Datasheet
HYMD1327258-H - HYMD1327258-H HYMD1327258-H Datasheet
CY62127DV18 - CY62127DV18 CY62127DV18 Datasheet
CAT4201EVAL2 - CAT4201EVAL2 CAT4201EVAL2 Datasheet
Privacy Policy | Disclaimer
© 2012 Datasheet Archive
