运算放大器：带宽4

Bandwidth–4带宽-4TIPL1214TIPrecisionLabs–OpAmpsPresentedbyJasonWuPreparedbyArtKay,PeteSemig,andTimGreenHello,andwelcometotheTIPrecisionLabdiscussingopampbandwidth,part4.Inthisvideowe’llcover5bandwidthrelatedtopics:First,adeeperlookathowtheslopeoftheAolcurveaffectsgainbandwidthSecond,Howanopampsinputcapacitancecanlimitthebandwidth.Third,HowtocalculatethepracticalgainversesfrequencyforamplifiercircuitsFourth,HowtolimitthebandwidthofacircuitonpurposeusingafeedbackcapacitorAndfinallyHowslewratecanaffecttheresponseoverfrequency.哈啰,欢迎来到TIPrecisionLabs(德州仪器高精度实验室).本次视频将介绍运算放大器的bandwidth(带宽)第四部分.我们将讨论5个带宽相关的主题。第一,深入探讨Aolcurve的斜率如何影响增益带宽第二,为何运算放大器的input电容能限制带宽第三,如何计算放大器电路的实际增益与带宽第四,如何运用feedbackcapacitor来限制电路的带宽第五,了解slewrate如何影响不同带宽的响应GainBandwidthChanges2PARAMETERCONDITIONSMINTYPMAXUNITGainBandwidthProductGBWGain=+118MHzOPA2092Atthispointwehavediscussedthegainbandwidthproductingreatdetail.WeknowthatitisonlyvalidwheretheslopeoftheAolcurveis-20dB/decade.Unfortunately,sometimesitisnotobviouswhetherthegainbandwidthproductisvalidornot.HerewehavethegainbandwidthproductspecificationfromtheOPA209datasheet.Thetypicalgainbandwidthproductislistedas18MHzgiventhatthegainis1V/V.But,whataboutotherclosedloopgains?Herewehavetheopenloopgainandphasecurvesversusfrequencyfromthedatasheet.Theopenloopgaincurveappearstobelinearanddecreasingataconstantrateof-20dB/decade.Thus,onemightassumethatthegainbandwidthproductisvalidforallgain.However,simulationandreal-worldmeasurementswillshowthatsuchanassumptionisincorrect.Why?我们已经很详细的讨论了增益带宽积。我们知道，只有当Aol曲线的斜率为-20dB/decade时是有效的。不幸的是，有时增益带宽积是有效还是无效,不是很明显。下面有OPA209数据表的增益带宽积规格。典型的增益带宽积列出当增益为1V/V时,带宽为18MHz。但是，其他闭回路增益的带宽呢？数据表中，我们有开回路增益,相位曲线与频率。开回路增益曲线似乎是线性的，以-20dB/decade的恒定速率下降。因此，大家可能会认为，增益带宽积适用于所有增益。然而，仿真和实际测量将表示，这假设是不正确的。为什么呢？Pole-ZeroCombination“Bend”inAolCurve3OPA2093Previouslyweassumedthattheopenloopgaincurvehadaconstantslopeof-20dB/decade.However,wefindthatthereisasmallbendintheAolcurvebetween1MHzand10MHzduetoapole-zeropair.Apolenear1MHzcausestheopenloopgaintodecreasegreaterthan-20dB/decadeoverasmallrangeoffrequency,butthepoleisthenquicklycancelledbyazero.Duetothelogarithmicscale,itisnotpossibletoseethissmallbendintheAolcurve.Asamatteroffact,thebendisprobablysmallerthanthethicknessofthelineinthecurve.Zoominginonthebendinthecurveyouwillnoticethatthegainbandwidthis18MHzatthespecifiedgainof1V/V,butincreasesto23.7MHzforclosed-loopgainsgreaterthan20dB.WhilethebendintheAolcurveisdifficult,ifnotimpossibletosee,thepole-zeropairismoreapparentinthephasecurve.Noticethedipinthephasecurvenear1MHz.Thisisduetothepole-zeropair.Thereforeitisrecommendedthatonealwaysinspectthephasecurveinadditiontothegaincurveinanopampdatasheet.Thesetwocurvesdepicthowthegainbandwidthproductchangeswithfrequencyandclosed-loopgain.Noticethatataclosed-loopgainof0dB,or1V/V,thegainbandwidthproductis18MHzasstatedinthedatasheet.Astheclosed-loopgainincreases,thebandwidthincreasesto23.7MHz.Ultimatelytherearethreelessonstolearnfromthisdiscussion:First,itisnotunusualtoseesomedeviationingainbandwidthfordifferentclosedloopgainsandthattheamountofdeviationwillvarydependingontheamplifier.Second,thephasecurveiskeytolocatingtheexistenceofapole-zeropair.So,alwaysbesuretoinspectthephasecurveinadditiontotheopenloopgaincurve.Finally,don’texpecttheexactbandwidthperformancelistedinadatasheet.Typicalbandwidthspecificationscanvarybyapproximately+/-30%overprocessandanadditional+/-30%overtemperature.Thereforeitishighlyrecommendedtoincludesignificantmarginforbandwidthinyourdesign.先前我们假设开回路增益曲线有-20dB/decade的恒定斜率。然而，我们发现由于有一对零点,极点在1MHz和10MHz之间,Aol曲线有一个转折。接近1MHz的极点在很小的频率范围内,造成开回路增益下降的速率大于-20dB/decade，不过极点很快速的被零点补偿。由于对数坐标，是不可能在Aol曲线看到这个小弯。事实上，此弯曲可能比曲线厚度还小。在曲线的弯曲处放大你会发现，1V/V的增益下,增益带宽为18MHz,但当闭回路增益大于20dB,带宽增大到23.7MHz。然而,发现Aol曲线的弯曲是困难的，在相位曲线发现零点,极点较为明显。请注意在1MHz附近的相位曲线倾角。这是由于在零点,极点对的原故。因此，建议大家除了检查算放大器数据表中的增益曲线,也要检查相位曲线。这两条曲线描述增益带宽积是如何随频率和死循环路增益变化。请注意，在0dB或1V/V的闭回路增益时，在数据表所列增益带宽积为18MHz。当闭路回增益增加，带宽增加至23.7MHz。最终这个讨论学习到三件事：第一,看到增益带宽在不同死循环增益有些偏差(GBP不为定值)是不常见的，而该偏差量将取决于放大器而异第二，寻找一个零点,极点对,相位曲线是关键。所以，除了检查开回路增益曲线,也要检查相位曲线。最后，不要指望在数据表中列出带宽性能的切确性。因为,典型带宽规格在不同的制程约有+/-30％,和不同的温度下+/-30％的变异。因此，强烈建议设计时对Bandwidth要有显著的裕度。InputCapacitance4fc12π⋅Rin⋅Ccm⋅12π⋅1MΩ()⋅6.4pF()⋅24.87kHz4Uptothispointwehaveseenhowtheopamp’sgainbandwidthlimitationsetsthebandwidth.However,insomecasesotherfactorscaneffectbandwidth.Thisslidefocusesontheeffectofinputcapacitanceonbandwidth.Allopampswillhaveadifferentialandcommonmodeinputcapacitanceandimpedance.Thisparasiticcapacitanceisfromthesemiconductorjunctionsontheinputstagetransistors.Thedifferentialcapacitanceisconnectedbetweentotwoinputsandthecommonmodecapacitanceisconnectedoneachinputwithrespecttoacground.Thetableatthetopoftheslideshowshowinputcapacitanceistypicallyspecified.Inthisexamplethedifferentialcapacitanceis1.6pFandthecommonmodecapacitanceis6.4pF.Theseinputcapacitancesarerelativelysmall,soitisunlikelythatyouwillseebandwidthlimitationsunlesstheinputsignalsourcehasalargeseriesresistance.Inthisexample,thesourceresistance