医用仪器放大器.docx
- 文档编号:3627125
- 上传时间:2022-11-24
- 格式:DOCX
- 页数:8
- 大小:22.24KB
医用仪器放大器.docx
《医用仪器放大器.docx》由会员分享,可在线阅读,更多相关《医用仪器放大器.docx(8页珍藏版)》请在冰豆网上搜索。
医用仪器放大器
3AMPLIFIERSUSEDININSTRUMENTION
3.1INTRODUCTION
Theelectrochemicalsignalsgeneratedbyapatient’sphysiologicalsystemsareusuallysmallandrequireamplificationAsimplifieddiagramofattypicalphysiologicalmonitorisFig.3.1.
Thetransducerdetectsandconvertstheelectrochemicalsignalfromthebarriercreatedbythepatient’sskinsurfaceintoanelectronicsignal,whichisthenusedtodrivetheamplifiercircuits,whichincreasethesignalactiveelements,suchastransistorsorintegratedcircuits,usedinthedesignoftheamplifiercircuits.Amplifiersusedwithinphysiologicalmonitorshavethefollowingcharacteristics:
1.Highinputimpedance,generallyover10MΩ
2.High-gainopenloop,generallyover1000
3.Moderateoutputimpedance
4.Lowwhitenoise
5.Lowharmonicdistortion
6.Bandwidthsdefinedbythefrequencycontentofsignal
7.Highlinearity
3.2INPUTISOLATION
Inthecaseofphysiologicalmonitors,thesignal-conditioningamplifiercircuitsconsistofpreamplifierinputisolationcircuits,electrodeselectionswitching,voltageamplifiers,andthepoweramplifiersusedtodrivethedisplay.
Preamplifierisolationcircuitsareusedtoincreasetheinputimpedanceofthemonitoringsystemsinordertoisolatethepatientfromtheinstrument.Thepatientmustbeisolatedbecausemostmonitorsarelineoperated.Thestrayaccircuitsthatmayleakthroughtheunit’spowersupplytothepatientthroughthetransducerelectrodesmustbereducedtominimum.Theisolationamplifiers,byraisingthesystem’sinputimpedancetoabout50MΩ,achievethisobjective.AtypicalisolationamplifiercircuitisshowninFig.3.2.
Inthiscircuit,twoidenticaltransistors(Q1andQ2)areconnectedincascadeemitter-followercircuits.Theinputimpedanceasseenbytheelectrodetransducersisfoundbymultiplyingthecommon–emittercurrentgainbeta(β)bytheoutputimpedance.Becausethetransistorsareidentical,theexpressionfortheinputimpedancemaybewrittenas
Z1=β2ZO
TheemitterresistanceR1isusedtomaintainthebiasQ1anddoesnotaffecttheoutputZtothedegreethattheemitterresistorR2does.theoutputimpedancebecomestheequivalentparallelresistanceformedbyR2andtheinputimpendenceofthenextstage,inthiscasetheswitchingcircuits.Thenormalswitchingcircuitimpedancesareontheorderof10MΩ.NormalR2valuesareontheorderof1KΩorlower.The10:
1ratioresultsinthevalueofR2beingthedeterminingfactorinestablishingtheoutputimpedanceoftheDarlingtonpair.thebetaoftypicaltransistorusedinDarlingtonpairsisontheorderof98to99,whichresultsinaninputimpedanceofabout10KΩ,thisisanextremelyhighimpedance,butnothighenough.Toincreasetheimpedanceevenfurther,afeedbacknetworkmaybeconnectedbetweentheemitterofthesecondtransistorandthecollectorofthefirst.thisfeedbackloop,calledbootstrap,isshowninFig.3.3.
Thefeedbackvoltagecreatedbythebootstrapvoltage-dividingnetwork,R1andR2,isinjectedintothecollectorcircuitofthetransistor.Anincreaseinsignallevelattheinputofthecircuitcausesanincreaseinsignalthroughthedivideratthecollector,changingthetransistor’sbiaspoint,ineffectincreasingtheinputimpedance.ThefeedbackdevelopedisproportionaltothevaluesofR1andR2inseries.theinputimpedanceisnormallyin-creasedto50MΩusingthismethod
ItshouldbenotedthatR4infig.3.3andfig.3.2areusedtolimitthecurrentflowingthroughQ2whereasthevoltagegainofthesecircuitsisapproximately0.9,thecurrentandpowergainsarequitehigh.Thesecondtransistorwouldburnoutifwerenotlimited.
Fromthreetofiveelectrodetransducersareusedwithaphysiologicalmonitor.the“waves”detectedbythesetransducersareactuallytheelectro-chemicalvoltagedifferencebetweenanytwooftheelectrodes.Eachelectroderequiresanisolationamplifierbetweenitandtheswitchingcircuits
Signalamplifiersmaybeclassifiedaseitherdcoraccircuits
3.3DCAMPLIFIERS
Thedcamplifieroperatesfromzerofrequencyupintotheaudiofrequencieswithoutappreciablelossofagain.Toillustratethedcamplifier,wechosetousethreemetal-oxidesemiconductorfield-effecttransistors(MOSFETs)connectedincascade(seeFig.3.4).TheMOSFETsusedhavenominaldraincurrentsof3mA.Thedrainpotentialissetat10Vwithrespecttoground.TheinputsignalisappliedtothegateofthefirstMOSFET,amplified,andpassedontothenexttransistor’sgatefromthefirstdrain.Theoutputvoltageistakenfromthecentertapofthevoltagedividersformedbythe100-kΩresistorsatthelasttransistor’sdrain.Thelowerresistorisreturnedto-10Vandbecausethedrainsarebiasedat+10V,thequiescentoutputvoltagerestsatzero.Whenanassourcedrivesthisdcamplifier,anamplifiedoutputvoltageisachieved.
Themajorproblemwithdirect-coupledamplifieristhermaldrift.Thefirststageofamplifiermustbecompensatedsothatachangeintemperaturedoesnotcauseachangeinbiaslevels.Bias-levelchangeswillbeseenbytheamplifierasiftheyregularsignals.Theywillbeamplifierbythefirststageandeachstagefollowingthefirststage.Theaccumulativeeffectwillbealargeoutputswingcausedbyarelativelysmallthermalchange.Compensationcorrectsthiseffect.
Anamplifier’sdcdriftisstatedintermsofchangeinoutputvoltageasafunctionofachangeintemperaturestatedindegreesCelsius.Thetemperaturecompensationusedmustbeexactlyoppositethisdriftandofthesamemagnitude.
AlthoughwehavediscussedtheMOSFETdcamplifier,therearemanyothercircuitsthatfallintothisclass.Theywillallhavesimilaritiesregardlessoftheformtheytake.Alldcamplifiershavebiasinteractionbetweenstages,anabsenceofcouplingcomponents,andstabilizationattainedbyfeedbackcircuits.
Ahigh-gainsignalamplifiedmaybeconstructedbyconnectingthreeRCcoupledamplifierstagesincascade,asshowninFig.3.5.Theinputtothefirststageisplacedbetweenthebaseofthetransistorandthepowersupply’sgroundreference.Theoutputofthefirststageistakenbetweenthetransistor’scollectorsandthepowersupplyground.Thesignalvoltage,V,attheoutputofthefirststageiscoupledtothenextstageviathecouplingcapacitorsCc3.Eachstageconsistsofasinglebipolar-junctiontransistorinthecommonemitterconfiguration,usingauniversalbiascircuit.
TheconventionalNPNtransistorasusedineachstagearebasicallyoff-switchdevicesandmustbebiased“on”totheiroperatingpoint.Forthecurrenttoflow,eachNPNtransistorinthethreestagesmusthaveitsbasemademorepositivethantheemitter.Toincreasethevoltagegainofeachstage,theemitterresistorshavebeenshuntedtogroundbyanemitterbypasscapacitor.Thesecond-stagecouplingcapacitor,CC2,connectsthecollectorofQ1tothebaseofQ2.Asfarasthedcvoltagesareconcerned,CcbehavesasanopencircuitsothatthedccollectorvoltageofQ1doesnotaffectthebiasvoltageatthebaseofQ2.Thequiescentoperatingvoltageforeachstagecanbesetseparately.Theinputvoltage,Vin,capacitivelycoupledtothebaseofQ1,appearsinamplifiedformwitha180°phaseshiftasAVinorV1.Theoutputofthesecondstage,V2isA2Vin,andinthethirdstagetheoutputisV3orA3Vin.
Tomakeaquickinputresistancecalculationforthethree-stageRCcoupledamplifierofFig.3.5,calculatetheinputresistancesofQ1,Q2,andQ3andcalculatedtheemittercurrents.Sincethethreeuniversalbiascircuitsareidentical,calculationofthedcemittercurrentwillapplytothethreetransistors.Thedcbasevoltage,VB,ofthefirststageisdeterminedbymultiplyingtheparallelcombinationofR1andR2bythecollectorvoltage,Asanexample,forFig.3.5if
R1=60KΩ
R2=30KΩ
ECc=12V
then
VB=30(12)or4.0V/(30+60)
ThesmallforwardvoltagedropacrosstheemitterjunctionisneglectedsothattheemittervoltageVEisalso4.0V.TheemitterresistorforeachstageisREandisassumedtobe4.0KΩ.TheemittercurrentIEisthen4V/4KΩ,or1mAforeachstage.Thethreetransistoroprateunderidenticaldcconditionssothattheinputresistanceofeachstageisapproximately50β/IE(mA).Ifβ=100foreachstage,theinputresistanceis50(100)/1,or5kΩ.TheactualinputresistanceofthesecondstageistheparallelcombinationofR1,R2,andRc.IfRcis5kΩ,theinputresistanceofthesecondstageis(20)(5)(1000)/(20+5),or4kΩ.Theactualinputresistanceofthefirst,second,andthirdstageis4kΩ.
3.4POWERAMPLIFIERS
Theprimaryfunctionofpoweramplifiers,alsocalledlarge-signalamplifiers,istosupplydrivingpowerintoaloadwithminimumdistortion.maximumpowergainisachieved.AnamplifierbiasedclassAisthemostcommonlinearcircuitbecausethebiascircuitsarerelativelycomplexbutstable.TheclassAcircuitis,however,theleastpowerefficient.
TheclassABpush-pullamplifieristhemostdesirablewherefairefficiency,largeamountsofpower,andlowdistortionareessential.
ClassBamplifiersproducealargedistortioncharacteristicandarenotsuitableinamplifiersarethemostefficientbiasmodeandareusedinamplitude-insensitivesystems.
Poweramplifiersareusedextensivelyinclinicalapplications.someapplicationsincludeelectrosurgicaloutputcircuit,EKGstylusdrivingcircuits,andrelaydrivingandcontrolcircuits.
3.6DIFFERENTIALAMPLIFIERS
Theamplifiersusedforbipolarmeasurementsuchasencounteredwithelectrocardiogram(ECG),electroencephalogram(ECG),andelectromyogram(EMG)instrumentsaregenerallyclassifiedasdifferentialamplifiers.
Thedifferentialamplifieramplifie
- 配套讲稿:
如PPT文件的首页显示word图标,表示该PPT已包含配套word讲稿。双击word图标可打开word文档。
- 特殊限制:
部分文档作品中含有的国旗、国徽等图片,仅作为作品整体效果示例展示,禁止商用。设计者仅对作品中独创性部分享有著作权。
- 关 键 词:
- 医用 仪器 放大器