搜索
Chapter2ElectronicsSection4OperationalAmplifiersTextNewWordsandExpressionsExercisesEndTranslationofLongSentencesSection4OperationalAmplifiersIntroductionOperationalamplifiersarehigh-gaindifferenceamplifiers,whichwereperfectedduringWorldWarII.Theybecamethefoundationofanalogcomputers,atonetimeanalogcomputerswerecalleddifferentialanalyzersbecausetheyareusedtosolvedifferentialequations.运算放大器是一种高增益的差分放大器,它在第二次世界大战期间得到完善,并成为模拟计算机的基础.由于运算放大器被用来解差分方程,因此一段时期内模拟计算机被称为差分分析器.运算放大器也是很多重要仪器的主要组成部分.Operationalamplifiersarealsothebasisofmanyimportantinstruments.Section4OperationalAmplifiersTheanalogamplifierconsistsofabasicdifferenceamplifier,implementedbyfeedbackandothercompensatingamplifyingcircuitstogivelinearresponse,stability,freedomfromdrift,andotherdesirableproperties.Thusitismoredifficulttoisolatethelong-termchangesthatarisefromvariationsintemperatureandpower-supplyvoltageandfromothereffectsthatcausetheoutputvoltagetodrift.模拟放大器由基本的差分放大器组成,通过反馈以及其它补偿放大电路实现线性反应,稳定性,来自漂移的自由度以及其它我们所期望的性能.我们要求它具有复杂性,因为运算放大器放大直流以及交流信号,在放大阶段不允许有容性耦合.Thecomplexityisrequiredbecauseoperationalamplifiersamplifydcaswellasacsignals,capacitivecouplingbetweenamplifyingstagesisnotpermitted.因此我们很难隔离由温度的变化和电源电压以及其它引起输出电压漂移的因素而产生的长效变化.Section4OperationalAmplifiersAftertheinventionofthetransistor,solid-stateoperationalamplifierswereintroducedasintegratedcircuits.Nowoperationalamplifiersareusedtomakehigh-quality,low-poweranalogamplifier,anditispossibletoavoiddesigningindividualtransistoramplifierstagesformanyapplication.Formostamplifyingpurposesandformanymeasuringandcontrolapplications,simplearrangementsofoperationalamplifierswithfeedbackcircuitswillmeetthedesigner'sneeds.在发明了晶体管后,固态运算放大器被引入成为集成电路.现在运算放大器被用来制造高品质,低功率的模拟放大器,因而我们可以在很多应用场合免去了设计单个的晶体管放大器的过程.对于大多数要求放大和很多测量和控制应用的场合,具有反馈电路的简单的运算放大器将满足设计者的需要.Section4OperationalAmplifierspackages(DIPs)orinothercompactformsmakesthesolutionofanalogsignalproblemsanalogousinmanyrespectstothesolutionofdigitallogicproblems,thatis,throughtheinterconnectionofintegratedcircuits.Theavailabilityofoperationalamplifiersasintegratedcircuitsintheformofdualin-lineOperationalAmplifierFig.2-5showsthesymbolforanoperationalamplifier.Therearetwoinputs:theonemarkedwithaplussignisthenoninvertinginput,andtheonemarkedwithaminussignistheinvertinginput.以双列直插式封装或以其它密封形式出现的运算放大器做成的集成电路的可用性使模拟信号问题在很多方面类似数字逻辑问题那样得到解决,也就是通过集成电路的相互连接得到解决.运算放大器图2-5显示了运算放大器的符号.它有两个输入端:标有正号的为非倒向输入端,而标有负号的为倒向输入端由运算放大器放大的电压是两个输入端之间的电压Thevoltageamplifiedbytheoperationalamplifieristhevoltagedifferencebetweenthetwoinputs.Section4OperationalAmplifiersTheopen-circuitgainissolarge,l05tol06,thatavoltagedifferenceofonlyafewmicrovoltswillgiveanappreciableoutput.Becausetheoperationalamplifierisadifferenceamplifier,connectionsmustalwaysbemadetobothinputterminalsforproperoperation.Fig.2-5Asymbolforanoperationalamplifier由于开始增益是如此大,从l05到l06,所以仅仅是几微伏的电压差将产生相当大的输出电压.由于运算放大器是一种差分放大器,因此,在两个输入端必须都连接电压才能使之正常工作.Section4OperationalAmplifiersIftwodifferentpositivevoltagesareappliedseparatelytothetwoinputs,theoutputoftheoperationalamplifierwillbeatthemaximum(saturated)valueifthevoltageatthenoninvertinginputislargerthanthevoltageattheinvertinginput.Ifthevoltagesexchangetheirrelativevalues—thatis,iftheinvertinginputexceedsthenoninvertinginput—theoperationalamplifierwillswitchtoaminimumoutput,whichisthesaturatednegativevoltage.如果两个不同的正电压分别被加在两个输入端,而且非倒向输入端的电压大于倒向输入端的电压,那么运算放大器的输出端电压将处于最大值(饱和值)Theswitchingoccursveryquicklywhenevertherelativevaluesofthevoltageschange.如果电压进行相互交换其相对值——也就是,如果倒向输入端的电压大于非倒向输入端的电压———运算放大器将产生一个最小的输出,即为饱和负电压.每当相对电压值改变时这种转换就会很快地发生.Section4OperationalAmplifiersWewillseethatanoperationalamplifierisanexcellentvoltagecomparatorbecauseitproducesalargevoltageswingattheinstantofcrossing,whenanearlierrelationinvoltagemagnitudesisreversedbyonlyafewmicrovolts.Iftheoperationalamplifieristooperateinitslinearrange,averysmallvoltagedifferencemustexistbetweentheinvertingandthenoninvertinginputs.我们将看到运算器是非常好的电压比较器,因为它在瞬间电压变换时产生很大的电压起伏.此时仅仅几微伏的电压将使先前的电压大小关系产生逆转.如果运算放大器工作在其线性范围内,在其倒向输入端和非倒向输入端之间必须存在非常小的电压差.Inanalyzingoperationalamplifyingcircuits,weassume,becauseofthelargeopen-circuitgain,thatthevoltagedifferenceacrosstheinputsisnegligible.在分析运算放大电路时,我们假设由于存在很大的开路电压增益,所以输入端两端的电压差可以忽略不计.Section4OperationalAmplifiersInsomecaseswewillsetthevoltagetozero.Whenaportionoftheoutputsignalisreturnedtotheinputasnegativefeedback,thevoltagedifferencecannotgrowlargebecausetheoutputchangestominimizethevoltagedifferencebetweentheinputs.Twoadditionalpropertiesarepresentinoperationalamplifiers.Theyareaveryhighinputimpedance,approximatelyl06,andalowoutputimpedance,approximatelyl00.在某些场合我们将把电压设为零.当输出信号的一部分返回输入端作为负反馈,电压差就无法增大因为输出改变使输入端的电压差减小.另外两个性质也存在于运算放大器,它们是非常高的输入阻抗,大约l06,,以及很小的输出阻抗,大约l00.Section4OperationalAmplifiersThesepropertiescontributetotheusefulnessofoperationalamplifiersbyallowingsignalsourceswithsmallcurrentcapabilitiestodriveoperationalamplifiersdirectly.Inturn,operationalamplifiersmaydrivedevicesthathaveseveresignalrequirements.Tosummarize,agoodoperationalamplifierischaracterizedbya