外文翻译--信号放大器.doc

Atthemostbasiclevel,asignalamplifierdoesexactlywhatyouexpectitmakesasignalbigger!However,thewayinwhichitisdonevarywiththedesignoftheactualamplifier,thetypeofsignal,andthereasonwhywewanttoenlargethesignal.Wecanillustratethisbyconsideringthecommonexampleofa“Hi-Fi”audiosystem.InatypicalmodernHi-Fi:system,thesignalswillcomefromaunitlikeaCDplayer,FMtuner,oraTape/Minidiskunit.Thesignalstheyproducehavetypicallevelsoftheorderof100mVorsowhenthemusicismoderatelyloud.Thisisareasonablylargevoltage,easytodetectwithsomethinglikeanoscilloscopeoravoltmeter.However,theactualpowerlevelsofthesesignalsarequitemodest.Typically,thesesourcescanonlyprovidecurrentsofafewmilliamps,whichbyP=VImeanspowersofjustafewmilliwatts.AtypicalloudspeakerwillrequirebetweenafewWattsandperhapsover100Wattstoproduceloudsound.HencewewillrequiresomefromofPowerAmplifierto“boost”thesignalpowerlevelfromthesourceandmakeitbigenoughtoplaythemusic.Manypracticalamplifierchaintogetheraseriesofanalogamplifierstagestoobtainahighoverallvoltagegain.Forexample,aPAsystemmightstartwithvoltagesoftheorderof0.1mVfrommicrophones,andboostthistoperhaps10to100Vtodriveloudspeakers.Thisrequiresanoverallvoltagegainof10,soanumberofvoltagegainstageswillberequired.Inmanycaseswewishtoamplifythecurrentsignallevelaswellasthevoltage.Theexamplewecanconsiderhereisthesignalrequiredtodrivetheloudspeakersina“Hi-Fi”system.Thesewilltendtohaveatypicalinputimpedanceoftheorderof8Ohms.Sotodrive,say,100Wattsintosuchaloudspeakerloadwehavetosimultaneouslyprovideavoltageof28Vrmsand3.5Arms.Takingtheexampleofamicrophoneasaninitialsourceagainatypicalsourceimpedancewillbearound100Ohms.Hencethemicrophonewillprovidejust1nAwhenproducing0.1mV.Thismeansthattotakethisanddrive100Wintoaloudspeakertheamplifiersystemmustamplifythesignalcurrentbyafactorofover10atthesametimeasboostingthevoltagebyasimilaramount.Thismeansthattheoverallpowergainrequiredis10i.e.180dB!Thishighoverallpowergainisonereasonitiscommontospreadtheamplifyingfunctionintoseparatelyboxedpreandpoweramplifiers.Thesignallevelsinsidepoweramplifiersaresomuchlargerthantheseweakinputsthateventheslightest“leakage”fromtheoutputbacktotheinputmaycauseproblems.Byputtingthehighpower(highcurrent)andlowpowersectionsindifferentboxeswecanhelpprotecttheinputsignalsfromharm.Inpractice,manydeviceswhichrequirehighcurrentsandpowerstendtoworkonthebasisthatitisthesignalvoltagewhichdeterminesthelevelofresponse,andtheythendrawthecurrenttheyneedinordertowork.Forexample,itistheconventionwithloudspeakersthatthevolumeofthesoundshouldbesetbythevoltageappliedtothespeaker.Despitethis,mostloudspeakershaveanefficiency(theeffectivenesswithwhichelectricalpowerisconvertedintoacousticalpower)whichishighlyfrequencydependent.Toalargeextentthisarisesasanaturalconsequenceofthephysicalpropertiesofloudspeakers.Wewontworryaboutthedetailshere,butasaresultaloudspeakersinputimpedanceusuallyvariesinquiteacomplicatedmannerwiththefrequency.(Sometimesalsowiththeinputlevel.)Thiskindofbehaviorisquitecommoninelectronicsystems.Itmeansthat,ininformationtrms,thesignalpatternisdeterminedbythewaythevoltagevarieswithtime,andideallythecurrentrequiredisthendrawn.Althoughtheaboveisbasedonahighpowerexample,asimilarsituationcanarisewhenasenorisabletogenerateavoltageinresponsetoaninputstimulusbutcanonlysupplyaverylimitedcurrent.Inthesesituationswerequireeitheracurrentamplifierorabuffer.Thesedevicesarequitesimilar,andineachcaseweareusingsomeformofgaindeviceandcircuittoincreasethesignalcurrentlevel.However,acurrentamplifieralwaystriestomultiplythecurrentbyasetamount.Henceitissimilarinactiontoavoltageamplifierwhichalwaystriestomultiplythesignalcurrentbyasetamount.Thebufferdiffersfromthecurrentamplifierasitsetsouttoprovidewhatevercurrentlevelisdemandedfromitinordertomaintainthesignalvoltagetoldtoassert.Henceitwillhaveahighercurrentgainwhenconnectedtoamoredemandingload.Thenatureofelectricalsignalsinelectroniccircuitsreadilyenablesthetechnologytobedividedintoclasses.Oneoftheclassesisanalogelectronics；anotherisdigitalelectronics.Bothanaloganddigitalelectronicsusesimilarelectronicelements,butthemannerofuseisdifferent,andthetechnologiesappeartobequitedistinct.Forthisreasonweshallstudythemseparatelyuntilwebringthemtogetherastheyinvariablyuniteininstrumentationandapplications.Analogelectronicspertainstothosesystemsinwhichtheelectricalvoltageandelectricalcurrentareanalogoustophysicalquantitiesandvarycontinuously.Electroniccircuitsthatreproducemusicmusthavevoltagesandcurrentsthatareproportionaltothesound.Ahighfidelityamplifyingsystemattemptstokeeptheanalogyastrueaspossible.Analogelectroniccircuitsarecarefullydesignedtomaketheelectricalvoltagesandcurrentsfollowtheinputsignal.Ifaninputsignaldoublesinamplitude,theoutputvoltageorcurrentalsoshoulddouble；thisispossiblebecausethecircuitelementsaremadetooperatewithinlimitsthatpreservethelinearity.Anelectricalvoltagethatisproportionaltotemperatureandchangessmoothlyasthetemperaturechangesisananalogoftemperature.Ifthetemperaturerangeisdividedintosmallincrements,thenthetemperaturemaybeindicatedbyadigitaldisplay.Asthetemperature(voltage)changessmoothly,adecisionmustbemadebyanelectronicsystemastothenumericalvaluetobedisplayedasthetemperature.Thecircuitmakingthedecisioniscalledananalog-to-digitalconverter,ADC.Theinverseprocessisaccomplishedbyadigital-to-analogconverter,DAC.Digitalelectroniccircuitsdonotrequirethelinearityofanalogcircuits.Digitalcircuitsactaselectronicswitchesandswitchfromonestatetoanother.Theoutputstate,onoroff,istheonlysignalconditiontobeexamined.Indigitalcircuitstheoutputstateisdeterminedbytheinputsignalsinasdirectamannerastheoutputvoltageofananalogcircuitisrelatedtotheinputsignal.Indigitalcircuitstherelationbetweeninputandoutputstatesareexpressedaslogicequations；theelementsofdigitalelectronicsarecalledlogicgates.Logicgatesswitchbetweenstates,onoroff,veryquicklysothattheymayoperateatmanymegahertzincomputersandotherapplications.Astechnicaldevelopmentscontinuetoprovidenewandamazingintegratedcircuits,astheyhavesincethe1960s,bothanaloganddigitalsystemswillbemorecapable.Thedesignersofelectronicsystemsusingintegratedcircuitswillhaveunlimitedpossibilitiesforinnovation.一般情况下，信号放大器正如人们期望的一样工作-将信号放大。然而，信号的放大方式随着实际放大器的设计、信号的类型以及放大信号目的的不同而变化。这一点可以通过一个常见的高保真音响系统实例来加以说明。在典型的现代高保真系统中，信号是来自于CD播放器、调频收音机或磁带/小型磁盘机等设备。当音乐声大小适当时，这些设备产生信号的幅度大概在100毫伏左右。这种信号幅度相当高，易于用示波器或电压表等仪器检测到。但是，这些信号的实际能量水平并不高。典型情况下，这些信号源只能提供毫安级的电流。根据公式P=VI，其功率只有几十毫瓦。典型的扬声器需要几十瓦到数百瓦的功率才能产生足够大的声音。因此，我们需要某些形式的功率放大器来提高来自信号源的信号功率，使其足以播放音乐。许多实际的放大器将多个放大器级联起来，以获得较高的电压增益。例如，一个功率放大系统的输入是来自于麦克风的0.1毫伏的电压，将其放大到10伏到100伏才能推动扬声器。这就要求电压的总增益达到10的九次方，因此就需要很多放大器级联起来。在很多情况下，除了信号的电压外，我们还要放大信号的电流。这里我们考虑的例子是高保真系统中用来驱动扬声器的信号，其典型的输入电阻约为8欧姆。因此，要驱动100瓦的扬声器负载，就要同时提供28伏的电压和3.5安的电流信号。仍以麦克风作为初始信号源为例，典型的源阻抗在100欧左右。因此，麦克风在产生0.1毫伏的信号时，提供的电流仅为1纳安。这就表示要接受这种输入信号并去驱动100瓦的扬声器，放大电路就必须将信号的电流和电压同时放大10的九次方倍。这也就意味着总的功率增益为10的18次方，即180分贝。一般都将放大功能分散到单独设计的前级放大器和功率放大器中，其原因就在于功率增益很大。功率放大器中的信号幅度比微弱的输入信号大得多，即使输出的极微小的泄漏传输到输入端，都会引发一些问题。通过将大功率（大电流）和小功率放大电路分置在不同的单元中，就可以避免输入信号受到干扰。实际上，许多需要大电流和大功率的设备往往都在特定的条件下工作，即由信号的电压决定响应的幅度，继而由设备吸收其所需要的电流而工作。例如，扬声器的音量通常是由所加电压控制。此外，大多数扬声器的效率（电能被转换为声能的效能）基本上与频率无关。在很大程度上，这是由扬声器的物理特性所产生的自然结果。这里不必考虑具体的细节，但扬声器的输入阻抗随频率的不同而呈复杂的变化（有时也与输入信号的幅度有关）。这种特点在电子系统中很常见。用信息术语来说就是信号类型取决于电压随时间变化的情况，且在理想情况下就能吸收所需的电流。尽管上述情况是基于大功率的例子，但当传感器在输入的激励下做出响应而产生一定的电压，却只能提供有限的电流时，类似的情况也会出现。这时我们就需要一个电流放大器或缓冲器。这些装置非常相似，在各种情况下都可采用一定形式的增益装置或电路来提高信号电流的大小。不过，电流放大器总是设法对电流进行一定的放大。这与电压放大器的功能相似。缓冲器总是可以提供任何你所需要的电流，以便维持其