放大器设计电路

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1、Sheet1of14BalancedAmplifiersThesingleamplifiermeetsthespecificationfornoisefigureandagainbutfailstomeetthereturnlossspecificationduetothelargemis-matchesontheinput&outputs.Toovercomethisproblemonesolutionistouseabalancedamplifiertopography,whichisshowninFigure1.NOTEDashedlinesarereflectedsignalsFigu

2、re1Schematicdiagramofabalancedamplifierusingtwoquadraturehybrids(egLangeCouplers).Thebalancedamplifieremploystwoquadraturehybridsinthiscasetwolangecouplers(althoughbranchlinecouplerscanbeused).Anyreflectionsofanincidentsignalontheinputduetothepoormatchoftheamplifierswillbechannelledbackthroughtheinp

3、utlangetothe50ohmloadwheretheywillbeabsorbed,andsimilarilyonthetheoutput.Thereforeifwelookintotheamplifierwewilleffectively‘see’the50ohmloadsandwillthereforepresentagoodmatchmatch.Inadditionthisconfigurationwillgiveusanextra3dB’sofoutputpower(lesstheinsertionlossoftheLangecoupler~typically<0.25dB)an

4、dalsothe1dBcompressionwillbeapproximately3dB’shigher(Inotherwordsthecircuitwillbeabletohandledoublethepowerwithoutdistortion).Themaindrawbackofthiscircuitisthepowerrequiredfortwoamplifiersinsteadofone.AmplifierlinearityAllamplifiersaredesignedtoworkoveragivendynamicrangewheretheamplifiershouldbehave

5、linearily,andgenerallywithLNA’sthisisthecasetheinputsignalsbeingreceivedareverysmall.Howevertheremaybelargesignalsoutofbandthatmayde-sensitizetheLNA(iereduceit’sgainandthereforeeffectivelyincreasetheNoisefigureoftheoverallsystem)ormaygenerateintermodulationproductsthatbecomemixedwithotherproductsfur

6、therdownthereceiverinthemixerformingspuriousresponses.Sheet2of14ThegraphinFigurexxshowsatypical1dBcompressioncharacteristicforanamplifier.TheDynamicrangeischaracterisedbylineargainiewheretheoutputpowerriseslinearlywithaninputpower.Butasthe1-dBcompressionpointisnearedtheoutputpowerbeginstoleveloutatt

7、hesaturatedoutputpowerlevel.Atthispointfurtherincreasesininputpowerfailtoraisetheoutputpowerandineffectthegainhasfallento0dB.OfagreaterconsequencetoLNAdesignisthelevelofintermodulationproductsthatarep