Balun_Design

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1、Sheet1of6BalunDesignInthedesignofmixers,push-pullamplifiers,balunsareusedtolinkasymmetrical(balanced)circuittoaasymmetrical(unbalanced)circuit.Balunsaredesignedtohaveaprecise180-degreephaseshift,withminimumlossandequalbalancedimpedances.Inpoweramplifierslossofsymmetry

2、willdegradeefficiencyandthesymmetricalportmustbewellisolatedfromgroundtoeliminateparasiticoscillations.Thebasicconstruction/designofabalunconsistsoftwo90-degreephasinglinesthatprovidetherequired180-degreesplit,andthisinvolvestheuseofλ/4andλ/2.Awire-woundtransformerpro

3、videsanexcellentbalun.MiniaturewirewoundtransformersarecommerciallyavailablecoveringfrequenciesfromlowkHztobeyond2GHz.Theyareoftenrealisedwithacentre-tappedsecondarywinding,ifgroundedthisprovidesashortcircuittoeven-mode(common-mode)signalswhilsthavingnoeffectonthediff

4、erential(odd-mode)signal.Wire-woundtransformersaremoreexpensivethantheprintedorlumpedelementbalunsdescribedbelow,whichfindgreateradoptioninpracticalmixerdesigns.Itshouldbenotedthatmostoftheselumpedelementandprintedbalunsdonotprovidethecentre-tappedgroundtoevenmodesign

5、alsandthisfactmustbeaccountedforinthemixerdesign.(1)L-CBalunThisdesignisessentiallyabridgeandisknownasa‘lattice-type’balun.Itconsistsoftwocapacitorsandtwoinductors,whichproducethe±90degreephaseshifts.Thediagrambelow(Figure1)showsthecircuitdiagramoftheBalun.LRICBalance

6、dRLInputCUnbalancedOutputLFigure1SchematicdiagramofaL-Clumpedbalun.Atoperatingfrequencyω=2πfandZc=R.RILZc1L=;C=ωω.ZcSheet2of6Whendesigningthiscircuitmakesurethattheoperatingfrequencyiswellbelowtheself-resonantfrequenciesofthecomponentsandtakeaccountofpadcapacitances.O

7、neofthemainapplicationsofthiscircuitisontheoutputofapush-pullamplifier,whichprovidesabalancedsignalandwewanttoconverttoasingleun-balancedoutput.NormallyawoundtoroidstyleofbalunisusedasshowninFigure2.VccBalunUnbalancedO/PFigure2Useofawire-woundbalunontheoutputofapush-p

8、ullamplifierstagetoprovideabalancedtounbalancedconversion.Sheet3of6However,itmaybeconvenientespeciallyatchipleveltorealiseth