射频电路理论与技术-Lectrue-14-(noise-figure)课件.ppt

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1、射频电路理论与技术2021/9/3NoiseDesignMethods1.IntroductionInsomeapplicationsthedesignobjectiveisforaminimumnoisefigure.Sinceaminimumnoisefigureandmaximumpowergaincannotbeobtainedsimultaneously,constantnoisefigurecircles,togetherwithconstantavailablepower-gaincircles,can

2、bedrawnontheSmithchart,andreflectioncoefficientscanbeselectedthatcompromisebetweenthenoisefigureandgainperformance.ThenoiseperformanceoftheGaAsFETissuperiortothatoftheBJTabove4GHz.AminimumnoisefigureinbothBJTsandGaAsFETsisobtainedatlowcollectorordraincurrent.20

3、21/9/32.NoiseinTwo-PortNetworksInamicrowaveamplifier,evenwhenthereisnoinputsignal,asmalloutputvoltagecanbemeasured.Werefertothissmalloutputpowerastheamplifiernoisepower.Thetotalnoiseoutputpoweriscomposedoftheamplifiednoiseinputpowerplusthenoiseoutputpowerproduc

4、edbytheamplifier.ThenoiseinputpowercanbemodeledbyasourceresistorthatproducesthermalorJohnsonnoise.Thisnoiseisproducedbytherandomfluctuationsoftheelectronsduetothermalagitation.2021/9/3ThermsvalueofthenoisevoltageproducedbythenoisyresistorRoverafrequencyrangeisg

5、ivenbykisBoltzmann’sconstant,i.e.,1.374×10-23J/0KTistheresistornoisetemperature,andBisthenoisebandwidth(i.e.,B=fH-fL).Thethermalnoisepowerdependsonthebandwidthandnotonagivencenterfrequency.Suchadistributionofnoiseiscalledwhitenoise.TheavailablenoisepowerfromRis

6、2021/9/3Example2.1CalculatetheavailablenoisepowerfromaresistoratthestandardtemperatureofT=T0=2900Kina1-Hzbandwidth.Calculatethenoisevoltageandtheavailablenoisepowerproducedbya2-MWresistoratastandardtemperature(T=2900K)ina5-kHzbandwidth.Solution.(a)orSincethisno

7、isepowerisfoundina1-Hzbandwidth,itiscommontoexpressitas-174dBm/Hz.(b)and2021/9/3Thenoisefigure(F)describesquantitativelytheperformanceofanoisymicrowaveamplifier.Thenoisefigureofamicrowaveamplifierisdefinedastheratioofthetotalavailablenoisepowerattheoutputofthea

8、mplifiertotheavailablenoisepowerattheoutputduetothermalnoisefromtheinputterminationR,whereRisatthestandardtemperatureT=To=2900K.thetotalavailablenoisepowerattheoutputtheavai