三,网络元件与偏置电路对噪声的影响

三、网络元件与偏置电路对噪声的影响Theinfluencesofnetcomponents&biascircuitsonnoise1、串、并联网络对噪声的影响Theinfluenceofseries、parallelnetworkonnoisea.并联耦合网络b.串联耦合网络parallelcouplingnetworkseriescouplingnetwork图中：：源阻抗，；SourceimpedancesZsssjXRZ：并联网络阻抗，；Parallelnetworkimpedance：串联网络阻抗，；Seriesnetworkimpedance：源电阻热噪声电压，；Thermalnoisevoltageofsourceresistance：并联阻抗实部噪声电流；Therealpartofthenoisecurrentofparallelimpedance.：串联阻抗实部噪声电压；Therealpartofthenoisevoltageofseriesimpedance.、：放大器噪声模型；Noisemodelofaamplifier：放大器电压增益；Amplifier’svoltagegain：输出噪声电压。OutputnoisevoltagecscscsjXRZcpZcsZcpcpcpjXRZnsEfkTREsns42cpIcsEnEnIvAnoE（1）并联网络Parallelnetwork各噪声源在输出端开路时的分量为Thecomponentsofeachnoisesourceonoutputend(opencircuit)are总输出噪声均方值为Thetotaloutputnoisemeansquarevalueis(3.1.75)vcpscpnsEnoAZZZEEns)(vcpscpscpvcpscpInoAZZZZIAZZIEcp)//()(vnEnoAEEn)(vcpscpsnvcpsnInoAZZZZIAZZIEn)//()(22)(2)(2)(2)(2vInoEnoInoEnonoAEEEEEnncpns22222222vcpscpsnncpscpscpcpscpnsAZZZZIEZZZZIZZZE网络电压传输增益为Thenetworkvoltagetransmissiongainis(3.1.76)等效输入噪声为Theequivalentinputnoiseis(3.1.77)(3.1.78)表明Showingthat若欲减小并联网络对噪声的影响，需使并联网络阻抗的直流电阻尽可能增大，同时使上的噪声电流尽可能减小。Toreducetheinfluenceofparallelnetworkonnoise,theDCresistanceoftheparallelnetworkimpedanceshouldbeincreasedaslargeaspossible,andthenoisecurrentonthedecreasedassmallaspossible.vcpscpsvcpscpsssovAZZZVAZZZVVVK222222222)(1sncpcpsnnsvnoniZIIZZEEKEE222222222)()1(1nssncpnscpsnnsniEZIIEZZEEEFcpRcpRcpIcpRcpRcpI中包含两个部分Theincludestwoparts：1热噪声电压，它与成反比；Thethermalnoisevoltage,whichisinverselyproportionaltothe.2流过的低频噪声电流，它与流过的直流电平成正比。Thelow-frequencynoisecurrentflowingthroughthe,whichisdirectlyproportionaltotheDClevelofthe.故欲减小，还需尽量减小上的直流压降。Therefore,theDCvoltagedroponthehastobereducedtoreducethe.（2）串联网络Seriesnetwork先求出各噪声源在输出端开路时输出的分量分别为：Theoutputcomponentsofeachnoisesource(opencircuit)arecpIcpRcpRcpRcpRcpIcpRcpRcpRcpIcpIcpR得总输出噪声电压均方值为Thenthetotaloutputnoisevoltagemeansquarevalueis电压传输增益为Thevoltagetransmissiongainis等效输入噪声为Theequivalentinputnoiseis(3.1.79)vnsEnoAEEns)(vcsEnoAEEcs)(vnEnoAEEn)(vcssnInoAZZIEn)()(2222222)(vcssnncsnsnoAZZIEEEEvsvsssovAVAVVVK22222222)(cssnncsnsvnoniZZIEEEKEE表明Showingthat:欲减小串联网络对噪声的影响，应使尽可能小，并尽量减小上的噪声电压，上的直流压降也应尽量减小。Allthe,thenoisevoltageontheandtheDCvoltagedropontheshouldbedecreasedassmallaspossibletoreducetheinfluenceoftheseriesnetworkonthesystemnoise.（3）并串联网络（导出过程留在习题课中解决）ThenoiseanalysisofParallel-SeriesNetworks(Thederivationwillbesolvedinexercises-lesson)(3.1.79)csEcsRcsRcsRcsEcsRcsRcsR22222222211]Z)ZZ(Z[IZI)ZZ)(EE(EEscpscsnscpcpsncsnsni（4）串并联网络ThenoiseanalysisofSeries-ParallelNetworks(3.1.80)上二式表明:若欲减小网络对噪声的影响，对并联网络，要尽可能提高并联阻抗的数值；对串联网络，要尽可能减小串联电阻抗的数值。同时，两者都应尽可能减小网络阻抗上的压降，以减小低频噪声。Toreducetheinfluenceofanetworkonnoise,theparallelimpedanceshouldbeincreasedaslargeaspossibleforaparallelnetwork;theseriesimpe-danceshouldbedecreasedassmallaspossibleforaseriesnetwork.Meanwhile,thevoltagedroponthenetworkimpedanceofthetwoshouldbedecreasedtoreducethelow-frequencynoise.22222222))(()1(cssncpcpcssncsnsniZZIIZZZEEEE2、偏置电路噪声的影响和元件的选取Theinfluenceofbiascircuitnoise&component’sselection偏置电路对放大器噪声的影响不可忽视，偏置元件的热噪声和低频噪声通过放大器放大可使系统噪声特性变坏。Theinfluenceofbiascircuitonanamplifier’snoisecannotbeignored.Thethermalnoiseofbiascomponentsandlow-frequencynoisecandegene-ratesystem’snoisecharacteristicswhentheypassthroughtheamplifier.（1）晶体管共发放大器电路及其噪声模型Thecommon-emittercircuitofatransistor&itsnoisemodel图中：、、、：上下偏置电阻及其噪声电压；Up-downbiasresistancesandtheirnoisevoltages、：发射极反馈电阻及其噪声电压；Emitterfeedbackresistanceanditsnoisevoltage、：发射极电阻及其噪声电压；Emitterresistanceanditsnoisevoltage：集电极电阻；Collectorresistance：基极耦合电容；Basecouplingcapacitance：发射极旁路电容；Emittershuntcapacitance（2）噪声分析Noiseanalysis依对串并联网络的噪声分析(3.1.80)及晶体管共发放大器等效电路可知:1R2R1E2EfRfEeReEcRcCeCAccordingtothenoiseanalysis(3.1.80)ofseries-parallelnetworkandtheequivalentcircuitofthetransistorcommon-emitteramplifier,weknowthat1偏置电阻、与源电阻并联，阻值越大对噪声影响越小，当满足时，偏置电阻对噪声的影响可略；Biasresistances、areinparallelwithsourceresistance.Thelargertheresistanceis,thesmallertheinfluenceonnoiseis.Whenfulfillingtheconditionof,theinfluenceonnoisecanbeomitted.2为使偏置电阻有良好的温度稳定性，取值要低，一般需使其保证流过的电流为基极电流的倍。Toobtaingoodtemperaturestabilityofthebiasresistance,theshouldbelowenough.Usually,itshouldbeensuredthatthecurrentflowingthroughtheistimesofthatpassingthebase.综上二条，低噪放大器的设计应兼顾上述两方面要求。nnsoptIERRR21//1R2R10~521//RR21//RRnnsoptIERRR21//1R2R10~521//RR21//RRAsasummary:thedesignoflownoiseamplifiershouldsatisfytheabovetwopoint.（3）实际电路Apracticalcircuit为减小偏置电阻对放大器噪声的影响，可采用“无噪声偏置电路”如下：Toreducetheinfluenceofbiasresistanceonamplifier’snoise,thefollowing“non-noisebiascircuit”canbeadopted:a、无噪声偏置电路b、噪声等效电路Non-noisebiascircuitNoiseequivalentcircuit[电路等效输入噪声均方值及噪声系数的推导可留做作业或试题]这种电路在满足一定数量关系时可减小偏置电路对噪声的影响。由共发放大器等效噪声分析结论1可知其数量关系为：Thiskindofcircuitscanreducetheinfluenceofthebiascircuitonthenoise,whencertainmagnituderelationissatisfied.Fromtheconclusion1oftheequivalentnoiseanalysisofthecommon-emitteramplifier,weknowtherelationis(3