双极性三极管开关特性

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1、双极性三极管开关特性晶体三极管工作于截止区时，内阻很大，相当于开关断开状态；工作于饱和区时，内阻很低，相当于开关接通状态。三极管开关电路如图2.2.2(a)示。输入控制信号为矩形电压脉冲，电源电压,输出信号为,三极管开关电路输入输出波形如图2.2.2(b)。下实例中为12V的开关控制信号，为单片机可接收的TTL信号，为了与输入控制信号一致，加入反相器74LS14。TransistorsATransistorisasolid-statedevicedesignedtocontrolDCcurrent.Transistorsaremostcomm

2、onlyfoundinlowDCpoweredsensorsastheoutputswitch.Therearetwotypesoftransistors-NPNandPNP.ThefigurebelowshowsaNPN(CurrentSink)OpenCollectorTransistorFigure1:SensorNPNOutputOutputStyleDependingonmodel,incrementalencodersareavailablewithseveraldifferentelectricaloutputstyles.Ch

3、oiceofsignaldependsonreceivinginstrumentandcabledistance.Linedriveroutputswithcomplimentaryoutputscanbeusedwithlongercablesasnoisespikescanbecancelled.NPNUsesanNPNtypetransistorandaninternalresistorpullinguptothepowersupplyrail. Theoutputisanactivevoltage. NPNOpenCollectorU

4、sesanNPNtypetransistorbutwithoutaninternalpullupresistortothesupplyrail.Theoutputispassivesoaseparatepowersupplycanbeused. PNPUsesaPNPtypetransistorandaninternalresistorpullingdowntozerovolts. PNPOpenCollectorUsesaPNPtypetransistorbutwithoutaninternalpulldownresistortozerov

5、olts. PushPullAproblemwithNPNandPNPtypeoutputsisthehighoutputimpedance.Thiscanbesolvedbyacomplementaryoutputallowingbetterswitchingtozeroandpositivesupplyrails. LineDriverThisoutputstylehastwocomplimentaryoutputsperchannelallowingbettertransmissioninnoisyenvironmentsandlong

6、cablelengths.Thereceivercanprocessthesignal,eliminatingnoisespikes. PTCprotectionApositivetemperaturecoefficientresistorcanbeaddedtotheoutputofaNPNorPNPencoder,protectingitfromoutputshortcircuits. Sinking/SourcingSinkingsensorsallowcurrenttoflowintothesensortothevoltagecomm

7、on,whilesourcingsensorsallowcurrenttoflowoutofthesensorfromapositivesource.Forbothofthesemethodstheemphasisisoncurrentflow,notvoltage.Byusingcurrentflow,insteadofvoltage,manyoftheelectricalnoiseproblemsarereduced.Whendiscussingsourcingandsinkingwearereferringtotheoutputofth

8、esensorthatisactinglikeaswitch.Infacttheoutputofthesensorisnormallyatransistor,tha