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1、The Bipolar TransistorPhysical structureThe Modes of OperationIf the BE voltage is zero or reverse biased(VBE0),then majority carrier electrons from the emitter will not be injected into the base.The BC junction is also reverse biased;thus,the emitter and collector currents will be zero for this cas
2、e.This condition is referred to as cutoffall currents in the transistor are zero.cutoffThe npn transistor in a simple circuit.In this configuration,the transistor may be biased in one of three modes of operation.The Modes of OperationWhen the BE junction becomes forward biased,an emitter current wil
3、l be generated as we have discussed,and the injection of electrons into the base results in a collector current.We may write the KVL equations around the collectoremitter loop as If VCCis large enough and if VRis small enough,then VCB0,which means that the BC junction is reverse biased for this npnt
4、ransistor.Again,this condition is the forward-active region of operation.forward-active region of operation As the forward-biased BE voltage increases,the collector current and hence VRwill also increase.The increase in VRmeans that the reverse-biased CB voltage decreases,or|VCB|decreases.At some po
5、int,the collector current may become large enough that the combination of VRand VCCproduces 0 V across the BC junction.The Modes of OperationA slight increase in ICbeyond this point will cause a slight increase in VRand the BC junction will become forward biased(VCB0).This condition is called satura
6、tion.In the saturation mode of operation,both BE and BC junctions are forward biased and the collector current is no longer controlled by the BE voltage.SaturationAn npn bipolar transistor in a common-emitter circuit configuration.The Modes of OperationWhen the collectoremitter voltage is large enou
7、gh so that the basecollector junction is reverse biased,the collector current is a constant in this first-order theory.For small values of CE voltage,the basecollector junction becomes forward biased and the collector current decreases to zero for a constant base current.Bipolar transistor common-em
8、itter currentvoltage characteristics with load line superimposed.The Modes of OperationLoad line：Equation(13.8)shows a linear relation between collector current and collector emitter voltage.This linear relation is called a load line.The load line,superimposed on the transistor characteristics,can b
9、e used to visualize the bias condition and operating mode of the transistor.Cutoff mode：The cutoff mode occurs when IC=0；Saturation：saturation occurs when there is no longer a change in collector current for a change in base current；Forward-active mode：the forward-active mode occurs when the relatio
10、n IC=IBis valid.Writing a Kirchhoffs voltage equation around the CE loop,we find(13.8)The Modes of OperationThis fourth mode,known as inverse active,occurs when the BE junction is reverse biased and the BC junction is forward biased.In this case the transistor is operating“upside down,”and the roles
11、 of the emitter and collector are reversed.We have argued that the transistor is not a symmetrical device;therefore,the inverse-active characteristics will not be the same as the forward-active characteristics.Inverse activeJunction voltage conditions for the four operating modes of a bipolar transi
12、stor.Common-emitter npn bipolar circuit configuration with a time-varying signal voltage Vi included in the baseemitter loop.The Modes of OperationVoltages and currents can be amplified by bipolar transistors in conjunction with other elements.An npn bipolar transistor in a common-emitter configurat
13、ion.The dc voltage sources,Vbband VCC,are used to bias the transistor in the forward-active mode.The voltage source Virepresents a time-varying input voltage(such as a signal from a satellite)that needs to be amplified.Amplification with Bipolar TransistorsiC=iBCommon-emitter npn bipolar circuit con
14、fi guration with a time-varying signal voltage Vi included in the baseemitter loop.Input sinusoidal signal voltageSinusoidal base and collector currents superimposed on the quiescent dc values.Sinusoidal voltage across the RC resistor super imposed on the quiescent dc value.Amplification with Bipola
15、r TransistorsSince iC=iB,then a relatively large sinusoidal collector current is superimposed on a dc value of collector current.The time-varying collector current induces a time-varying voltage across the RCresistor which,by Kirchhoffs voltage law,means that a sinusoidal voltage,superimposed on a d
16、c value,exists between the collector and emitter of the bipolar transistor.The sinusoidal voltages in the collectoremitter portion of the circuit are larger than the signal input voltage Vi,so that the circuit has produced a voltage gain in the time-varying signals.Hence,the circuit is known as a voltage amplifier.iC=iBAmplification with Bipolar Transistors