1、The Bipolar TransistorPhysical structureThe Modes of Operation If 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 th
2、is case. This condition is referred to as cutoffall currents in the transistor are zero.cutoffThe npn transistor in a simple circuit. In this conguration, the transistor may be biased in one of three modes of operation.The Modes of OperationWhen the BE junction becomes forward biased, an emitter cur
3、rent will 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 VCC is large enough and if VR is small enough, then VCB 0, which means that the BC junction is reverse biase
4、d for this npntransistor. 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 VR will also increase. The increase in VR means that the reverse-biased CB voltage decreases, or |V
5、CB | decreases. At some point, the collector current may become large enough that the combination of VR and VCCproduces 0 V across the BC junction.The Modes of Operation A slight increase in IC beyond this point will cause a slight increase in VR and the BC junction will become forward biased (VCB 0
6、) .This condition is called saturation. 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 conguration.The Modes of Operation When the col
7、lectoremitter voltage is large enough so that the basecollector junction is reverse biased, the collector current is a constant in this rst-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
8、current.Bipolar transistor common-emitter currentvoltage characteristics with load line superimposed.The Modes of Operation Load 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, superimp
9、osed on the transistor characteristics, can be 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 m
10、ode ： the forward-active mode occurs when the relation IC=IB is valid. Writing a Kirchhoffs voltage equation around the CE loop, we nd(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 ca
11、se the transistor is operating “upside down,” and the roles 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 volt
12、age conditions for the four operating modes of a bipolar transistor.Common-emitter npn bipolar circuit conguration with a time-varying signal voltage Vi included in the baseemitter loop.The Modes of Operation Voltages and currents can be amplied by bipolar transistors in conjunction with other eleme
13、nts. An npn bipolar transistor in a common-emitter conguration. The dc voltage sources, Vbb and VCC, are used to bias the transistor in the forward-active mode. The voltage source Vi represents a time-varying input voltage (such as a signal from a satellite) that needs to be amplied.Amplification wi
14、th Bipolar TransistorsiC=iBCommon-emitter npn bipolar circuit con 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 s
15、uper imposed on the quiescent dc value.Amplification with Bipolar Transistors Since 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 RC resistor which, by Kirc
16、hhoffs voltage law, means that a sinusoidal voltage, superimposed on a dc 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 amplier.iC=iBAmplification with Bipolar Transistors
