(8.1.10)--chapter3 Bipolar Junction Transi模拟电子技术基础.ppt

Chapter 3Bipolar Junction Transistors(BJT)OutlineTransistor Construction Common-Base ConfigurationCommon-Emitter Configuration3.1 Transistor ConstructionA simplified structure of the npn transistor.A dual of the npn is called pnp type.This is the simplified structure of the pnp transistor.Transistor Construction3.2 Common-Base ConfigurationThe emitter is distinguished by the arrowhead.Modes of OperationModesEBJCBJApplicationCutoffReverseReverseSwitching application in digital circuitsSaturationForwardForwardActiveForwardReverseAmplifierReverse activeReverseForwardPerformance degradationBasic CharacteristicsFar more useful than two terminal devices(such as diodes)The voltage between two terminals can control the current flowing in the third terminal.We can say that the collector current can be controlled by the voltage across EB junction.Much popular application is to be an amplifierOperation in the Active ModeCurrent flowCurrent equationGraphical representation of transistors characteristicsCurrent FlowCurrent flow in an npn transistor biased to operate in the active mode.Collector CurrentCollector current is the drift current.Carriers are successful excess minority carriers.The magnitude of collector current is almost independent of voltage across CB junction.This current can be calculated by the gradient of the profile of electron concentration in base region.Base CurrentBase current consists of two components.Diffusion current Recombination current Recombination current is dominant.The value of base current is very small.Emitter CurrentEmitter current consists of two components.Both of them are diffusion currents.Heavily doped in emitter region.Diffusion current produced by the majority in emitter region is dominant.Profiles of Minority-Carrier ConcentrationsCurrent EquationCollector current Base currentEmitter currentExplanation for Saturation CurrentSaturation current is also called current scale.Expression for saturation current:Has strong function with temperature due to intrinsic carrier concentration.Its value is usually in the range of 10-12A to 10-18A.Explanation for Common-Emitter Current GainExpression for common emitter current gain:Its value is highly influenced by two factors.Its value is in the range 50 to 200 for general transistor.Explanation for Common-Base Current GainExpression for common base current gain:Its value is less than but very close to unity.Small changes in correspond to very large changes in.RecapitulationCollector current has the exponential relationship with forward-biased voltage vBE as long as the CB junction remains reverse-biased.To behave as an ideal constant current source.Emitter current is approximately equal to collector current.Graphical Representation of Transistors CharacteristicsCharacteristic curve relates to a certain configuration.Input curve is much similar to that of the diode,only output curves are shown here.Three regions are shown in output curves.Early Effect is shown in output curve of CE configuration.Output Curves for CB ConfigurationOutput Curves for CB ConfigurationActive regionEBJ is forward-biased,CBJ is reverse-biased;Equal distance between neighbouring output curves;Almost horizontal,but slightly positive slope.Saturation regionEBJ and CBJ are not only forward-biased but also turned on;Collector current is diffusion current not drift current.Turn on voltage for CBJ is smaller than that of EBJ.Breakdown regionEBJ forward-biased,CBJ reverse-biased;Great voltage value give rise to CBJ breakdown;Collector current increases dramatically.3.3 Common-Emitter Configuration(a)Conceptual circuit for measuring the iC vCE characteristics of the BJT.(b)The iC vCE characteristics of a practical BJT.The Early EffectCurves in active region are more sloped than those in CB configuration.Early voltage.Effective base width and base width modulation.The Early Effect(contd)Assuming current scale remains constant,collector current is modified by this term:Narrow base width,small value of Early voltage,strong effect of base width modulation,strong linear dependence of on .