(8.1.1)--Chapter5 BJT AC Analysis模拟电子技术基础.ppt
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1、Chapter5 BJT AC Analysis Chapter 5 BJT AC AnalysisChapter5 BJT AC AnalysisOutline IntroduceAmplification in the AC DomainBJT Transistor Modeling The re Transistor ModelHybrid ModelCommon-EmitterFixed-Bias ConfigurationEmitterfollower ConfigurationCommon-Base ConfigurationCollector Feedback Configura
2、tionCollector DC Feedback ConfigurationChapter5 BJT AC Analysis5.1 Introduce by fixing VBEby fixing IB.Chapter5 BJT AC AnalysisClassical Discrete Circuit Bias ArrangementBoth result in wide variations in IC and hence in VCE and therefore are considered to be“bad.”Neither scheme is recommended.Chapte
3、r5 BJT AC AnalysisClassical Biasing for BJTs Using a Single Power SupplyCircuit with the voltage divider supplying the base replaced with its Thvenin equivalent.Stabilizing the DC emitter current is obtained by considering the negative feedback action provided by REChapter5 BJT AC AnalysisClassical
4、Biasing for BJTs Using a Single Power SupplyTwo constraintsRules of thumbChapter5 BJT AC AnalysisTwo-Power-Supply VersionResistor RB can be eliminated in common base configuration.Resistor RB is needed only if the signal is to be capacitively coupled to the base.Two constraints should apply.Chapter5
5、 BJT AC AnalysisBiasing Using Current Source(a)Q1 and Q2 are required to be identical and have high.(b)Short circuit between Q1s base and collector terminals.(c)Current source isnt ideal due to finite output resistor of Q2Chapter5 BJT AC Analysis5.2 Amplification in the AC DomainThis is the two-port
6、 network of amplifier.Voltage signal source.Output signal is obtained from the load resistor.Chapter5 BJT AC AnalysisDefinitionsInput resistance with no loadInput resistanceOpen-circuit voltage gainVoltage gainChapter5 BJT AC AnalysisDefinitions(contd)Short-circuit current gainCurrent gainShort-circ
7、uit transconductanceChapter5 BJT AC AnalysisDefinitions(contd)Open-circuit overall voltage gainOverall voltage gainChapter5 BJT AC AnalysisDefinitions(contd)Output resistance of amplifier properOutput resistanceChapter5 BJT AC Analysis5.3 BJT Transistor Modeling Voltage amplifierTransconductance amp
8、lifierVoltage amplifierChapter5 BJT AC Analysis5.4 The re Transistor Model a.Determine the DC operating point of BJT and in particular the DC collector current IC(ICQ).b.Calculate the values of the small-signal model parameters,such as gm=IC/VT,r=/gm=VT/IB,re=/gm=VT/IE.c.Draw ac circuit path.d.Repla
9、ce the BJT with one of its small-signal models.The model selected may be more convenient than the others in circuits analysis.e.Determine the required quantities.Chapter5 BJT AC AnalysisRelationshipsVoltage divided coefficientChapter5 BJT AC AnalysisBasic StructureBasic structure of the circuit used
10、 to realize single-stage,discrete-circuit BJT amplifier configurations.Chapter5 BJT AC Analysis5.5 Hybrid Model Internal capacitanceThe base-charging or diffusion capacitanceJunction capacitancesThe base-emitter junction capacitanceThe collector-base junction capacitanceHigh frequency small signal m
11、odelCutoff frequency and unity-gain frequencyChapter5 BJT AC AnalysisThe Base-Charging or Diffusion CapacitanceDiffusion capacitance almost entirely exists in forward-biased pn junctionExpression of the small-signal diffusion capacitanceProportional to the biased currentChapter5 BJT AC AnalysisJunct
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