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1、Ch6 Basic BJT Amplifiers Circuits6.1 Bipolar junction transistors (BJTs)6.2 Single-Stage BJT Amplifiers6.3 Frequency Response6.4 Power AmplifiersReadingsReadings: Gao-Ch7 ; Floyd-Ch3,5,6 Circuits and Analog ElectronicsCircuits and Analog ElectronicsCh6 Basic BJT Amplifiers Circuits6.1 Bipolar juncti
2、on transistors (BJTs)Key WordsKey Words: Construction of BJT BJT in Active Mode BJT DC Model and DC Analysis C-E Circuits I-V Characteristics DC Load Line and Quiescent Operation Point BJT AC Small-Signal ModelCh6 Basic BJT Amplifiers Circuits6.1 Bipolar junction transistors (BJTs)This lecture will
3、spend some time understanding how the bipolar junction transistor (BJT) works based on what we know about PN junctions. One way to look at a BJT transistor is two back-to-back diodes, but it has very different characteristics.Once we understand how the BJT device operates, we will take a look at the
4、 different circuits (amplifiers) we can build with them.Ch6 Basic BJT Amplifiers Circuits6.1 Bipolar junction transistors (BJTs)Ch6 Basic BJT Amplifiers Circuits6.1 Bipolar junction transistors (BJTs)Construction of Bipolar junction transistors Base region(very narrow)Emitter regionCollector regionC
5、ollectorBaseEmitterEmitter-base junctionCollector-base junctionCh6 Basic BJT Amplifiers Circuits6.1 Bipolar junction transistors (BJTs)NPN BJT shown 3 terminals: emitter, base, and collector 2 junctions: emitter-base junction (EBJ) and collector-base junction (CBJ) These junctions have capacitance (
6、high-frequency model) BJTs are not symmetric devices doping and physical dimensions are different for emitter and collectorConstruction of Bipolar junction transistors Ch6 Basic BJT Amplifiers Circuits6.1 Bipolar junction transistors (BJTs)Standard bipolar junction transistor symbolsDepending on the
7、 biasing across each of the junctions, different modes of operation are obtained cutoff, active, and saturationCh6 Basic BJT Amplifiers Circuits6.1 Bipolar junction transistors (BJTs)BJT in Active ModeTwo external voltage sources set the bias conditions for active mode EBJ is forward biased and CBJ
8、is reverse biasedCh6 Basic BJT Amplifiers Circuits6.1 Bipolar junction transistors (BJTs)BJT in Active Mode Forward bias of EBJ injects electrons from emitter into base (small number of holes injected from base into emitter) IEIENIEP IEN Ch6 Basic BJT Amplifiers Circuits6.1 Bipolar junction transist
9、ors (BJTs)BJT in Active Mode Most electrons shoot through the base into the collector across the reverse bias junction Some electrons recombine with majority carrier in (P-type) base regionIBIBNIEP Ch6 Basic BJT Amplifiers Circuits6.1 Bipolar junction transistors (BJTs)BJT in Active Mode Electrons t
10、hat diffuse across the base to the CBJ junction are swept across the CBJ depletion region to the collector.IC=ICN+ICBO Ch6 Basic BJT Amplifiers Circuits6.1 Bipolar junction transistors (BJTs)BJT in Active ModeIEIENIEP IEN IC=ICN+ICBO IE=IB+IC Let ICNIE -common-base current gainIC (1)= IB+ICBO IBIBNI
11、EPCh6 Basic BJT Amplifiers Circuits6.1 Bipolar junction transistors (BJTs)BJT in Active ModeIEIENIEP IEN IC=ICN+ICBO IE=IB+IC IC (1)= IB+ICBO IBIBNIEPLet -common-emitter current gain Beta:Ch6 Basic BJT Amplifiers Circuits6.1 Bipolar junction transistors (BJTs)BJT Equivalent CircuitsBJT DC modelUse a
12、 simple constant-VBE model Assume VBE = 0.7VCh6 Basic BJT Amplifiers Circuits6.1 Bipolar junction transistors (BJTs)BJT DC Analysis Make sure the BJT current equations and region of operation match VBE 0, VBC 0, VE VB VC Utilize the relationships ( and ) between collector, base, and emitter currents
13、 to solve for all currentsCh6 Basic BJT Amplifiers Circuits6.1 Bipolar junction transistors (BJTs)C-E Circuits I-V CharacteristicsBase-emitter Characteristic(Input characteristic)Ch6 Basic BJT Amplifiers Circuits6.1 Bipolar junction transistors (BJTs)C-E Circuits I-V Characteristics Collector charac
14、teristic (output characteristic)Ch6 Basic BJT Amplifiers Circuits6.1 Bipolar junction transistors (BJTs)C-E Circuits I-V Characteristics Collector characteristic (output characteristic)SaturationVsatCh6 Basic BJT Amplifiers Circuits6.1 Bipolar junction transistors (BJTs)C-E Circuits I-V Characterist
15、ics Collector characteristicSaturation occurs when the supply voltage,VCC, is across the total resistance of the collector circuit,RC. IC(sat)=VCC/RCOnce the base current is high enough to produce saturation, further increases in base current have no effect on the collector current and the relations
16、hip IC=IB is no longer valid. When VCE reaches its saturation value, VCE(sat), the base-collector junction becomes forward-biased.Ch6 Basic BJT Amplifiers Circuits6.1 Bipolar junction transistors (BJTs)C-E Circuits I-V Characteristics Collector characteristicCutoffWhen IB=0, the transistor is in cut
17、off and there is essentially no collector current except for a very tiny amount of collector leakage current, ICEO, which can usually be neglected. IC0.In cutoff both the base-emitter and the base-collector junctions are reverse-biased.Ch6 Basic BJT Amplifiers Circuits6.1 Bipolar junction transistor
18、s (BJTs)C-E Circuits I-V Characteristics Collector characteristiclinearityCh6 Basic BJT Amplifiers Circuits6.1 Bipolar junction transistors (BJTs)DC Load Line and Quiescent Operation PointDC load line.QQ-pointICQVCEQVCCCh6 Basic BJT Amplifiers Circuits6.1 Bipolar junction transistors (BJTs)BJT AC Sm
19、all-Signal Model We can create an equivalent circuit to model the transistor for small signals Note that this only applies for small signals (vbe VT) We can represent the small-signal model for the transistor as a voltage controlledcurrent source( )or a current-controlled current source(ic=ib). For
20、small enough signals, approximate exponential curve with a linear line.Ch6 Basic BJT Amplifiers Circuits6.2 Single-Stage BJT AmplifiersKey WordsKey Words: Common-Emitter Amplifier Graphical Analysis Small-Signal Models Analysis Common-Collector Amplifier Common-Base AmplifierCh6 Basic BJT Amplifiers
21、 Circuits6.2 Single-Stage BJT AmplifiersC-E AmplifiersTo operate as an amplifier, the BJT must be biased to operate in active mode and then superimpose a small voltage signal vbe to the base.DC + small signalcoupling capacitor (only passes ac signals)Ch6 Basic BJT Amplifiers Circuits6.2 Single-Stage
22、 BJT AmplifiersC-E AmplifiersCh6 Basic BJT Amplifiers Circuits6.2 Single-Stage BJT AmplifiersC-E AmplifiersvBE=vi+VBEApply a small signal input voltage and see ibCh6 Basic BJT Amplifiers Circuits6.2 Single-Stage BJT AmplifiersC-E Amplifiers vi=0， IB、IC、VCE vo out of phase with vi iC=ic+IC vCE=vce+VC
23、ESee how ib translates into vce.Ch6 Basic BJT Amplifiers Circuits6.2 Single-Stage BJT AmplifiersC-E AmplifiersCh6 Basic BJT Amplifiers Circuits6.2 Single-Stage BJT AmplifiersC-E AmplifiersCh6 Basic BJT Amplifiers Circuits6.2 Single-Stage BJT AmplifiersGraphical AnalysisVCCCan be useful to understand
24、 the operation of BJT circuits. First, establish DC conditions by finding IB (or VBE) Second, figure out the DC operating point for ICCan get a feel for whether the BJT will stay in active region of operation What happens if RC is larger or smaller?Ch6 Basic BJT Amplifiers Circuits6.2 Single-Stage B
25、JT AmplifiersGraphical AnalysisVCCCh6 Basic BJT Amplifiers Circuits6.2 Single-Stage BJT AmplifiersGraphical AnalysisQ-point is centered on the ac load line:VCCVCCCh6 Basic BJT Amplifiers Circuits6.2 Single-Stage BJT AmplifiersGraphical AnalysisClipped at cutoff(cutoff distortion)Q-point closer to cu
26、toff:VCCCh6 Basic BJT Amplifiers Circuits6.2 Single-Stage BJT AmplifiersGraphical AnalysisClipped at cutoff(saturation distortion)Q-point closer to saturation:Ch6 Basic BJT Amplifiers Circuits6.2 Single-Stage BJT AmplifiersGraphical AnalysisCh6 Basic BJT Amplifiers Circuits6.2 Single-Stage BJT Ampli
27、fiersSmall-Signal Models AnalysisSteps for using small-signal models1. Determine the DC operating point of the BJT in particular, the collector current2. Calculate small-signal model parameters: rbe3. Eliminate DC sources replace voltage sources with shorts and current sources with open circuits4. R
28、eplace BJT with equivalent small-signal models5.AnalyzeCh6 Basic BJT Amplifiers Circuits6.2 Single-Stage BJT AmplifiersSmall-Signal Models AnalysisICIB, IE=IC+IB=(1+)IBExemple 1,Ch6 Basic BJT Amplifiers Circuits6.2 Single-Stage BJT AmplifiersSmall-Signal Models AnalysisExemple 1,Ch6 Basic BJT Amplif
29、iers Circuits6.2 Single-Stage BJT AmplifiersSmall-Signal Models AnalysisExemple 2, Find Ri、Ro；Av、 Avs 、vo Ch6 Basic BJT Amplifiers Circuits6.2 Single-Stage BJT AmplifiersSmall-Signal Models AnalysisThere are three basic configurations for single-stage BJT amplifiers: Common-Emitter Common-Base Commo
30、n-CollectorCh6 Basic BJT Amplifiers Circuits6.2 Single-Stage BJT AmplifiersCommon-Collector AmplifierCh6 Basic BJT Amplifiers Circuits6.2 Single-Stage BJT AmplifiersCommon-Collector AmplifierThe last basic configuration is to tie the collector to a fixed voltage, drive an input signal into the base
31、and observe the output at the emitter.Ch6 Basic BJT Amplifiers Circuits6.2 Single-Stage BJT AmplifiersCommon-Collector Amplifier1 Lets find Av， Ai：Ch6 Basic BJT Amplifiers Circuits6.2 Single-Stage BJT AmplifiersCommon-Collector AmplifierLets find Ri：Ch6 Basic BJT Amplifiers Circuits6.2 Single-Stage
32、BJT AmplifiersCommon-Collector AmplifierLets find Ro：Ch6 Basic BJT Amplifiers Circuits6.2 Single-Stage BJT AmplifiersCommon-Collector AmplifierCh6 Basic BJT Amplifiers Circuits6.2 Single-Stage BJT AmplifiersCommon-Collector AmplifierC-C amp characteristics: Gain is less than unity, but close (to uni
33、ty) since is large and rbe is small. Also called an emitter follower since the emitter follows the input signal. Input resistance is higher, output resistance is lower. - Used for connecting a source with a large Rs to a load with low resistance.1 Ch6 Basic BJT Amplifiers Circuits6.2 Single-Stage BJ
34、T AmplifiersCommon-Base AmplifierGround the base and drive the input signal into the emitterCh6 Basic BJT Amplifiers Circuits6.2 Single-Stage BJT AmplifiersCommon-Base AmplifierFor RLRC,Ri=RoRC Ch6 Basic BJT Amplifiers Circuits6.2 Single-Stage BJT AmplifiersCommon-Base AmplifierFor RLRC,Ri=RoRC CB a
35、mp characteristics: current gain has little dependence on is non-inverting most commonly used as a unity-gain current amplifier or current buffer and not as a voltage amplifier: accepts an input signal current with low input resistance and delivers a nearly equal current with high output impedance m
36、ost significant advantage is its excellent frequency responseCh6 Basic BJT Amplifiers Circuits6.3 Frequency ResponseKey WordsKey Words: Basic Concepts High-Frequency BJT Model Frequency Response of the CE AmplifierCh6 Basic BJT Amplifiers Circuits6.3 Frequency ResponseBasic Concepts Time0.5ms1.0ms1.
37、5ms2.0ms2.5ms3.0ms3.5ms4.0msV(1)V(2)-1.0V-0.5V0V0.5V1.0VCh6 Basic BJT Amplifiers Circuits6.3 Frequency ResponseBasic Concepts Time0.5ms1.0ms1.5ms2.0ms2.5ms3.0ms3.5ms4.0msV(1)V(2)-1.0V-0.5V0V0.5V1.0VCh6 Basic BJT Amplifiers Circuits6.3 Frequency ResponseBasic Concepts Frequency0Hz2KHz4KHz6KHz8KHz10KH
38、z12KHz14KHz16KHz18KHz20KHzV(2)V(1)0V200mV400mV600mV800mVCh6 Basic BJT Amplifiers Circuits6.3 Frequency ResponseBasic ConceptsLower-frequencyUpper-frequencyCh6 Basic BJT Amplifiers Circuits6.3 Frequency ResponseIn BJTs, the PN junctions (EBJ and CBJ) also have capacitances associated with themrbeCCCr
39、beCHigh-Frequency BJT ModelCh6 Basic BJT Amplifiers Circuits6.3 Frequency ResponseFrequency Response of the CE AmplifierCrbeCThere are three capacitors in the circuit. At the mid frequency band, these are considered to be short circuits and internal capacitors and are considered to be open circuits.
40、C,CCh6 Basic BJT Amplifiers Circuits6.3 Frequency ResponseFrequency Response of the CE AmplifierAt low frequencies, C1, C2 are an open circuit and the gain is zero. Thus C1 has a high pass effect on thegain, i.e. it affects the lower cutoff frequency of the amplifier.2 is the time constant for C2. T
41、he worst case time constant for the calculation of the lower cutoff frequency is the smallest value, i.e. the value which predicts the highest pole frequency. For this to be the case, the collector input resistance must be calculated. -is neglectedCh6 Basic BJT Amplifiers Circuits6.3 Frequency Respo
42、nseFrequency Response of the CE Amplifier-is neglectedCapacitor Ce is an open circuit. The pole time constant is given by the resistance multiplied by Ce.This equation gives the worst case value for fL. That is, the actual lower cutoff frequency cannot be larger than the value predicted by this equa
43、tion.The frequency that dominates is the highest pole frequency.Ch6 Basic BJT Amplifiers Circuits6.3 Frequency ResponseFrequency Response of the CE AmplifierCrbeCAt high frequencies, C1, C2 Ce are all short circuit. The frequency that dominates is thelowest pole frequency.The time constant is neglec
44、ted forCCrbeCCh6 Basic BJT Amplifiers Circuits6.3 Frequency ResponseFrequency Response of the CE AmplifierCh6 Basic BJT Amplifiers Circuits6.3 Frequency ResponseCrbeCFrequency Response of the CE AmplifierCh6 Basic BJT Amplifiers Circuits6.3 Frequency Responsedecadedecade0Frequency Response of the CE
45、 AmplifierCh6 Basic BJT Amplifiers Circuits6.4 Power AmplifiersKey WordsKey Words: Power Calculation Class-A, B, AB Amplifiers Complementary Symmetry(Push-Pull) Amplifier Biasing the Push-Pull Amplifier (OCL) Single-Supply Push-Pull Amplifier (OTL)Ch6 Basic BJT Amplifiers Circuits6.4 Power Amplifier
46、sPower AmplifiersVoltage AmplifiersSensorLoadAn Analog Electronics System BlockEnergy conversionSignal AmplifiersEnergy conversionCh6 Basic BJT Amplifiers Circuits6.4 Power AmplifiersThe average power delivered by the supply:The output power delivered to the load RL:The efficiency in converting supp
47、ly power to useful output power is defined asCh6 Basic BJT Amplifiers Circuits6.4 Power AmplifiersPower CalculationThe DC power by the supplyThe DC power delivered to BJT by the supplyCh6 Basic BJT Amplifiers Circuits6.4 Power AmplifiersPower CalculationThe average power dissipated as heat in the BJ
48、T:Ch6 Basic BJT Amplifiers Circuits6.4 Power AmplifiersClass-A AmplifiersClass-B AmplifiersCh6 Basic BJT Amplifiers Circuits6.4 Power AmplifiersClass-AB AmplifiersCh6 Basic BJT Amplifiers Circuits6.4 Power AmplifiersComplementary Symmetry Power Amplifier (Class-B)Ch6 Basic BJT Amplifiers Circuits6.4
49、 Power AmplifiersComplementary Symmetry Power Amplifier (Class-B)Ch6 Basic BJT Amplifiers Circuits6.4 Power AmplifiersAssumingforComplementary Symmetry Power Amplifier (Class-B)Ch6 Basic BJT Amplifiers Circuits6.4 Power Amplifiers=78.5% Complementary Symmetry Power Amplifier (Class-B)Ch6 Basic BJT A
50、mplifiers Circuits6.4 Power AmplifiersCrossover distortionComplementary Symmetry Power Amplifier (Class-B)Ch6 Basic BJT Amplifiers Circuits6.4 Power AmplifiersBiasing the Push-Pull Amplifier (Class-AB)(OCL) To overcome crossover distortion, the biasing is adjusted to just overcome the VBE of the tra
51、nsistors; this results in a modified form of operation called class AB. In class AB operation, the push-pull stages are biased into slight conduction, even when no input signal is present.Power Calculation is the same as class-BVCCVCCCh6 Basic BJT Amplifiers Circuits6.4 Power AmplifiersSingle-Supply
52、 Push-Pull Amplifier (OTL)The circuit operation is the same as that described previously, except the bias is set to force the output emitter voltage to be VCC/2 instead of zero volts used with two supplies. Because the output is not biased at zero volts, capacitive coupling for the input and output is necessary to block the bias voltage from the source and the load resistor.
