Bjts

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Bjts

  1. 1. CB E Kristin Ackerson, Virginia Tech EE Spring 2002
  2. 2. Table of ContentsThe Bipolar Junction Transistor_______________________________slide 3BJT Relationships – Equations________________________________slide 4DC  and DC α _____________________________________________slides 5BJT Example_______________________________________________slide 6BJT Transconductance Curve_________________________________slide 7Modes of Operation_________________________________________slide 8Three Types of BJT Biasing__________________________________slide 9 Common Base______________________slide 10-11 Common Emitter_____________________slide 12 Common Collector___________________slide 13Eber-Moll Model__________________________________________slides 14-15Small Signal BJT Equivalent Circuit__________________________slides 16The Early Effect___________________________________________slide 17Early Effect Example_______________________________________slide 18Breakdown Voltage________________________________________slide 19Sources__________________________________________________slide 20 Kristin Ackerson, Virginia Tech EE Spring 2002
  3. 3. The BJT – Bipolar Junction Transistor Note: It will be very helpful to go through the Analog ElectronicsDiodes Tutorial to get information on doping, n-type and p-type materials. The Two Types of BJT Transistors: npn pnpE n p n C E p n p CCross Section C Cross Section C B B B B Schematic Schematic Symbol Symbol E E • Collector doping is usually ~ 106 • Base doping is slightly higher ~ 107 – 108 • Emitter doping is much higher ~ 1015 Kristin Ackerson, Virginia Tech EE Spring 2002
  4. 4. BJT Relationships - Equations IE IC IE IC - VCE + + VEC -E C E C - - + + VBE IB VBC VEB VCB IB + + - - B B npn pnp IE = I B + I C IE = I B + I C VCE = -VBC + VBE VEC = VEB - VCB Note: The equations seen above are for the transistor, not the circuit. Kristin Ackerson, Virginia Tech EE Spring 2002
  5. 5. DC  and DC α  = Common-emitter current gain α = Common-base current gain  = IC α = IC IB IEThe relationships between the two parameters are:α=  = α +1 1-αNote: α and  are sometimes referred to as α dc and  dcbecause the relationships being dealt with in the BJTare DC. Kristin Ackerson, Virginia Tech EE Spring 2002
  6. 6. BJT Example Using Common-Base NPN Circuit Configuration C Given: IB = 50 µ A , IC = 1 mAVCB + _ IC Find: IE ,  , and α IB B Solution:VBE + _ IE IE = IB + IC = 0.05 mA + 1 mA = 1.05 mA  = IC / IB = 1 mA / 0.05 mA = 20 E α = IC / IE = 1 mA / 1.05 mA = 0.95238 α could also be calculated using the value of  with the formula from the previous slide. α=  = 20 = 0.95238 +1 21 Kristin Ackerson, Virginia Tech EE Spring 2002
  7. 7. BJT Transconductance Curve Typical NPN Transistor 1 Collector Current:IC IC = α IES eVBE/η VT8 mA Transconductance: (slope of the curve)6 mA gm =  IC /  VBE4 mA IES = The reverse saturation current of the B-E Junction.2 mA VT = kT/q = 26 mV (@ T=300K) η = the emission coefficient and is 0.7 V VBE usually ~1 Kristin Ackerson, Virginia Tech EE Spring 2002
  8. 8. Modes of OperationActive: • Most important mode of operation • Central to amplifier operation • The region where current curves are practically flatSaturation: • Barrier potential of the junctions cancel each other out causing a virtual shortCutoff: • Current reduced to zero • Ideal transistor behaves like an open switch * Note: There is also a mode of operation called inverse active, but it is rarely used. Kristin Ackerson, Virginia Tech EE Spring 2002
  9. 9. Three Types of BJT BiasingBiasing the transistor refers to applying voltage to get the transistor to achieve certain operating conditions.Common-Base Biasing (CB) : input = VEB & IE output = VCB & ICCommon-Emitter Biasing (CE): input = VBE & IB output = VCE & ICCommon-Collector Biasing (CC): input = VBC & IB output = VEC & IE Kristin Ackerson, Virginia Tech EE Spring 2002
  10. 10. Common-Base Although the Common-Base configuration is not the most common biasing type, it is often helpful in the understanding of how the BJT works. Emitter-Current Curves IC ActiveSaturation Region Region IE Cutoff IE = 0 VCB Kristin Ackerson, Virginia Tech EE Spring 2002
  11. 11. Common-Base VCE IC IECircuit Diagram: NPN Transistor C E VCB VBEThe Table Below lists assumptions IBthat can be made for the attributesof the common-base biased circuitin the different regions of _ + + _operation. Given for a Silicon NPN B VCB VBEtransistor.Region of IC VCE VBE VCB C-B E-BOperation Bias BiasActive  IB =VBE+VCE ~0.7V  0V Rev. Fwd.Saturation Max ~0V ~0.7V -0.7V<VCE<0 Fwd. Fwd. =VBE+VCE  0V NoneCutoff ~0  0V Rev. /Rev. Kristin Ackerson, Virginia Tech EE Spring 2002
  12. 12. Common-Emitter Circuit Diagram VCE Collector-Current Curves IC IC + ActiveVCC _ IB Region IBRegion of DescriptionOperationActive Small base current VCE controls a large collector current Saturation Region Cutoff RegionSaturation VCE(sat) ~ 0.2V, VCE IB = 0 increases with ICCutoff Achieved by reducing IB to 0, Ideally, IC will also equal 0. Kristin Ackerson, Virginia Tech EE Spring 2002
  13. 13. Common-Collector Emitter-Current CurvesThe Common- IECollector biasingcircuit is basicallyequivalent to the Active Regioncommon-emitterbiased circuit exceptinstead of looking at IBIC as a function of VCEand IB we are lookingat IE. VCEAlso, since α ~ 1, andα = IC/IE that means Saturation RegionIC~IE Cutoff Region IB = 0 Kristin Ackerson, Virginia Tech EE Spring 2002
  14. 14. Eber-Moll BJT Model The Eber-Moll Model for BJTs is fairly complex, but it is valid in all regions of BJT operation. The circuit diagram below shows all the components of the Eber-Moll Model: IE ICE C α RIC α RI E IF IR IB B Kristin Ackerson, Virginia Tech EE Spring 2002
  15. 15. Eber-Moll BJT Modelα R = Common-base current gain (in forward active mode)α F = Common-base current gain (in inverse active mode)IES = Reverse-Saturation Current of B-E JunctionICS = Reverse-Saturation Current of B-C JunctionI C = α FI F – I R IB = I E - I CIE = I F - α RIRIF = IES [exp(qVBE/kT) – 1] IR = IC [exp(qVBC/kT) – 1] If IES & ICS are not given, they can be determined using various Tech EE Kristin Ackerson, Virginia Spring 2002
  16. 16. Small Signal BJT Equivalent CircuitThe small-signal model can be used when the BJT is in the active region. The small-signal active-region model for a CB circuit is shown below: iB iC B C rπ  iB rπ = ( + 1) * η VT iE IE @ η = 1 and T = 25° C Erπ = ( + 1) * 0.026 Recall: IE  = IC / IB Kristin Ackerson, Virginia Tech EE Spring 2002
  17. 17. The Early Effect (Early Voltage) ICNote: Common-Emitter Configuration IB-VA VCE Green = Ideal IC Orange = Actual IC (IC’) IC’ = IC VCE + 1 VA Kristin Ackerson, Virginia Tech EE Spring 2002
  18. 18. Early Effect Example Given: The common-emitter circuit below with IB = 25µ A, VCC = 15V,  = 100 and VA = 80. Find: a) The ideal collector current b) The actual collector current Circuit Diagram VCE IC  = 100 = IC/IB a) + VCC _ IB IC = 100 * IB = 100 * (25x10-6 A) IC = 2.5 mAb) IC’ = IC VCE + 1 = 2.5x10-3 15 + 1 = 2.96 mA VA 80 IC’ = 2.96 mA Kristin Ackerson, Virginia Tech EE Spring 2002
  19. 19. Breakdown Voltage The maximum voltage that the BJT can withstand.BVCEO = The breakdown voltage for a common-emitter biased circuit. This breakdown voltage usually ranges from ~20-1000 Volts.BVCBO = The breakdown voltage for a common-base biased circuit. This breakdown voltage is usually much higher than BVCEO and has a minimum value of ~60 Volts. Breakdown Voltage is Determined By: • The Base Width • Material Being Used • Doping Levels • Biasing Voltage Kristin Ackerson, Virginia Tech EE Spring 2002
  20. 20. SourcesDailey, Denton. Electronic Devices and Circuits, Discrete and Integrated. Prentice Hall, New Jersey: 2001. (pp 84-153) 1 Figure 3.7, Transconductance curve for a typical npn transistor, pg 90.Liou, J.J. and Yuan, J.S. Semiconductor Device Physics and Simulation. Plenum Press, New York: 1998.Neamen, Donald. Semiconductor Physics & Devices. Basic Principles. McGraw-Hill,Boston: 1997. (pp 351-409) Web Sites http://www.infoplease.com/ce6/sci/A0861609.html Kristin Ackerson, Virginia Tech EE Spring 2002
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