Bipolar Junction Transistors

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Kristin Ackerson, Virginia Tech EE Spring 2002

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The Bipolar Junction Transistor_______________________________slide 3 BJT Relationships – Equations________________________________slide 4 DC  and DC  _____________________________________________slides 5 BJT Example_______________________________________________slide 6 BJT Transconductance Curve_________________________________slide 7 Modes of Operation_________________________________________slide 8 Three Types of BJT Biasing__________________________________slide 9 Common Base______________________slide 10-11 Common Emitter_____________________slide 12 Common Collector___________________slide 13 Eber-Moll Model__________________________________________slides 14-15 Small Signal BJT Equivalent Circuit__________________________slides 16 The Early Effect___________________________________________slide 17 Early Effect Example_______________________________________slide 18 Breakdown Voltage________________________________________slide 19 Sources__________________________________________________slide 20Table of ContentsKristin Ackerson, Virginia Tech EE Spring 2002

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The BJT – Bipolar Junction TransistorKristin Ackerson, Virginia Tech EE Spring 2002 Note: It will be very helpful to go through the Analog Electronics Diodes Tutorial to get information on doping, n-type and p-type materials.The Two Types of BJT Transistors:npnpnpnpnEBCpnpEBCCross SectionCross SectionBCESchematic SymbolBCESchematic SymbolCollector doping is usually ~ 106 Base doping is slightly higher ~ 107 – 108 Emitter doping is much higher ~ 1015

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BJT Relationships - EquationsKristin Ackerson, Virginia Tech EE Spring 2002BCEIEICIB-+VBEVBC+-+-VCEBCEIEICIB-+VEBVCB+-+-VECnpn IE = IB + IC VCE = -VBC + VBEpnp IE = IB + IC VEC = VEB - VCBNote: The equations seen above are for the transistor, not the circuit.

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DC  and DC Kristin Ackerson, Virginia Tech EE Spring 2002  = Common-emitter current gain  = Common-base current gain  = IC  = IC IB IE The relationships between the two parameters are:  =   =   + 1 1 -  Note:  and  are sometimes referred to as dc and dc because the relationships being dealt with in the BJT are DC.

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BJT ExampleKristin Ackerson, Virginia Tech EE Spring 2002Using Common-Base NPN Circuit Configuration+ _+ _Given: IB = 50  A , IC = 1 mA Find: IE ,  , and  Solution: IE = IB + IC = 0.05 mA + 1 mA = 1.05 mA = IC / IB = 1 mA / 0.05 mA = 20  = IC / IE = 1 mA / 1.05 mA = 0.95238  could also be calculated using the value of  with the formula from the previous slide.  =  = 20 = 0.95238  + 1 21ICIEIBVCBVBEECB

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BJT Transconductance CurveKristin Ackerson, Virginia Tech EE Spring 2002Typical NPN Transistor 1VBEIC2 mA4 mA6 mA8 mA0.7 VCollector Current: IC =  IES eVBE/VT Transconductance: (slope of the curve) gm =  IC /  VBE IES = The reverse saturation current of the B-E Junction. VT = kT/q = 26 mV (@ T=300K)  = the emission coefficient and is usually ~1

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Modes of OperationKristin Ackerson, Virginia Tech EE Spring 2002Most important mode of operation Central to amplifier operation The region where current curves are practically flatActive:Saturation: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.

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Three Types of BJT BiasingKristin Ackerson, Virginia Tech EE Spring 2002Biasing the transistor refers to applying voltage to get the transistor to achieve certain operating conditions. Common-Base Biasing (CB) : input = VEB & IE output = VCB & IC Common-Emitter Biasing (CE): input = VBE & IB output = VCE & IC Common-Collector Biasing (CC): input = VBC & IB output = VEC & IE

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Common-BaseKristin Ackerson, Virginia Tech EE Spring 2002Although 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 CurvesSaturation RegionIEICVCBActive RegionCutoff IE = 0

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Common-BaseKristin Ackerson, Virginia Tech EE Spring 2002Circuit Diagram: NPN TransistorThe Table Below lists assumptions that can be made for the attributes of the common-base biased circuit in the different regions of operation. Given for a Silicon NPN transistor.

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Common-EmitterKristin Ackerson, Virginia Tech EE Spring 2002Circuit Diagram+ _VCCICVCEIBCollector-Current CurvesVCEICActive RegionIBSaturation RegionCutoff Region IB = 0

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Common-CollectorKristin Ackerson, Virginia Tech EE Spring 2002Emitter-Current CurvesVCEIEActive RegionIBSaturation RegionCutoff Region IB = 0The Common-Collector biasing circuit is basically equivalent to the common-emitter biased circuit except instead of looking at IC as a function of VCE and IB we are looking at IE. Also, since  ~ 1, and  = IC/IE that means IC~IE

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Eber-Moll BJT ModelKristin Ackerson, Virginia Tech EE Spring 2002The 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:ECBIRIFIEICIBRIERIC

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Kristin Ackerson, Virginia Tech EE Spring 2002Eber-Moll BJT ModelR = Common-base current gain (in forward active mode) F = Common-base current gain (in inverse active mode) IES = Reverse-Saturation Current of B-E Junction ICS = Reverse-Saturation Current of B-C Junction IC = FIF – IR IB = IE - IC IE = IF - RIR IF = IES [exp(qVBE/kT) – 1] IR = IC [exp(qVBC/kT) – 1]  If IES & ICS are not given, they can be determined using various BJT parameters.

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Kristin Ackerson, Virginia Tech EE Spring 2002Small 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:iBr iEiCiBBCEr = ( + 1) * VT IE@  = 1 and T = 25C r = ( + 1) * 0.026 IE Recall:  = IC / IB

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The Early Effect (Early Voltage)Kristin Ackerson, Virginia Tech EE Spring 2002VCEICNote: Common-Emitter Configuration-VAIBGreen = Ideal IC Orange = Actual IC (IC’) IC’ = IC VCE + 1 VA

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Kristin Ackerson, Virginia Tech EE Spring 2002Early Effect ExampleGiven: The common-emitter circuit below with IB = 25A, VCC = 15V,  = 100 and VA = 80. Find: a) The ideal collector current b) The actual collector currentCircuit Diagram+ _VCCICVCEIB = 100 = IC/IB a) 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

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Breakdown VoltageKristin Ackerson, Virginia Tech EE Spring 2002The 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

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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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Last Updated: 8th March 2018

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