Modern Transistors NPN Bipolar Junction Transistor

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Electronic Principles 7th Edition Albert Malvino & David J Bates Rizwan H. Alad & Vasim A. VohraRef. Books Electronic devices and circuit theory – Robert Boylestad Basic Electronics – A. P. Godse & U. A. Bakshi

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Part 1 SyllabusChap. 6 Bipolar Junction Transistors Chap. 7 Transistor Fundamentals Chap. 8 Transistor Biasing Chap. 9 Transistor AC Models Chap. 10 Voltage Amplifiers Chap. 11 CC and CB Amplifiers Chap. 12 Power Amplifiers Chap. 23 Oscillators

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TransistorThree doped regions Emitter, Base and Collector Base region is much thinner as compared to the collector and emitter npn and pnp Emitter is heavily doped, Base is lightly and collector is intermediate Collector regions is physically largest

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Before DiffusionAfter DiffusionEach of Dep. Layer barrier potential app. 0.7 V at 25o CUnbiased transistor is like two back-to-back diodes

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Bipolar Junction Transistors A bipolar transistor essentially consists of a pair of PN Junction diodes that are joined back-to-back. There are therefore two kinds of BJT, the NPN and PNP varieties. The three layers of the sandwich are conventionally called the Collector, Base, and Emitter.

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Modern Transistors

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NPN Bipolar Junction TransistorOne N-P (Base Collector) diode one P-N (Base Emitter) diode

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PNP Bipolar Junction TransistorOne P-N (Base Collector) diode one N-P (Base Emitter) diode

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Analogy with Transistor :Fluid-jet operated Valve

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*The Biased TransistorHeavily doped emitter inject free electrons into the base Lightly doped base pass electrons on to the collector Collector collects or gathers electrons from the base Biasing method – Emitter junction FB Collector junction RB

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SummaryForward biased emitter diode, forcing the free electrons in the emitter to enter the base Thin and lightly doped base diffuse electrons into collector Collector, through RC and into the positive terminal of VCC

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Figure 5.3 Current flow in an npn transistor biased to operate in the active mode. (Reverse current components due to drift of thermally generated minority carriers are not shown.)

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Transistor CurrentsIE – Largest emitter current Emitter electrons flow to the collector, IC ≈ IE IB ≤ 0.01 IC KCL, IE = IC + IB

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BJT  and From the previous figure IE = IB + IC Define dc = IC / IE DC alpha is slightly less than 1 Low power transistor αdc > 0.99 and High power transistor αdc > 0.95 Define dc = IC / IB - known as a current gain

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BJT  and  Then dc = IC / (IE –IC) = dc /(1- dc) Assignment – Derive dc = dc /(1+ dc) Then IC = dc IE & IB = (1-dc) IE Solved Example 6.1, 6.2, 6.3

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NPN BJT Current flow

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The CE connectionCE, CC and CB CE because emitter is common to both VBB and VCC Left loop – Base loop Right loop – collector loop

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The CE connectionBase Loop, VBB source and RB – current limiting resistor Changing VBB or RB, change base current and IB Change than IC change IB controls IC

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NotationDouble SubscriptsVoltage source – VBB and VCC VBE – voltage between points B and E VCE – voltage between points C and ESingle SubscriptsUsed for Node voltages VB – voltage between base and ground VC and VE VCE = VC – VE VCB and VBE

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The Base Curve / Input Characteristics Graph IB versus VBE Like ordinary diode Ohm’s low to Base loop Ideal diode VBE = 0 and second app. VBE = 0.7 V

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Collector Curve / output Characteristics Graph IC versus VCE Ohm’s low to Collector loop Fixed value of based current, vary VCC and measure IC and VCE

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Transistor CharacteristicsInput CharacteristicsOutput Characteristics

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Active Region, Constant collector currentAfter collector diode reverse biased, it collect all the electrons that reach its deplation layer Further increased VCE cannot increased IC Collector can collect only those free electrons that emitter injects VCE > VCE(max), collector diode break down Power Dissipation PD = VCEIC PD < PD(max)

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Operating Region of TransistorActive region, middle region – normal operation of transistor Emitter diode – FB and Collector diode – RB Breakdown region – transistor will be destroyed Saturation region – rising part of curve, VCE between zero and few tenth of volt Collector diode has insufficient positive voltage to collect all the free electrons injected into the base

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Operating Region of TransistorCut off region – IB = 0 but still small collector current Because collector diode RB – Reverse minority carrier + Surface leakage current

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Last Updated: 8th March 2018

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