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THE BJT (BIPOLAR JUNCTION TRANSISTOR)
INTRODUCTION
2015-1
Prof. Gustavo Patiño. M.Sc. Ph.DMJ 12- 1407-05-2015
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THE BJT: THE BASICS
The transistor (name itself) comes from the phrase “transferring and electrical signal across a resistor”.
It is a three-terminal device that is capable of controlling current flow.
A small current at the input of the device can control larger currents at its output. Thus, we are able to build circuits that can amplify.
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PHYSICAL STRUCTURE OF TRANSISTORS
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BJT DEFINED BY PN JUNCTIONS
Junction:Thin layer where p-type and n-type materials meet.
Terminals:E - Emitter - Injects electrons (or holes)B - Base - Controls current flowC - Collector - Collects electrons (or holes)
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TRANSISTOR OPERATION: CURRENT FLOW
Current Flow in an NPN transistor biased to operate in the active mode: For an NPN emitter electrons are
the majority carriers (n-type material has an excess of free electrons).
Electrons begin to move from E to B when a forward voltage VBE is applied (VBE ≥ 0.7V).
A tiny fraction of this injected stream of electrons are recombined in the p-type base region and flow out of the B terminal.
But the majority of the electrons are attracted to the n-type collector, because of the larger (+) voltage at the C terminal.
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TRANSISTOR OPERATION: CURRENT FLOW (2)
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BJT OPERATING REGIONS
Operating
Region
Parameters Mode
Cut OffVBE < Vcut-in VCE > Vsupply
IB = IC = 0Switch OFF
LinearVBE = Vcut-in
Vsat < VCE < Vsupply
IC = β*IB
Amplification
Saturated
VBE = Vcut-in,VCE < Vsat
IB > IC,max/β, IC,max > 0
Switch ON
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I-V CHARACTERISTICS OF BJTBJT OPERATING REGIONS
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BJT OPERATING REGIONS (2)
ModeEmitter-Base Junction
Collector-Base Junction
ACTIVE (partially ON) FORWARDREVERSE (useful for amplification)
CUTOFF (OFF) REVERSEREVERSE (useful for switching in digital circuits)
SATURATION (fully ON)
FORWARDFORWARD (useful for switching in digital circuits)
Operating region = Operating mode
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ALGORITHM TO DETERMINE OPERATION REGION FOR NPN TRANSISTOR
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TRANSISTOR SYMBOLS AND CONVENTIONS
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TRANSISTOR OPERATION: BJT PARAMETERS
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TRANSISTOR OPERATION: CURRENT GAIN
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TRANSISTOR OPERATION: CURRENT GAIN (2)
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ACTIVE MODE OPERATION
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SUMMARY
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CONCEPTUAL CIRCUIT FOR MEASURING THE IC-VCE CHARACTERISTICS OF THE
BJT.
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THE TRANSISTOR AS A SWITCH
While there are limitations as to what we can switch on and off, transistor switches offer lower cost and substantial reliability over conventional mechanical relays.
The secret to making a transistor switch work properly is to get the transistor in a saturation state.
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THE TRANSISTOR AS A AMPLIFIER
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THE TRANSISTOR AS AN AMPLIFIER
Parameter Common EmitterCommon Collector
Common Base
Voltage gain Av High (about 100) Unity (1) Medium (10-50)
Current Gain High (50 - 800) High (50 -800)Less than unity (<1)
Input ImpedanceMedium (about 3 to 5k)
High (several k) Low (about 50R)
Output Impedance
Medium, Approx = Load resistor value
Low (a few ohms) High (about 1M)
Summary of the three types transistor connection
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DC OPERATION OF THE COMMON-EMMITER AMPLIFIER
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GRAPHICAL CONSTRUCTION FOR THE DETERMINATION OF THE DC BASE CURRENT
IN THE CIRCUIT
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GRAPHICAL CONSTRUCTION FOR DETERMINING THE DC COLLECTOR CURRENT IC AND THE
COLLECTOR-TO-EMMITER VOLTAGE VCE
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OPERATION POINT AND SIGNAL COMPONENTS
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DC ANALYSIS: A FEW SIMPLE ANALYSIS RULES
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DC ANALYSIS: A FEW SIMPLE ANALYSIS RULES (…
CONT)
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EJERCICIO PARA LA PRÓXIMA CLASE
Encuentre todas las corrientes y voltajes en el siguiente circuito:
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BIBLIOGRAPHY
http://people.senecac.on.ca/john.kawenka/EDV255/bjt.html
Sedra/Smith. Microelectronics Circuits. Fourth Edition.
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APORTE DEL ESTUDIANTE
Para el estudiante: En tu tiempo extra-clase, de qué manera puedes
complementar el contenido dado en esta clase ? Qué información adicional complementa y ayuda a
comprender mejor el contenido de estas diapositivas ?
Qué preguntas te surgen de esta clase? Qué respuestas le das a dichas preguntas?
Busca más bibliografía e información adicional que complemente tus respuestas y el contenido de esta clase. Consulta oportunamente al profesor del curso para
complementar tus respuestas y resolver tus dudas restantes.
Electrónica Analógica I. Facultad de Ingeniería. Universidad de Antioquia. 2015-1