THE BJT (BIPOLAR JUNCTION TRANSISTOR)

INTRODUCTION

2015-1

Prof. Gustavo Patiño. M.Sc. Ph.DMJ 12- 1407-05-2015

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.

PHYSICAL STRUCTURE OF TRANSISTORS

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)

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.

TRANSISTOR OPERATION: CURRENT FLOW (2)

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

I-V CHARACTERISTICS OF BJTBJT OPERATING REGIONS

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

ALGORITHM TO DETERMINE OPERATION REGION FOR NPN TRANSISTOR

TRANSISTOR SYMBOLS AND CONVENTIONS

TRANSISTOR OPERATION: BJT PARAMETERS

TRANSISTOR OPERATION: CURRENT GAIN

TRANSISTOR OPERATION: CURRENT GAIN (2)

ACTIVE MODE OPERATION

SUMMARY

CONCEPTUAL CIRCUIT FOR MEASURING THE IC-VCE CHARACTERISTICS OF THE

BJT.

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.

THE TRANSISTOR AS A AMPLIFIER

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

DC OPERATION OF THE COMMON-EMMITER AMPLIFIER

GRAPHICAL CONSTRUCTION FOR THE DETERMINATION OF THE DC BASE CURRENT

IN THE CIRCUIT

GRAPHICAL CONSTRUCTION FOR DETERMINING THE DC COLLECTOR CURRENT IC AND THE

COLLECTOR-TO-EMMITER VOLTAGE VCE

OPERATION POINT AND SIGNAL COMPONENTS

DC ANALYSIS: A FEW SIMPLE ANALYSIS RULES

DC ANALYSIS: A FEW SIMPLE ANALYSIS RULES (…

CONT)

EJERCICIO PARA LA PRÓXIMA CLASE

Encuentre todas las corrientes y voltajes en el siguiente circuito:

BIBLIOGRAPHY

http://people.senecac.on.ca/john.kawenka/EDV255/bjt.html

Sedra/Smith. Microelectronics Circuits. Fourth Edition.

APORTE DEL ESTUDIANTE

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Electrónica Analógica I. Facultad de Ingeniería. Universidad de Antioquia. 2015-1