Bipolar Bipolar Junction Transistors Junction Transistors...

1

Bipolar Bipolar Junction Transistors Junction Transistors (BJTs)(BJTs)

1

(BJTs)(BJTs)

Lecture # 6Lecture # 6

BJT at a GlanceBJT at a Glance

The most important use of BJT is under severe environmentpcondition for reliability.

Also, in analogue applications still BJT is the best in businesswhich is true for high frequency applications, radio frequency forwireless systems.

Emitter Coupled logic is also based on BJT.

Copyright 2004 by Oxford University Press, Inc.2

BJT has been combined with MOSFET to have some innovativecircuits, the resulting technology is called BiMOS or BiCMOS.

2

npn Transistornpn Transistor


pnp Transistorpnp Transistor


3

BJT ModesBJT Modes

ModeMode EBJEBJ CBJCBJCutoffCutoff ReverseReverse ReverseReverse

ActiveActive ForwardForward ReverseReverse

Reverse ActiveReverse Active ReverseReverse ForwardForward

SaturationSaturation ForwardForward ForwardForward

IfIf aa transistortransistor isis toto bebe usedused aa anan amplifieramplifier itit hashas toto bebe operatedoperated inin activeactivemodemode whichwhich isis alsoalso calledcalled forwardforward activeactive modemode..


SwitchingSwitching applicationsapplications (logic(logic circuits)circuits) useuse cutoffcutoff andand saturationsaturation modemode..

TheThe reversereverse activeactive alsoalso calledcalled inverseinverse activeactive isis alsoalso usedused butbut inin veryverylimitedlimited applicationsapplications..

Active ModeActive Mode


Current in BJT is consists of two components Electrons (injected from emitter to base) &Holes (from base to emitter) that is why it is called BJT.

Base is normally very thin and less doped (lightly doped) as compared to emitter which isvery heavily doped. The emitter current is due to these two carriers but as electrons are highin numbers due to doping it will be dominated by electron components.

4

Concentration of Carriers in BaseConcentration of Carriers in Base

where2

/

BC

A

inES

vvSC

ii

WN

nqDAIeIi TBE

1

and /

EC

vvSECBE

CB

BC

ii

eI

iiii

ii

TBE


α is called Common Base Current Gain and β is called Common Emitter Current Gain.

Profiles of minority-carrier concentrations in the base and in the emitter of an npn transistor operating in the active mode: vBE 0 and vCB 0.

TBE vvpp enn /

0)0(

1

Large Signal Equivalent Circuit ModelsLarge Signal Equivalent Circuit ModelsCollector current is independent ofcollector voltage as long as vCB ≥0.Therefore, in active mode it behaves asan ideal current source which iscontrolled by vBE .

TBE vvSEF eIi /

TBE vvSE e

Ii /


Large-signal equivalent-circuit models of the npn BJT operating in the forward active mode.

It is a non-linear voltage controlled current source model. It can be converted into a Current Controlledcurrent source model as in (b) still it is non-linear due to the diode.

The current in collector is controlled by vBE (exponential term, non-linearity).

5

Exercise 5.2, 5.3, 5.4 & 5.5Exercise 5.2, 5.3, 5.4 & 5.5

5.1 Consider an npn transistor with vBE = 0.7 V at iC = 1mA. Find vBE at iC = 0.1 mA and 10 mA.

Ans. 0.64 V; 0.76 V


R

RR

R

FSCRSEF Ii

1 , ,

Transistor StructureTransistor Structure

It is not a symmetrical device, as collector surrounds the emitter area so it is not possiblefor electrons to escape which are injected into the base so all of them are collected, that iswhy beta is large and alpha is close to unity.


Therefore, if emitter and collector are interchanged and it is operated in the reverse activemode the value of beta and alpha will be different from active mode values.

6

Reverse Active Circuit ModelReverse Active Circuit Model

III

1. - 0.01 range in the is , 0.5 - 0.01 range in the is

large. is andunity toclose is

RR

FF

SSCRSEF III


Model for the npn transistor when operated in the reverse active mode (i.e., with the CBJ forward biased and the EBJ reverse biased).

EbersEbers--Moll Circuit ModelMoll Circuit Model

iii

RS

VvSC

vvSCDC

vvSEDE

DCRDEE

DCRDEFB

DEFDCC

IeIi

eIi

eIi

where

iii

iii

iii

TBE

TBC

TBE

11

)1(

)1(

)1()1(

/

/


FS

Vv

F

SE

RFS

Vv

F

SB

IeI

i

IeI

i

TBE

TBE

11

11

7

iiCC

–– vvCB CB

CharacteristicsCharacteristics

To be operated in the active mode VCB has to be greater or equal to zero to ensure that CBJ is reversed


To be operated in the active mode, VCB has to be greater or equal to zero to ensure that CBJ is reversedbiased, we know that a pn junction cannot be forward biased until the voltage is 0.5 volts, so this meansthat CBJ will remain reversed biased until -0.4 volts. After that it will enter into saturation and the currentwill change.

The iC –vCB characteristic of an npn transistor fed with a constant emitter current IE. The transistor entersthe saturation mode of operation for vCB –0.4 V, and the collector current diminishes.

Electron Concentration in Saturation ModeElectron Concentration in Saturation Mode

TBC

R

STBE vvIvvSC eeIi //

We have shown earlier that concentration of electrons in the base is almost zero at the collector end of the base, infact thingsh d i ll i i h if h ll b j i i f d bi d (V i ) h h ll


changes dramatically in saturation that if the collector base junction is forward biased (VCB negative) then the collector currentreduces.

When BJT goes to saturation the second term in the above expression becomes larger so more amount is subtracted so IC reduces,the fact is that when the collector base junction is forward biased, the concentration of electrons is no more zero near the basecollectore junction so the slope changes (less) so causing reduction in the collector current.

Saturation mode of BJT is completely different from the MOSFET, for BJT it is equal to the triode region of the characteristics,where as the saturation region of MOSFET corresponds to the active region of BJT.

8

Exercise 5.6Exercise 5.6

VvS

RFS

Vv

F

SB

RS

VvSC

I

IeI

i

IeIi

TBE

TBE

1

11

11


FS

Vv

F

SE Ie

Ii TBE

1

1

pnp Transistorpnp Transistor


9

pnp Transistor Circuit Modelpnp Transistor Circuit Model


npn & pnp Transistor Symbolsnpn & pnp Transistor Symbols


10

Exercise 5.8 & 5.9Exercise 5.8 & 5.9

TBE VvSC

EC

eII

II


Voltage Polarities in the Active ModeVoltage Polarities in the Active Mode


11

Summary: Current Voltage RelationshipsSummary: Current Voltage Relationships

/

/

VvSC

B

VvSC

eIi

i

eIi

TBE

TBE

is which voltage, thermalis

KT

VT

)1(

1)1(

/

BE

EEB

EC

VvSCE

ii

iii

ii

eIi

i TBE re. temperaturoomat 25.0q

KT


1

1

Example 5.1Example 5.1


12


EC II




13

iiC C ––vvBEBE Characteristic npn Transistor.Characteristic npn Transistor.

TBE VvSC eIi /


For small value of base emitter voltage the current is small up to 0.5 V, over most of thenormal current range base emitter voltage changes between 0.6 to 0.8 V, the constant ofthe exponent is quite high (1/VT =40) so the current rise sharply.

Temperature EffectTemperature Effect

F 1 d i i h l i b j i


For 1 degree rise in temperature the voltage across emitter base junctiondecreases approximately by 2 mV.

Which is shown for 3 different temperatures.

14

Common Base CharacteristicsCommon Base Characteristics

Base held ConstantSo common terminal for input


terminal for input & output

The curves deviates in the active region in two ways, curves are not horizontal showing that iC is slightlydependent on vCB. Secondly, the collector current shows a rapid increase (basically breakdownphenomenon) at a large value of vCB. One can determine the value of alpha (incremental or small signalalpha) by measuring change in collector current obtained as a result of change in emitter current.


W k h f ifi l f b i l h b 2 V/C0We know that for specific value of current base emitter voltage change by -2 mV/C0 .

BEv


15

Dependence of IDependence of ICC on Collector Voltageon Collector Voltage

TBE vvSC eIi /

For a given base emitter voltage which can be adjusted we obtain IC – VCE characteristics at each point byi th ll t itt lt B i ll h i th V f i V th bi


varying the collector emitter voltage. Basically, when we increase the VCE for a given VBE the reverse biasat collector base increases which in turn increases the width of the depletion region, and we know thatsaturation current is inversely proportional to the width.

Therefore, saturation current increases which is directly proportional to collector current so that increasesand we see this slope increases, it is called early effect discovered by J. M. Early on his name. We extendthe slope we see that it meets at a point VA which is a parameter for the transistor in the range 50 – 100.

Equivalent Circuit ModelsEquivalent Circuit Models

The slight slope in the characteristics shows that the output resistance looking into thecollector is not infinite, rather it is finite, so we can include that in the circuit model, it


, , ,should be noted that dependence of IC on VCE is rarely considered in dc bias design analysis,however, the finite resistance can have significant effect on the gain of a transistoramplifier.

The above models differ in a sense that (a) is a voltage controlled current source where as(b) is a current controlled current source as IB is controlling things.

16


C

A

I

Vr 0

CC

C

C

r

VI

rI

V

0

0


CCC III 0

Common Emitter CharacteristicsCommon Emitter Characteristics

CAC

BQ

CQDC

i

i

I

I

tconsvBCE

itan


Another alternative for common emitter configuration is to use base current ratherthen the base emitter voltage parameter. The magnitude of beta for ac and dcdiffer by 10% to 20%.

The characteristics are the same however, here breakdown occurs.

17

ββ Dependence on IDependence on ICC


Typical dependence of on IC and on temperature in a modern integrated-circuit npn silicon transistor intended for operation around 1 mA.

Expanded ViewExpanded View


An expanded view of the common-emitter characteristics in the saturation region.

18

DiscussionDiscussionThe steep slope indicate that the saturated transistor showslow collector to emitter resistance.

CEsatCsatCEoffCEsat RIVV

R 0 1 0 3 V


(a) An npn transistor operated in saturation mode with a constant base current IB. (b) The iC–vCE characteristic curve corresponding to iB = IB. The curve can be approximated by a straight line of slope 1/RCEsat. (c) Equivalent-circuit representation of the saturated transistor. (d) A simplified equivalent-circuit model of the saturated transistor.

Range 0.1 – 0.3 V

Transistor BreakdownTransistor Breakdown

The maximum voltages that can be applied to EBJ and CBJ are limited by the avalanche multiplicationThe maximum voltages that can be applied to EBJ and CBJ are limited by the avalanche multiplicationmechanism discussed earlier.

For the common base configuration, BVCB0 is breakdown voltage at IE = 0 (Emitter open circuit), when IE isgreater than zero breakdown will occur at lower voltage typically, it is greater than 50 V.

For common emitter configuration it is BVCE0 , it is also known as sustaining voltage LVCE0 given bymanufacturer.

The Breakdown of CBJ either in common base or common emitter configuration is not destructive as long asthe power dissipation is kept in the safe limit, this is however, not the case for EBJ. The EBJ breaks down at a


voltage BVEB0 much smaller than BVCB0 , BVEB0 is in that range 6 – 8 V. The breakdown is destructive in thesense that beta of the transistor permanently destroyed.

19

Exercise 5.16, 5.17 & 5.18Exercise 5.16, 5.17 & 5.18

5.18 What is the output voltage of the following circuit, if the transistor VBC0 = 70 V ?

Ans. -60 V

CEsatCsatCEoffCEsat

CEsatCE

C

RIVV

SlopeR

V

ISlope

1

,


Bipolar Bipolar Junction Transistors Junction Transistors...

Documents

Transcript of Bipolar Bipolar Junction Transistors Junction Transistors...