Lecture - BJTs1.ppt

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    Bipolar JunctionBipolar JunctionTransistors (1)Transistors (1)

    Dr. Wojciech Jadwisienczak EE314

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    Introduction

    Your goal is to explain the transistor.t is assu!ed that EE314 students to which thispresentation is ai!ed" ha#e not a clue to how these little$uggers work and%or how to use the!.& real pro'le! with pre#ious explanations( )or the sake

    o) *)idelit+* authors, include con)using details until theconcept" or thread--o) how the+ actuall+ work how touse the!--is lost.

    /he )ollowing presentation is co!prised o) se#eraldi))erent explanations. You should read chapter 13 andthis presentation se#eral ti!es" 'ecause an+ insightgained )ro! one will help in understanding another.

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    0hapter 13( $ipolarJunction /ransistors

    pp. 24-4

    1.History of BJT2.First BJT3.Basic symbols and features.! little bit of p"ysics#

    $.%urrents in BJT&'.Basic confiurations.%"aracteristics

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    T"e transistor *asprobably t"e mostimportant in+ention oft"e 2,t" %entury- andt"e story be"ind t"ein+ention is one ofclas"in eos and topsecret researc".

    First BJTs

    /eference0Bell Labs Museum

    B. . treetman . Baner4ee 5olid tate 6lectronic 7e+ices&- 8rentice Hall 1999.

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    5icture )ro! pre#ious slide shows the work'ench o) John $ardeen

    and Walter $rattain at $ell 6a'oratories. /he+ were supposed to 'edoing )unda!ental research a'out cr+stal sur)aces. /he experi!entalresults hadn,t 'een #er+ good" though" and there,s a ru!or that their'oss" Willia! 7hockle+" ca!e near to canceling the project. $ut in1849" working alone" the+ switched to using tre!endousl+ pure

    !aterials. t dawned on the! that the+ could 'uild the circuit in thepicture. t was a working a!pli)ier: John and Walter su'!itted apatent )or the )irst working point contact transistor.

    7hockle+ was )urious and took their work and in#ented thejunction transistor and su'!itted a patent )or it 8 da+s later./he three shared a ;o'el 5rize. $ardeen and $rattain continued inresearch

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    8oint%ontact Transistor :first transistor e+er made

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    ;ualitati+e basic operation of pointcontact

    transistor

    8roblems *it" first transistor#

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    First Bipolar Junction Transistors

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    >nderstandin of BJT

    force : +oltae?current*ater flo* : current amplification

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    Basic models of BJT

    7iode

    7iode

    7iode

    7iode

    npn transistor

    pnp transistor

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    ;ualitati+e basic operation of BJTs

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    Basic models of BJT

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    BJTs : Basic %onfiurationsFluid Flo* !naloy

    7ifference bet*een F6T (field effect transistor)and BJTTec"noloy of BJTs

    pnp BJT npn BJT

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    BJTs : 8ractical !spects

    @eat sink

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    BJTs : Testin

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    BJTs : Testin

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    ! little bit of p"ysics#

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    ! little bit of p"ysics#

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    ! little bit of p"ysics#

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    ! little bit of p"ysics#

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    ! little bit of p"ysics#

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    @ore accurate p"ysical description#

    pnp BJT

    1. In4ected "Acurrent from 6 to B2. ein4ected across t"e for*ardbiased 6B 4unction (current from

    B to 6)3. esupplied by t"e B contact for recombination *it" "A

    (recombination current). "Areac"in t"e re+ersebiased % 4unction$-'.T"ermally enerated e "Ama=in up t"e re+erse saturation

    current of t"e % 4unction

    i6

    iB

    i%

    %6

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    Co*- you can try#

    npn BJT

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    BJTs : Basic confiurations

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    npn BJTs : Dperation @odes

    For*ard re+erse polariEedpn4unctions

    7ifferent operation modes0

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    npn BJTs : Dperation @odes

    A

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    BJTBasic operation

    npn BJTpnp BJT

    (nA)- (pA) : "ea+y doped reions 7opin in 6GBG%

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    Dperation mode0 +B6is for*ard +B%is re+erse

    T"e "oc=ley euation

    = 1exp

    T

    BEESE

    V

    vIi

    I6:saturation I (1,121,1'!) T=T? t"ermal (2'me)

    7 : diffusion coefficient cm2?sK : carrier mobility cm2?sKT"e Lirc""off&s la*s

    BCE iii +=

    EiIt is true reardless of t"e biasconditions of t"e 4unction

    >sefulparameter

    B

    C

    i

    i=

    t"e commonemitter current ainfor ideal BJT is infinite

    0=++ CEBCBE VVV

    BJTs : %urrent oltae /elations"ips

    q

    kTD=

    6instein relation

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    = 1exp

    T

    BEESC

    V

    vIi

    E

    C

    i

    i=>seful

    parameter

    t"e commonbase current ain

    for typical BJT is M,.99

    T"e "oc=ley euationonce more

    If *e define t"e scale current

    ESS II =

    T

    BESC

    V

    vIi

    ! little bit of mat"# searc" for iB

    ( ) EB ii = 1 ( )

    = 1exp1

    T

    BEESB

    V

    vIi

    ==1B

    C

    i

    i

    BC ii =Finally#

    BJTs : %urrent oltae /elations"ips

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    BJTs : %"aracteristics

    c"ematic

    %ommon6mitter

    Input

    Dutput

    B%N, or eui+alently %6GB6

    If %6NB6t"e B% 4unction isfor*ard bias and I%decreases/emember B6"as to be reater

    t"an ,.',

    BC ii =

    6Oample 13.1

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    BJTs : Poad line analysis%ommon6mitter !mplifier

    )()()( tvtiRtvV BEBBinBB +=+Input loop

    if iB, inBBBE vVv += if +B6, BinBBE RvVi /)( +=

    smaller+in(t)

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    BJTs : Poad line analysis

    CECCCC

    viRV +=

    Dutput loop

    %ommon6mitter !mplifier

    6Oample 13.2

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    %ircuit *it" BJTs

    Dur approac"0 Dperatin point dc operatin point

    !nalysis of t"e sinals t"e sinals to be amplified

    %ircuit is di+ided into0 model for laresinal dc analysis of BJT circuitbias circuits for BJT amplifiersmallsinal models used to analyEe circuits forsinals bein amplified

    /emember Q

    P l d l / @ d l

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    Pareinal dc !nalysis0!cti+e/eion @odel

    Important0 a currentcontrolled current source models t"edependence of t"e collector current on t"e base current

    T"e constrains for IBand %6must be satisfy to =eep BJT in t"e

    acti+emode

    B6for*ard bias

    %Bre+erse bias R

    R

    P i l d ! l i i / i @ d l

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    Pareinal dc !nalysis0aturation/eion @odel

    B6for*ard bias

    %Bfor*ard bias

    R

    R

    P i l d ! l i % ff / i @ d l

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    Pareinal dc !nalysis0%utoff/eion @odel

    %B

    re+erse bias

    B6

    re+erse bias

    R

    R

    If small for*ardbias +oltae of up to about ,.$ are applied- t"ecurrents are often neliible and *e use t"e cutoffreion model.

    P i l d ! l i " t i ti f BJT

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    Pareinal dc !nalysis0c"aracteristics of an npn BJT

    P i l d ! l i

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    Pareinal dc !nalysis

    8rocedure0 (1) select t"e operation mode of t"e BJT (2) use selected model for t"e de+ice to sol+e t"e circuit

    and determine I%- IB- B6- and %6(3) c"ec= to see if t"e solution satisfies t"e constrains fort"e reion- if so t"e analysis is done

    () if not- assume operation in a different reion andrepeat until a +alid solution is found

    T"is procedure is +ery important in t"e analysis and desinof t"e bias circuit for BJT amplifier.

    T"e ob4ecti+e of t"e bias circuit is to place t"e operatin point int"e acti+e reion.

    Bias point : it is important to select I%- IB- B6- and %6independent of t"e and operation temperature.

    6Oample 13.- 13.$- 13.'

    P i l d ! l i Bi %i it

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    Pareinal dc !nalysis0 Bias %ircuit

    From 6Oample 13.'

    /emember0 t"at t"e ; point s"ould be independent of t"e stabi

    lity issue)

    BB %%pro+ide t"is stability- "o*e+er t"is impractical solution

    BB acts as a s"ortcircuit for ac sinals

    P i l d ! l i F / i t Bi %i it

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    Pareinal dc !nalysis0 Four/esistor Bias %ircuit