Lecture - BJTs1.ppt

7/21/2019 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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@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

1

2

3

T"e+enineui+alent

21 RRRB= ( )( )212/ RRRVV CCB +=

6ui+alentcircuit for

acti+ereionmodel

olution of t"e bias problem0

Input

Dutput

EEBEBBB IRVIRV ++=

( ) BE II 1+= VVBE 7.0

( ) EBBEB

BRR

VVI

1++=

EECCCCCE IRIRVV =


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npn

BJTs : 8ractical !spects


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R

VI=


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Lecture - BJTs1.ppt

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