1. BJT

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3Transistor lng cc

(Bipolar JunctionTransistor)

3-1 Gii thiu

Transistor l mt linh kin rt quan trng trong in t, bao gm c cc mch in t ri rc vcc mch tch hp. S quan trng ca thit b ny xut pht t kh nng ca n trong vic to ra ccb khuch i. Mt mch c xem l mch khuch i khi n c kh nng s dng cc thay inh ca dng hoc p ng vo to ra cc thay i ln hn ng ra. Tn hiu nh c xem lng vo ca b khuch i, tn hiu ln nhn c l ng ra ca b khuch i.

Hai dng transistor quan trng nht l transistor lng cc tnh (Bipolar Junction TransistorBJT) v transistor hiu ng trng (Field Effect Transistor FET). BJT s dng hai loi ht dn to ra dng in l l trng v electron t do, do n c gi l lng cc. Chng ta s tm hiuBJT trong chng ny. Hot ng ca FET s c cp nhng chng sau.

BJT l loi transistor c pht trin u tin v k t n c s dng rng ri trong int. Ngy nay, BJT vn cn gi mt vai tr quan trng trong cng nghip bn dn. Tuy nhin, kthut FET ngy nay pht trin rt nhiu v thm ch n c s dng nhiu hn c BJT trong ccmch tch hp.

3-2 L thuyt hot ng ca BJT

Transistor lng cc tnh (BJT) l mt linh kin ba cc c to nn t hai chuyn tip PN. Nc th c to nn t mt thanh bn dn c kch thch sao cho mt ht dn thay i dn t Nsang P v tr li N hoc t P chuyn sang N ri tr li P. Trong c hai trng hp, mi chuyn tip

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s c hnh thnh ti ranh gii ca s chuyn i tnh cht bn dn t loi N (hoc P) sang loi P(hoc N). Hnh 3-1 cho thy hai dng BJT.

Khi BJT c to nn bng cch t bn dn loi P gia hai bn dn loi N nh hnh 3 -1(a), nc gi lBJT loi NPN. Ngc li, hnh 3-1(b) cho thy cu trc ca BJT loi PNP.

Vng bn dn nm gia c gi l min nn (base). Hai vng hai bn, mt vng c gi lmin pht (emitter) v mt vng c gi l min thu (collector). cc phn sau ta s dng c thutng ting Vit hoc ting Anh ch cc cc v cc min ca transistor. Thng thng, trong ccBJT ri, cc min ny c gn vi cc chn linh kin ni ra bn ngoi c th thc hin cc ktni vi mch ngoi. Cc BJT trong cc mch tch hpc th khng c cc chn kt ni ny. Ccchn linh kin c t theo tn ca min m n kt ni vo. Hnh 3-2 trnh by cc chn linh kinc kt ni vi cc vng trong BJT.

Trong thc t, BJT c ch to vi min nn rt hp v mt ht dn trong n cng rt thpdo n c kch thch vi rt t tp cht. C hai c im ny u rt quan trng i vi mttransistor.

V c hai loi BJT ny u c c tnh ging nhau do ta ch xem xt trn loi NPN. Cc tnhcht c loi PNP c th suy ra t NPN bng cch thay i loi ht dn, cc tnh ca in p cngnh chiu dng in nh ta s thy trong phn sau. BJT c th hot ng bnh thng trong ch khuch i, cn phi phn cc c hai chuyn tip ca BJT. Chuyn tip gia min nn v minpht jE phi phn cc thun v chuyn tip gia min nn v min thujCphi phn cc ngc.Hnh 3-3 trnh by cch thc phn cc cho c hai chuyn tip.

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Ta c th thy l trong hnh 3-3(a), chuyn tip c phn cc thun bi ngun p . Khichuyn tip ny c phn cc thun, dng khuch tn ca cc electron t do s c pht i tmin pht emitter, b rng vng ngho thu hp. Ta bit n iu ny khi xt phn cc ca mtchuyn tip trong chng 2. Ta ni rng ht dn c phun (injected) temitter vo min nn base.

Tht ra khi c phn cc thun, cn c dng l trng i t base sang emitter, tuy nhin nh ta cp phn trn, v mt ht dn trong min nn rt thp nn ta c th b qua dng ny so vidng electron t do pht i t emitter.

Hnh 3-3(b) trnh by phn cc ngc chuyn tip c thc hin bng ngun . Kt quca phn cc ngc l b rng vng ngho m rng, dng in ch c th i t min nn base sang

min thu collector v l dng ca cc ht dn thiu s. Tuy nhin, nh phn tch phn trn, ccelectron t do c phun vo min nn s tr thnh ht dn thiu s, cc electron ny s tip tc trisang min thu collector di tc dng ca phn cc ngc.

Hnh 3-4 cho thy transistor NPN khi c phn cc ng thi c hai chuyn tip . Ch lmin nn base c ni t, tc l im c in th c qui c l 0 volts. Min pht emitter m sovi min nn base v min thu collector dng so vi min nn base. y l iu kin cn thit

phn cc thun v phn cc ngc .

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V min nn hp v mt ht dn rt thp do rt t electron b ti hp trong min ny. Cc

electron ny s khuch tn sang min thu di tc dng ca phn cc ngc . Chng ta kt lunl dng electron l dng chi phi trong transistor NPN. i vi transistor PNP, dng l trng s ldng chi phi ch yu.

Trong thc t, mc d mt l trng trong min nn rt thp, qu trnh ti hp vn c th xyra. Khi mi electron ti hp vi mt l trng, mt electron s ri min nn thng qua cc nn B sinh

ra mt dng nn rt nh, gi tr ca n ch khong dng electron pht i t emitter.

Trong hnh 3-4, mi tn c v ch hng qui c ca dng trong transistor NPN, hng

ny l ngc vi hng ca dng electron. Dng qui c chy t vo cc C c gi l dng

cc thu, hoc dng collector . Dng chy vo cc nn c gi l dng nn, hoc dng base ,

v dng t chy vo cc pht c gi l dng cc pht, hoc dng emitterIE. Hnh 3-5(a) trnhby biu tng mch ca mt transistor NPN. Hnh 3-6(a) l biu tng ca transistor PNP. So snhhnh 3-5 v hnh 3-6, chng ta cn phi chiu ca mi tn ti cc E, d nh, ta c th xem l

mi tn ny ch chiu qui c ca dng in. Hn na, cc tnh ca ngun v l ngcnhau cho BJT loi NPN v PNP.

nhn mnh v lm r hn hot ng ca BJT, hnh 3 -7 thay biu tng BJT bng mt khiv ch r chiu dng in chy vo v ra khi khi. p dng nh lut Kirchhoff ta c:

(3-1)

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3-2-1 Dng ngc

Trong chng 2 ta bit l nu mt chuyn tip PN b phn cc ngc th trong chuyn tipxut hin mt dng in ngc rt nh. Khi inp phn cc ngc tng dn th dng ngc ny

tin ti gi tr bo ha . V chuyn tip b phn cc ngc nn cng xut hin dng in

ngc, dng in ny cng chiu vi dng collector to ra do cc ht dn t min pht phun vomin nn. Do , dng collector tng cng s l tng ca dng do ht dn c phun vo min nnv dng ngc.

Nu ta gi s l in p phn cc thun c h mch v vn duy tr phn cc ngcnh hnh 3-8 th vn c dng in qua cc thu, chnh l dng ngc. Dng inngc ny c

k hiu l do n c chiu t collector n base khi h mch (Open) cc pht. Nh vy khi BJT iu kin hot ng bnh thng ta c:

(3-2)

vi l thnh phn dng cc thu do cc ht dn phun t min pht vo min nn gy ra.

Mt thng s quan trng ca transistor l , c nh ngha bng t s ca dng collector, docc ht dn c phun vomin nn gy ra, so vi dng emitter:

(3-3)

Thng s xc nh phn dng emitter tn ti sau khi i qua c min nn v tr thnh dngcollector. R rng lun lun nh hn 1. Ni chung, ta lun mun cng ln (cng gn 1) cng

tt. iu c ngha l ta mun transistor c dng base cng nh cng tt xp x . Gi

tr thng thng ca nm trong khong n .

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T biu thc 3-3 ta c . Do :

(3-4)

Biu thc ny chng t l dng collector tng cng bng mt phn ca dng emitter i qua

c min nn cng vi dng do bn thn phn cc ngc trn gy ra.Trong cc transistor ngy nay, c bit l i vi silicon, rt nh nn c th b qua trong

hu ht cc ng dng thc t. Tuy nhin, cn phi nh l thc ra chnh l dng in ngccachuyn tip PN. Dng ngc ny ph thuc nhiu vo nhit v in p phn cc ngc. V

chuyn tip ca transistor thng c phn cc ngc vi mt in p khong vi volts hoc

hn na nn gi tr ca thng xp x dng ngc bo ha . Khi nhit tng , gi tr

ca tng gp i do cng chu cng mt nh hng.

Trong transistor, ngoi dng ngc , transistor cn c dng r (leakage current) chy ngoib mt transistor thng c gi tr ln hn dng ngc rt nhiu. Trong cc transis tor silicon, dngr ny gn nh chi phi hon ton s thay i theo nhit ca dng ngc.

V rt nh nn ta c th vit

(3-5)

V d 3-1

Dng cc pht ca mt transistor NPN l . Nu ht dn b ti hp trong min nn v

dng r l . Tm (1) dng base, (2) dng collector, (3) gi tr chnh xc ca v (4) gi trxp x ca khi b qua dng r.

Hng dn

1.

2. T biu thc 3-1,

3. T biu thc 3-2,

Dng 3-3,

4. Dng biu thc xp x 3-5,

3-3 c tnh B chung (Common-Base)

Trong phn trc, ta thy mt mch phn cc (hnh 3-4) trong cc nn c ni vi t,tc l im tham kho chung ca mch. Cch phn cc ny c gi l cu hnh B chung (CB) catransistor. y ch l mt trong ba cch c th thit k phn cc cho transistor theo nguyn

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tc phn cc thun v phn cc ngc, v bt k cc no cng c th lm im tham khochung.

ngha ca vic c im tham kho chung trong mch l im ny c dng nh im thamkho cho c ng vo (input) v ng ra (output) cho transistor. Trong cu hnh CB, in p emitter-

base c xem nh ng vo v in p collector-base c xem nh ng ra, xem hnh 3-9. i vimt transistor NPN, dng v i vi PNP, ldng. Tng t, l dng i vi

transistor NPN v l dng i vi transistor PNP. Dng emitter l dng ng vo v dngcollector l dng ng ra.

Trong phn phn tch ny ta ch s dng cc ngun phn cc DC to ng vo v ng ra chocu hnh phn cc CB. Ta s xem xt p ng ca mch di tc ng ca nhng thay i nh trongng vo sau. Mc tiu trong phn ny ch l tm cc mi lin h gia dng in v in p ng vov ng ra. c tuyn ng vo s cho thy mi quan h gia dng in v in p ng vo, v ctuyn ng ra s cho thy mi quan h gia dng in v in p ng ra.

3-3-1 c tuyn ng vo B chung

Trong phn ny ta s xy dng c tuyn ca ng vo CB ca mt transistor NPN. V ng vo

l trn chuyn tip phn cc thun nn c tuyn s trnh by mi quan h gia dng ng

vo v in p ng vo . Tuy nhin, c tuyn ny cn ph thuc vo in p ng ra . L

do l nu cng ln th lng ht dn i qua c min nn cng nhiu dn n s gia tng trongdng ht dn t cc pht n cc thu v kt qu l gia tng dng emi tter. Hnh 3-10 trnh by h ctuyn ng vo cho cu hnh CB. Mc d vic tnh ton dng cc c tuyn ny trong thc t rt t,tuy nhin, khi hiu c c tuyn ta s c mt ci nhn su hn v hn v hot ng ca transistor.

Trong hnh 3-10, mi ctuyn tng ng vi mt gi tr khc nhau, chng cho thy mi quan

h ca dng emitter v hiu in th gia cc nn v cc pht ti mt gi tr c nh. H ctuyn ny c th c xc nh bng cch t mt gi tr c nh, thay i v o

dng tng ng. Mi ln thay i gi tr l tng ng vi vic v mt c tuyn mi trongh c tuyn.

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Trong hnh 3-10, mi ng cong c tuyn c dng c tuyn diode phn cc thun. i vi

mt gi tr cho trc, ta c th thy l tng khi tng.Tuy nhin s thay i ny ch r rt

khi thay i nhiu, do , nh hng ca ln c tuyn ng vo c th b qua trong thc t.

Lc , c th dng c tuyn trung bnh tnh ton.c tuyn ng vo CB ca mt transistor PNP c dng ging nh ca transistor NPN, tuy nhin,

in p ng vo dng phi l ch khng phi .

V d 3-2

Transistor trong hnh 3-11 c c tuyn nh trong hnh 3-10. Khi l , dng .

1. Tm ca transistor (b qua ).

2. Lp li nu khi ngn mch .

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Hng dn

1. Trong hnh 3-11 ta thy l . T hnh 3-10, ng thng tng ng

vi ct ng cong ti . Do

,

2. Khi ngn mch ngun , . T hnh 3-10, ti v .

Do ,

3-3-2 c tnh ng ra B chung

By gi ta th mt th nghim trong dng collector (dng ng ra) c o khi thay

i (in p ng ra) tng ng vi mt gi tr dng emitter (dng ng vo) c nh. Hnh 3 -12v s mch v qui trnh c th c dng trong th nghim cho mt transistor NPN. Lu l s mch trong hnh 3-12 khng phi l mt mch thc t, mch ny ch c s dng xy dng

c tuyn cho transistor. Cc mch transistor thc t phi cha cc in tr phn cc, do , in png vo v ng ra ca transistor khc vi cc ngun in p phn cc trong mch. Tuy nhin, nlc ny ta ch tp trung vo vic tm hiu s lin quan gia dng in v in p ca linh kin, cha

cn phi quan tm nhiu n cc mch phn cc bn ngoi. Khi c v theo vi cc gi

tr khc nhau, chng ta c h c tuyn c trnh by trong hnh 3-13. Chng c gi l hc tuyn ng ra ca cu hnh CB.

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u tin, trong hnh 3-13, ta c th thy l mi c tuyn bt u ti , sau tng ln rt

nhanh i vi nhng thay i nh ca . V c gi c nh nn iu ny cng ng ngha

vi vic t s cng gia tng. Ta bit , nh vy khng phi l mt hng

s. s bt u t 0 sau tng dn khi tng. L do l v ch mt phn rt nh ca cc ht dn

pht i t cc E s n c cc C cho n khi in p phn cc ngc ln gia tc cho

cc ht dn ny vt qua min nn B. Khi ang c gi tr m th chuyn tip ang phn cc

thun, nn dng in lc ny ph thuc trc tip vo . Qu trnh ny tip tc xy ra cho n

khi khng cn phn cc thun chuyn tip . Vng tng ng vi m c gi l vng

bo ha (saturation). Mt transistor c gi l b bo ha khi c v u c phn ccthun.

Mt khi ln m bo l phn ln ht dn i vo c trong min thu th vi mt gi

tr c nh, dng collector duy tr mt gi tr khng i, c lp vi gi tr . Trong c

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tuyn ta cng c th thy gi tr ca khi l hng s th xp x vi . iu ny cho thy gitr rt gn vi 1 v l mt hng s. Vng ny c gi l vng tch cc (active). Trong vng tchcc ny, BJT c nhng c tnh nh ta phn tch trong phn trc.

Trong c tuyn cn c mt vng khc, vng ny tng ng vi vng nm di ng c

tuyn ng vi v c gi l vng tt (cutoff). c tuyn ny nm rt gn vi trc honh, v

khi ny chuyn tip b h mch nn dng in ch l dng in ngc do phn cc ngc

trn gy ra. chnh l dng nh ta thy phn trc. Mt transistor lm vic trongvng tt khi c hai chuyn tip u b phn cc ngc.

V d 3-3

Mt transistor NPN c c tuyn ng vo CB nh trong hnh 3 -10 v c tuyn ng ra nh tronghnh 3-13.

1. Tm dng cc thu khi v .

2. Lp li khi v .

Hng dn

1. T hnh 3-10, ta thy l ti v . Trong hnh 3-13, ng

thng ct ng cong ti .

2. Vi cc iu kin cho, ta c th suy ra rng c tuyn ng ra nm gia hai ng ng

vi v . Giao im ca ng cong ny vi ng cho kt

qu xp x . Phng php ny khng th t chnh xc cao, trong thc t, ta c th

xem .

3-3-3 nh thng BJT

Cng nh i vi diode khi phn cc ngc, chuyn tip vi phn cc ngc trn n c thb nh thng nu in p phn cc ngc ln. S gia tng dng in ny thng xut hin do cch nh thng thc l nh kho st trong phn trc. Tuy nhin, mt transistor cn c th bnh thng bi mt hin tng c gi l punch through. Dng nh thng ny xy ra khi b rng

vng ngho, trn chuyn tip b phn cc ngc, ln lm cho vng ngho ny m rng n

vng ngho ca chuyn tip c phn cc thun. Lc ny, min pht v min thu b ngn mch

v do xut hin mt dng in ln. Hin tng punch through cn t ra mt gii hn v nng tp cht cng nh b rng min nn khi thit k BJT. Hnh 3 -14 trnh by h c tuyn ng ra baogm c on nh thng ca c tuyn.

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3-4 c tnh E chung (Common-Emitter)Trong phn ny ta s xem xt cu hnh phn cc E chung c minh ha trong hnh 3-15. Cn lu

l ngun c s dng phn cc thun cho chuyn tip v ngun c dng phn

cc ngc cho chuyn tip . Bin ca phi ln hn m bo phi phn cc

ngc.

Hnh 3-16 chng t l in p ng vo trong cu hnh CE l in p gia cc nn v cc pht (

i vi NPN v i vi PNP), v in png ra l in p gia cc thu v cc pht ( i

vi NPN v i vi PNP). Dng ng vo l dng v dng ng ra l . Cu hnh CE l cuhnh c s dng rng ri nht do chng s c xem xt mt cch chi tit trong phn ny.

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3-4-1 v

Trc khi xy dng c tuyn vo ra cho cu hnh CE ta s xy dng mi quan h

gia v . Mc d mi lin h ny hon ton khng ph thuc vo cu hnh phn cc, tuynhin mi lin h ny s cho ta mt s cc thng s mi rt hu ch trong vic d on hot ng

ca cu hnh CE. Biu thc 3-4

hay

Chia hai v cho , ta c:

ThayIE:

(3-6)

S dng biu thc 3-6 ta c th c mt biu thc cho dng r ngc trong cu hnh CE. Hnh 3-17 trnh by cc transistor NPN v PNP trong ng vo BE b h mch. Lc ny dng ng ra ch

c dng ngc trn chuyn tip . Dng ny i t min thu C qua min nn B v vo min pht E.

N c k hiu l . V phi l 0 khi h mch ng vo nn ta c

(3-7)

V rt gn 1 nn l kh ln. Do , biu thc 3-7 cho ta thy dng r CE ln hn

nhiu so vi dng r CB. iu ny cng c ngha l dng r CB c khuch i ln trong cu hnhCE. Kt qu ny c th gy ra cc vn khi mch hot ng nhit cao, c bit l i vi cctransistor loi germanium.

Tr li biu thc 3-6, ta thy c mt tham s quan trng khc ca tranistor l :

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(3-8)

l mt s ln hn 1 v c gi tr thay i trong khong t 20 n vi trm. Khi cng tin

gn n 1 th mt s thay i nh trong cng gy ra nhng thay i ln trong .

Khi , biu thc 3-6 c th vit li l:

(3-9)

(3-10)

Mc d ln hn rt nhiu so vi , ni chung, gi tr ny l kh nh khi so snh

vi . Do , gi tr ny c th b qua trong cc tnh ton mch thc t.

(3-11)

V d 3-4

Mt transistor c v .

1. Tm v .

2. Tm gi tr chnh xc ca dng collector khi .

3. Tm gi tr xp x ca dng collector khi b qua dng r.

Hng dn

1.

2.

3.

Biu thc 3-8 cho ta cng thc tnh ca mt transisotor khi bit ca n v ngc li sdng quan h sau

(3-12)

3-4-2 c tuyn ng vo E chung

V ng vo ca mt transistor trong cu hnh CE l ngang qua chuyn tip (hnh 3-16), ctnh ng vo CE l cc ng c tuyn ca diode phn cc thun. Mt tp hp cc ng c tuyn

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ng vo ca cu hnh CE c v trong hnh 3-18. Ch l tng khi gim nu gi c

nh . Gi tr ln s lm cho phn cc ngc trn mnh hn, do vng ngho m rngv min nn tr nn nh hn. Khi min nn cng nh, kh nng ti hp ht dn trong min ny cngt v do dng nn gim xung. c tuyn ng vo CE thng c gi l c tuyn nn.

3-4-3 c tuyn ng ra CE

c tuyn ng ra CE biu din dng cc thu theo in p cho cc gi tr c nhkhc nhau. c tuyn ny thng c gi l c tuyn collector. Hnh 3 -19 cho thy mt tp hpcc c tuyn ra tiu biu cho cu hnh CE.

Gi tr xp x ca c th c xc nh ti mt im bt k trn c tuyn trong hnh 3-19

bng cch tnh ti im . Hnh 3-19 minh ha iu ny, ti v , gi

tr ca l , do gi tr ca ti im ny l . R

rng khng phi l hng s nh , gi tr ca n ph thuc vo vng lm vic ca transistor.Vng c tuyn gn nh nm ngang c gi l vng tch cc trong cu hnh CE. Trong vng

ny, gn nh l hng s, tuy nhin, s tng theo khi c tuyn nng ln pha trn.

Khi (h mch ng vo), in p ti im nh thng c k hiu l . Gi trny l lun lun nh hn i vi cng mt transistor.

Khi quan st c tuyn trong hnh 3-19, cn phi nh l cc c tuyn ny c v tng ng

vi gi tr nh (khong i vi silicon). Hnh 3-20 minh ha iu ny.

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Ch l trong hnh 3-20, . V vy, nu gim xung cn

khong , s tin n 0 v chuyn tip s khng cn phn cc ngc. Kt qu ny c

th thy trong hnh 3-19, ta thy l cc c tuyn gn nh phng trong vng tch cc cho n khiin p gim xung xp x n . Nu tip tc gim , dng bt u gim

xung. Transistor c coi l bo ha khi chuyn tip c phn cc thun. Gi tr bo ha

ca , k hiu l , khong n ty theo gi tr ca dng .

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Trong hnh 3-19, dng khc khng mc d l 0. y chnh l thnh phn dng

ngc . Vng nm di c gi l vng ct (cutoff).

Cng trong hnh 3-19, cc ng c tuyn tng ng vi gi tr ln s tng nhanh hn

khi tng so vi cc c tuyn c nh. Nu cc ng ny c ko di sang bn tri nhhnh 3-21 chng s giao nhau ti cng mt im trn trc honh. im ny c k hiu l v

c gi l in p Early. D nhin l mt transistor khng bao gi hot ng vi bng in p

Early, ch n gin l mt thng s khc ca transistor. N rt hu dng cho cc chng trnhm phng mch nh SPICE.

V d 3-6

Mt transistor c c tuyn ng ra nh hnh 3-19.

1. Tm thay i ca khi thay i t n vi l .

2. Tm thay i ca khi thay i t n khi l .

Hng dn

1. Ti giao im ca ng thng vi ng cong , ta c . Do

, .

i dc theo ng cong n giao im ca n vi ng thng , ta

c . Do , .

thay i ca l

2. Ti giao im ca ng thng vi ng cong , ta c . Do

, .

i dc theo ng thng n giao im ca n vi ng cong , ta

c . Do , .

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thay i ca l

3-5 c tnh C chung (Common-collector)Trong cch phn cc th ba ny, cc thu c chn lm im tham kho chung. Cu hnh phn cc

CC c trnh by trong hnh 3-22. Ta c:(3-13)

Trong , , , v vy . V thng nh v l hng s i

vi phn cc thun (khong i vi silicon), nn:

(3-14)

Nh vy, gi chuyn tip phn cc ngc th phi ln hn .Hnh 3-23 cho thy in p base-collector l in p ng vo v dng base l dng ng vo. in pemitter-collector l in p ng ra v dng emitter l dng ng ra.

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Hnh 3-24 biu din mt tp hp tiu biu ca cc c tuyn ng vo cho mt transistor loiNPN trong cu hnh CC. R rng l chng khng phi l c tuyn ca mt chuyn tip PN phn

cc thun. Chng ta c th thy l mi ng cong c v cho mt gi tr c nh khc nhau,

v vi mi ng dng base s gim xung 0 rt nhanh khi tng. iu ny c th gii thch l

do phi xp x trong khong n th mi c dng base, nhng t biu thc 3-13 tac:

(3-15)

Do , nu tng n mt gi tr gn th tin n 0 v khng c dng base. Hnh 3-25 biu din mt tp hp cc c tuyn ng ra cho transistor loi NPN trong cu hnh CC. Chng

biu din dng emitter theo vi c gi c nh. Cc c tuyn ny c dng gn ging

vi cc c tuyn ng ra trong cu hnh CE hnh 3-19. iu ny r rng l v .

Khi b qua dng r, ta c:

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(3-16)

Biu thc 3-16 biu din quan h gia dng ng vo v dng ng ra trong cu hnh CC.

3-6 Cc dng mch phn cc

Trong phn phn tch trc, ta dng t phn cc (bias) ch vic ta dng mt in p bnngoi t ln chuyn tip PN nhm xc nh cc tnh ca n. Ta cng nhn mnh l chuyn

tip gia base v emitter phi phn cc thun v chuyn tip gia base v collector phi phncc ngc. Di tc ng ca cc ngun phn cc ny c th xc nh c mt gi tr c th cain p v ca dng in ng ra, ta ni l ta nh im phn cc ng ra ti cc gi tr .

3-6-1 Mch phn cc B chung

Trong thc t, phn cc c iu khin bng cch dng cc in tr mc ni tip vi cc

ngun v . T ta c th thay i gi tr ca in tr thay v thay i gi tr in p cangun iu khin dng v p tnh (dc) trong mch. Mch c s dng khi ny c gi l mchphn cc. Hnh 3-26 trnh by cu hnh mch phn cc CB trong in tr c mc ni tip vi

emitter v in tr c mc ni tip vi collector. Lu l ng vo v ng ra vn gi nguynnh trong phn tch phn trc v cu hnh phn cc CB (hnh 3 -9). S khc bit dng mch

phn cc ny l in p ng vo khng cn l bi v c in p ri trn in tr , v in p

ng ra khng cn l do in p ri trn . Cc ngun v c gi l cc ngun cungcp. D nhin l c tuyn ng vo v ng ra vn gi nguyn trong vic biu din mi quan h cadng v p ng vo v ng ra.

T hnh 3-26 ta c:

(3-17)

(3-18)

Khi ta xem v lbin, v l hng s, ta c th thy l biu thc 3-18 biu din

phng trnh ca mt ng thng. Nu v trn th c h trc l , ng thng ny c

dc l v giao vi trc ti . Biu thc 3-18 l biu thc ng ti (load line) chocu hnh CB (NPN). ng ti ny c cch hiu hon ton ging nh ng ti ca diode m ta

bit trong chng 3: n l tp hp ca tt c cc t hp c th c ca v trong mch. im

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phn cc tht s phi nm trn ng ti ny. V tr chnh xc ca im phn cc ph thuc vo

dng v p ng vo v .

T biu thc 3-18 c th thy l ng ti giao vi trc ti . Do , ng ti c th

c v bng cch v mt ng i qua hai im v .V d 3-7

Xc nh v v ng ti cho mch trong hnh 3-27.

Hng dn

ng ti c dc , giao vi trc ti , v giao vi trc ti . Hnh3-28 v ng ti ny.

Ta c th xc nh im phn cc bng cch v ng ti trn c tuyn ng ra ca transistorc s dng trong mch. minh ha, ngti c xc nh trong v d 3-8 c v trn mttp hp cc c tuyn ng ra ca cu hnh CB trong hnh 3-29.

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c th xc nh c im phn cc trn ng ti c v trong hnh 3 -29, chng ta phi

xc nh dng trong mch ca hnh 3-27. Mt cch tm l vit phng trnh ng ti ngvo trn mt c tuyn ng vo v xc nh gi tr ca da vo giao im ca hai ng ny.Cch ny ging vi cch lm i vi diode tm dng v p phn cc tnh cho diode. Tuy nhinphng php ny khng thc t v c tuyn ng vo thng khng c sn.

Cch thng dng nht xc nh l xem nh chuyn tip phn cc thun c in p

ri l khong (Si) v tm dng qua chuyn tip nh ta bit. Trong hnh 3-30 ta c

(3-19)

Ch l ta b qua nh hng hi tip (feedback) ca ln c tuyn ng vo. Cng cnphi lu l v c cc dng ni n im tham kho chung (tc l ground) nn ta thng xem

n nh ngun m. Tuy nhin, khi tnh ton trong biu thc 3 -19 gi tr ca phi c xem nh

l ln ca ngun. Tr li v d trong hnh 3-27, p dng biu thc 3-19 ta c: .

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Trong hnh 3-29, im phn cc, im Q, c xem l giao im ca ng ti v ng c

tuyn c . Ti im , v .

im phn cc thng c gi l im tnh (quiescent point), im Q, hoc im hot ng.N xc nh in p v dng ng ra dc khi khng c in p ac trn ng vo. Mch c s dngnh mt b khuch i ac bng cch kt ni mt ngun p ac ni tip vi cc pht. Khi in p ac

thay i, dng cc pht cng thay i theo, kt qu l dng v p ng ra thay i dc theo ng

ti trn gii hn c xc nh bi s thay i gi tr ca .

c tuyn ng vo v ng ra ca transistor rt tin li c th nhn thy hot ng bn trongca transistor v khi s dng ng ti, chng gip thy c s thay i trong dng v p ng ra.Tuy nhin, cch ny t khi c s dng thit k hoc phn tch cc mch dng transistor. Mt ldo l v khng phi tt c cc transistor cng kiu u c cng c tnh, do , cc nh ch to linhkin khng a ra mt tp hp cc ng c tuyn c th s dng c trong thit k. Hn na, chnh xc khi s dng xp x v phng php phn tch i s thng cho cc ng dng trongthc t. Ta thy phng php tnh xp x khi ta xem ng vo ca transistor CB tng t nh mtdiode c phn cc thun. By gi ta s phn tch ton b mch phn cc m khng dng ctuyn.

V v , do . Ta xc nh c dng biu thc 3-19, do c

th tnh xp x c . Sau , ta c th dng biu thc 3-17 tm

(3-20)

V d 3-8

Xc nh im phn cc cho mch trong hnh 3-27 m khng s dng c tuyn.

Hng dn

Ta c .

V nn t biu thc 3-20, . Ch l imphn cc tnh bng cch ny cho kt qu ging vi kt qu c tm thy bng th trong hnh 3-29.

Tm li, y l bn cng thc c th dng tm dng v p ng vo v ng ra cho cu hnhCB, NPN trong mch hnh 3-26(a):

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(3-21)

Biu thc 3-21 cng c th c dng cho transistor PNP (hnh 3-26(b)) bng cch

thay thnh v thnh , ng thi l gi tr tuyt i ca ngun.

3-6-2 Mch phn cc E chung

Hnh 3-31 trnh by mch phn cc cho transistor NPN v PNP trong cu hnh CE. Ch l cc

mch phn cc ny ch s dng ngun n . Gi tr ca v phi c chn sao cho in

p ri trn l ln hn in p ri trn nhm gi cho chuyn tip c phn cc ngc.

S hnh 3-31 tht ra t c s dng trong thc t, s ny ch c dng gip nhn ramt vng kn trong mch, hnh 3-32 trnh by mt s tng t nhng dng thng gp hn.

Dng nh lut Kirchhoff cho vng ng ra trong hnh 3-31(a) hoc 3-32(a), ta c phng trnhng ti ca mt transistor NPN trong cu hnh E chung:

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(3-22)

Biu thc ny c dng tng t nh biu thc ng ti ca cu hnh CB. ng ti CE c

dc , giao vi trc ti , v giao vi trc ti . Hnh 3-33 v th ca ng

ti.

Khi ng ti CE c v trn tp hp c tuyn ng ra CE, ta c th xc nh c im lm

vic ng ra khi bit trc gi tr . xc nh , ta xem ng vo ca transistor l mt chuyntip PN c phn cc thun, do c in p ri c nh nh c trnh by trong hnh 3 -34. Tac:

(3-23)

vi cho silicon v cho germanium. Trong tnh ton ny ta b qua nh

hng ca ln . Cng nh trong cu hnh CB, im phn cc ca CE cng c th c xcnh bng cc biu thc i s. Cc biu thc ny c th c tm tt nh sau:

(3-24)

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Biu thc 3-24 cng c th c p dng cho transistor loi PNP (mch phn cc hnh 3 -31(b))

bng cch thay bng v thay bng , dng gi tr tuyt i ca .

V d 3-9

Transistor silicon trong mch phn cc hnh 3-35 c l 100.

1. Gi s l transistor c c tuyn ng ra nh hnh 3-36, tm im phn cc bng cch dng th.

2. Tm im phn cc bng cch dngi s.

3. Lp li cu 1 v 2 khi l .

Hng dn

1. Biu thc ng ti l

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ng ti giao vi trc ti v trc ti . ng ti ny c v trn c tuyn

ng ra trong hnh 3-36. xc nh im phn cc ta cn tm :

Ti giao im ca ng cong vi ng ti, , ta c im phn cc

l .

2. T biu thc 3-24, ta c

Kt qu ny ph hp vi cc gi tr c tm bng th.

3. Thay i khng c nh hng trn ng ti. Lu l ng ti 3-22 khng ph

thuc . Tuy nhin, gi tr c thay i

V vy im phn cc dch chuyn dc theo ng ti n im trong hnh 3-36. Chng ta thy

l nm trong vng bo ha. Ti , . Kt qu ny cho thy im phncc c th b thay i bng cch thay i gi tr ca cc in tr phn cc trong mch ngoi.

Dng biu thc 3-24 tm im phn cc mi, ta c

R rng l kt qu ny l sai v gi tr ti a ca c th c l v gi tr ti thiu

ca l . Biu thc 3-24 khng c ngha trong trng hp im lm vic nm trong vngbo ha.

3-6-3 Mch phn cc C chung

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Hnh 3-37 biu din mch phn cc C chung cho transistor NPN v PNP. Biu thc ng til:

(3-25)

Nh trong cu hnh trc, chng ta phi tm xc nh im phn cc. Hnh 3-38 biu din

mch tng ng cho vng kn trong hnh 3-37(a), bt u ti , qua , qua chuyn

tip v . Ta c:

(3-26)

T biu thc 3-16 ta c . Thay vo biu thc 3-26 ta c:

(3-27)

Tm li, cc biu thc xc nh im phn cc trong cu hnh CC l

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(3-28)

i vi transistor loi PNP, thay bng v bng , s dng gi tr tuyt i

ca .

V d 3-10

Tm im phn cc ca transistor loi Ge cho mch trong hnh 3-39. Gi s l 120.

Hng dn

T biu thc 3-28

3-7 Thit k mch phn cc

3-7-1 Thit k phn cc B chung

Ta ch xem xt cch thc thit k phn cc cho cu hnh hai ngun (hnh 3-26). Thng thng,gi tr ca ngun v l c nh, v ta phi chn gi tr ca v theo gi tr phn cc

ca v bit trc. t , biu thc 3-21 c th vit li:

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(3-29)

Trong cc thit k mch thc t, cc gi tr in tr c thit k theo cc gi tr chun, do ,cc gi tr in tr chun gn nht vi kt qu thit k s c chn. T , phn tch li cc gi tr

phn cc v .Nu cc gi tr ny c sai s qu ln so vi yu cu, c th cn phi tnh li ccgi tr ca in tr phn cc.

V d 3-11

Mt mch phn cc B chung c thit k dng transistor NPN silicon. Cc ngun phn cc c gi

tr v . im phn cc l v .

1. Thit k mch dng cc in tr chun dung sai .

2. Gi tr phn cc tht s khi dng cc cc in tr chun l bao nhiu?

3. Tm gii hn ca v khi tnh c sai s trn in tr.

Hng dn


Da vo cc gi tr ca in tr chun , ta c th chn v .


3. Gii hn ca cc gi tr in tr c th ca v l

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T kt qu trn ta c th thy c s thay i trong im phn cc khi dng cc gi tr in trchun.

3-7-2 Thit k mch phn cc E chung

Mch phn cc hnh 3-31 c th c s dng nh l mch phn cc cho cu hnh CE. Gi s l

p ngun cung cp l bit trc (iu ny cng thng ng trong thc t). Biu thc 3-24 cth vit li cho trng hp thit k:

(3-30)

S kh khn vi thit k loi ny l im phn cc b ph thuc vo , trong khi phthuc rt nhiu vo nhit . Thm vo , vi cng mt loi transistor, gi tr ny cng thay i rt

nhiu. Chnh v vy, dng phn cc ny thng khng c ph bin trong thc t. V d 3-12

Mt transistor silicon NPN c ti u l 100, c s dng trong mch phn cc CE

vi . im phn cc l v .

1. Thit k mch dng cc in tr chun .

2. Tm gii hn c th c ca im phn cc nu ca transistor thay i t 50 n 150 (mt gii

hn thng gp trong thc t). Gi s l cc in tr c gi tr ti u.

Hng dn1. T biu thc 3-30, ta c

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Nu s dng cc in tr chun , ta c th chn v .

2. T biu thc 3-24

Trong hu ht gi tr c th thay i t n .

3-7-3 Thit k phn cc C chung

tm c cc gi tr in tr trong mch phn cc CC (hnh 3 -36), biu thc 3-28 c th vitli nh sau:

(3-31)

V d 3-13

Mt transistor silicon NPN c c dng trong cu hnh CC vi . im phn

cc cn thit k l v .1. Thit k mch phn cc dng cc in tr chun .

2. Tm im phn cc tht s khi cc in tr chun c s dng, gi s l chng c cc gitr ti u.

Hng dn

1. T biu thc 3-31,

Dng in tr chun v

2. T biu thc 3-28

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3-8 BJT Inverter (Transistor switch)

Transistor c s dng rng ri trong cc mch s v cc ng dng switching. Dng tn hiu

trong cc ng dng ny thay i gia hai mc in p cao v thp, v d nh v . Ccmch transistor c bn c dng trong cc ng dng switching c gi l inverter. Hnh 3-40 ldng mch ny i vi transistor loi NPN. Trong hnh ny, transistor c mc cu hnh CE, tuynhin, khng c in p phn cc thun c t ln cc base thng qua in tr. Thay vo , in

tr c mc ni tip vi cc base, mt tn hiu sng vung hoc xung vung s c t ng

vo ca inverter. Trong mch, v in p mc cao u l . Ng ra vn l in p gia

collector v emitter ( ).

Khi ng vo l mc cao, chuyn tip base-emitter c phn cc thun v dng in chy qua

in tr vo cc base ca transistor. Gi tr ca v c chn (khi thit k) sao cho dngbase lm cho transistor bo ha, tc l li transistor vo vng bo ha ca c tuyn ng ra.Hnh 3-41 v ng ti trn tp hp c tuyn ng ra v xc nh im trn ng ti ti

transistor b bo ha. Ch l gi tr ca tng ng vi im ny, c gi l , rt gn 0

(thng thng khong ). Dng ti im bo ha c gi l v rt gn vi giao im

ca ng ti vi trc tung, . Khi transistor b bo ha, n c xem nh l dn (ON). Phn

tch ny chng t l ng vo cao ( ) s to ra mt ng ra thp ( ).

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Khi ng vo ca mch l thp, tc l , chuyn tip base-emitter khng c phn cc

thun, do khng c dng base v v vy cng khng c dng collector. in p ri trn lkhng v phi bng . Trong trng hp ny, transistor nm trong vng tt ca c tuynng ra nh trong hnh 3-41, lc ny transistor c xem nh l tt (OFF). Mt ng vo mc thps to ra mt ng ra mc cao. cng l l do m mch c gi l inverter.

Trong thit k v phn tch cc mch inverter dng transistor, ta thng gi s

l v . Dng cc gi s ny ta c mi quan h ca dng v p trong mch

inverter. V transistor tt khi ng vo mc thp bt chp gi tr ca v , cc biu thc sauch dng khi ng vo mc cao.

(3-32)

(3-33)

(3-34)

vi l in p mc cao ca tn hiu vo, thng bng vi .

V d 3-14

Chng t l mch trong hnh 3-42 hot ng nh mch inverter khi ng vo thay i

gia v . Gi s transistor l loi silicon vi .

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Hng dn

Ta ch cn chng t l transistor sb bo ha khi . T 3-34 ta c

3-8-1 Transistor l mt cng tc

Mt mch inverter dng transistor thng c gi l cng tc transistor (transistor switch).iu ny l do trng thi ON v OFF ca transistor tng ng vi trng thi ng v m ca mt

cng tc. Khi transistor ON, tc l bo ha, in p collector emitter gn bng ging nh khi

cng tc ng li, dng ti a l . Khi transistor OFF, tc b tt, khng c dng chy tcollector n emitter v in p l ti a ging nh khi cng tc m. Cng tc ny c iu khinbng in p ng vo: mc cao th cng tc ng v mc thp th cng tc m. Hnh 3-45 vmt s s mch switch dng transistor.

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1. BJT