D X U N THI (ch hin) - NG VN C H Y T - N G U Y N V I T n g u y n

NGT]N V S S N - N G U Y N C THUN - NG L T H Y - N G VN TON

K THUT IN T( c hci ng mn hc ca B G io d c v o to thng qua

dng lnm ti liu ging y trong c c trng i h c k thut)

(T i h n ln t h ino i by)

NH XUT BN GIO DC VIT NAM

Chng 1

M U

Ki thu t in t v t in hc l mt ngnh mi nhn mi ph t tr in. Trong mtkhong thi gian tng i ngn, t khi I'a i tranzito (1948), n c nhng tinb nh;iy vt, mang li nhiu thay i ln v su sc trong hu ht mi linh vc r t kh' nhau ca i sng, dn tr thnh mt trong nhng cng c quan t rng n h t ca tch mng k th u t tr inh cao (m im t rung tm l t ng ha tng phn hoc hon t.on, tin hc ha, phng php cng ngh v vt liu mi).

D br u lm quen vi nhng vn c bn nh t ca ngnh mang ngha i cvng, chng ny cp li cc khi nim c s nhp mn v gii thiu cu trcvi h thng in t in hnh.

11, CC I LNG CO BN

l . l . l . in p v (n in l hai khi nim nh ng c bn ca mt mch in.Chng cho php xc dih trng thi din nhng im, nhng b phn khc nhauvo nhng thi im khc nhau ca mch in v do vy chng cn c gi l ccthng ri t r n g thi c bn ca nit mch in,

Kh nicni in p c r t ra t khi nim in th trong vt , ! hiu s' inth xia hai im khc nhau ca mch in. Thng mt im no ca mch cchn lm im gc c in th bng 0 (im t). Khi in th ca mi im khc trong mch c gi tr m hay dng c mang so vi im gc v c hiu l in p ti im tng ng. Tng qut, hn, in p gia hai im A v B ca mch (k hiu l .v ) xc nh bi:

Uah = v,x - v = - u,,.,vi v V| l in th ca A v B so vi gc.

Kh . 1 n im dng din l biu hin t r n g thi chuyn ng ca cc h t m ang introng vt cht do tc ng ca trng hay do tn ti mt gradien nng ht theokhng gian. Dng in t rong mch c chiu chuvn ng t ni c in th cao n ni cd in th thp v do vy ngc vi chiu chuyn ng ca in t.

T cc khi nim nu trn, cn r t ra m y nhn xt quan t rng sau :

a) in p lun c o gia hai im khc nhau ca mch t rong khi dng inc xc nh ch ti mt im ca mch.

b) bo ton in tch, tng cc gi tr cc dng in i vo mt im ca mch lun bng tng cc gi tr dng in i ra khi im {qu tc nt vi dng din) suy ra t rn mt on mch ch gm cc phn t ni ni t ip nhau, dng in ti mt im l nh nhau.

() Din p gia hai im A v B khc nhau ca nich nu o theo mi nhnh btki c in tr khc khng (xem khi nim nhnh l . l 4) ni gia A v B l gingnhau v bng ,\l- Ngha l in p gia 2 u ca nhiu phn t hay nhiu nhnh ni song song vi nhau lun bng nhau. (Quy tc vng di v din pi,

. . 2. Tinh chui in ca mi phn

. Dinh nghio : T n h cht (iin ca nit phn t bl k trong mt mch in cth hin qua mi quan h tng k giQ din p t ' trn hai u ca n r diq (iii chy qua n v c nh ngha l din fr (h;iy in tr phc - tr khng) 'a phn t. Ngha ! khi nim in tr gn lin vi qu;i tr inh bin i in p thnh dng in hoc ngc li t dng in thnh in p.

a) Neu mi quan h ny l ti l thun, ta c nh lut m :

u = R.I l - 1 )

y, R l mt hng s ti l c gi l in tr ca phn t v phn t tngng c gi l mt (in tr thun.

b) Nu in p trn phn t ti l vi tc bin i theo thi gian ca dng in trn n, tc l :

dl .. , u ( y L l mt hng s ti l) (1-2)

ta c phn t l mt cun dy c gi tr in cm l L.

c) Nu dng in t rn phn t t l vi tc bin i theo thi gian ca in p trn n, tc l :

d . . . .I = c ( y c l mt hng s ti l) (1-3)

ta c phn t l mt t din c gi tr in dung l c.

d) Ngoi cc quan h nu trn, trong thc t cn tn ti nhiu quan h tngh a dng v phc tp gia in p v dng in t rn mt phn t. Cc phn t ny gi chung l cc phn t khng tuyn t nh v c nhiu t nh cht c bit. in tr ca chng c gi chung l cc in tr phi tuyn, in hnh n h t l it, tranzito ,thiristo... v s c cp ti cc phn tip sau.

2. Cc t inh cht quan trng ca phn t tuyn t inh l :

a) c tuyn Vn - Ampe (th hin quan h U(D) l mt ng thn g ; in trl mt i lng c gi tr khng thay i mi im.

b) Tun theo nguyn l chng cht. Tc ng tng cng bng tng cc tc ngring l ln nd.

p ng tng cng (kt qu chung) bng tng cc kt qu thnh phn do tc ng thnh phn gy ra.

c) Khng pht sinh thnh phn t n s l khi lm vic vi t n hiu xoay chiu (khng gy mo phi tuyn).

( i ) ( ih c h : khi n i m ) \v m t d;y l t o n g cLil. d i d i n c h o m l yu l C;'L1 lh ;m h m c h i n h;iy nit

l hOi nhiLi you t MI) n n m t h phin m.'ich in.

)\ l) li , vi phn t ph i tuvcn. ta c cc t inh cht sau :

at Dc tuvn VA l mt n^ cong ulin tr thay i theo im lm vic).

b) Khng p dng c nguyn l ('hng cht.

c) Ain pht, s inh tnn s l (u vo khng c) khi c t n hiu xoay chiu tc ng.

3. ng diig - Cc phn t tuyn t nh (R, L, C). cd mt s ng dng quan trng sau ;

a) Dn tr lun thng s c trng cho hin tng tiu kao nng ng (ch yu di dng nhi) v mt thng s khng qun t.ink, Mc tiu hao nang lng ca in tr c nh gi hng cng sut trn n, xc nh bi:

p ^ u . = ' r = U-/R (1-4)

khi c5, [*un dv v i din l cc phn t v c bn khng tiu haonng lng (xt l tng) v c qiin tnh. Chng c tr i ng cho hin tng tch luy nan^' lng t trng hay in trng ra mch khi c dng in hay in p bin t.hin qua chn. 0 y t(' bin i ca c' thng s t rng thi (in p, dng i ( n) C'C va i t r ( uyt c nh g i t r t r k h n g c a c h n g , n g h a l c h n g c ( i n t r phu hc vo tn .s (vo tc hipn i ca in p hay dng in tnh t rong mtn v thi gian). Vi t in. t h thc ( L3). dung khng ca n gim khi tng tn0 v ngc: lai vi cun dy, t ( 1- 2 ) cm khng ca n tng thoo tn s.

b Gi tr in tr tng cng ca nhiu in tr ni tip nhau lun ln hn ca tng ci v c tnh cht cng tuvn tnh. Din dn (gi tr nghch o ca in tr) ca nhiou in tr ni song song nhau lun ln hn in dn ring r ca tng ci v cung (* tnh cht, cng tuyn tnh.

l i Cu i :

( th thc hin vic chia nh mt ii p (hay dng in) hay cn gi l thc hin vic dch mc in th (hay mc dng in) gia cc im khc nhau ca mch bng cch ni ni tip (hay song song) cc in tr.

''M 'lYong cch ni ni tip, in tr no lB hn S* quyt nh gi tr chung cady. Ngc li, t rong cch ni song song, in tr no nh hn s quyt nh.

Vic ni ni tip (hay song ong) cc cun dy s dn ti kt qu tng t nh i vi cc in tr : s lm tng (hay gni) tr s in cTi chung. Di vi t in, khi ni song song chng, in dung tng cng tng :

c , , = c , + + ... c , (1-5)

c khi ni ni tip, in dung tng cng gim :

1/C^, = 1/Cj + I /C2 + ... + 1/C^ (1-6)

c) Nu ni ni tip hay song song R vi L hoc c s nhn c mt kt cu mch c t inh cht cin lc tan s (tr khng chung ph thuc vo tn s gi l cc mch lc tn s).

Nu ni ni t ip hay song song L vi c s dn ti mt kt cu mch va cd t nh cht chn lc t n s, va c kh nng thc hin qu tr nh trao i qua li gia hai dng nng lng in - t trng, tc l kt cu c kh nng pht sinh dao ng in p hay dng in nu ban u c mt ngun nng lng ngoi kch thch, (vn ny s gp mc 2.4).

\!wn in p v n^un dn din

a) Nu mt phn t t n hay khi chii cc tc ng khng c bn cht in (. f kh nng to ra in p hay dng in mt im no ca mch in thi n() c gi l mt ngin sc din dng (s.c.). Hai thng s c t rng cho ml ngun s.. l ;

i tr in p gia hai u lc h mch (khi khng ii vi bt k mt phnt no khac t ngoi n hai u ca n gi l in p lc h mch ca ngiii kh i u l Ul ini -

Gi tr dng in ca ngun a ra mch ngoi c mch ngoi dn in honton ; gi l gi tr dng in ngn lch ca ngiin k hiu l Inyni

Mt ngun s.. c coi l Iv tng nu ir p hav dng in do n cung capcho mch ngoi khng ph thuc vo tnh cht ca mch ngoi mch ti).

b) Trn t.hr t. vi nhng ti c g tr khc nhau, irn p trn hi u ngunhay dng in do n cung cp c gi tr khc nhau v phu t.huc vo ti. Diu chng t. bn t rong n^un c xy ra qu tr nh bin i dng in cung cp thnhgim p trn chnh n. ngha l ln ti gi tr din tr bn t rong gi l din fr trong ca igun k hiu

,,.' ' ~ Iriin

Nu gi v I * cc gi Ir in p v dng in do ngun cung cp khi c ti hu hn 0 < Rj< 0- th :

hn, - R

T (1-7) v (1-8) suy ra :

Innm u

R

( 1- 8)

(1-9)

T cc h thc trn, ta c cc nhn xt sau :

1. Nu R.. 0 th t h thc (1-8) ta c 1. iNeu u ini t n t.nc ( i - ) ta co u khi ngun s.. l mtngun in p l tng. Ndi cch khc mt ngun in p cng gn l tng khi intr trong ca n c gi tr cng nh.

2. Nu Rng co, t h thc (1-9) ta c I Ingm, ngun s khi c dng lmt ngun dng in l tng hay mt ngun dng in cng gn l tng khi RniTca n cng ln.

3. Mt ngun s... t rn thc t c coi l mt ngun in p hay ngun dng in ty theo bi cht cu to ca nd gi tr Rn. l nh hay ln. Vic ih gi Rne ty thuc tng quan gia n vi gi tr in tr ton phn ca mch ti ni ti hai u ca ngun

Rf xut pht t cc h thc (1-8) v (1-9) c hai cch biu din k hiu ngun (s) thc t nh trn hinh 1.1 a v b.

I n

o-----'R-^

u

I~o----->I

u\ Rt

-o-

6 )

l n l . a ) l i r i ( i i c n m ; ( / f \ ^ / I i ! ; u n ( i i p

h ) . h . ( y H ^ i ( ( i i n .

1, Mt b phn ht ki ca mch c cha ngun, khng c lin h h cm vi phnCull li c i a m ach n chi ni vi phn cn l i ny hai i i i , lun c th th a y th

nt nii ng doig vi mt in tr trong l ii tr tng ng ca bphn larh in^ xt. Trng h) ring. nu b phn mch bao gni nhiu ngun inap ni vi nhiu in tr theo mt cch bat ki. c 2 u ra s c thay th bng rhi nt ngiin in p tng ng vi mt in tr troig tng ng (nh l v nguon tng i!g iia Tovnin)

. .4 . B i i ivn tach din hiti cc k iiu v hnh v (s i)

( () nhiu ct'h hiu dion mt inch in t, trong n gin v thun li hn cl cch hu dien bng s d gni tp hp cc k hiu quy c hay ki hiu tng ng ca ct' phn t c ni vi nhau theo nt cch no * (ni tip, riong song, hn hp ni tip soig song hav phi ghp thc^h hp) nh cc ng ni c in trbang 0- Khi bien dien nh vy, xut hin mt .vi vu t hnh hc cn lm r khinini [ :

N h a n h (ca s mch) l nit b phn ca s . trong chi bao gm ccphn t ni ni tip nhau, qua n chi c mt dng in duy nht.

N l mt im ca mch chung cho t ba nhnh tr ln.

Vng l mt, phn ca mch hi\o gm mt s nt v nhnh lp thnh mt ngkn ni dc theo no' mi nhnh v nt phi v chi gp mt ln (tr nt c chn lm ini xut phtK

Cv l mt phn ca mch bao gm ton b s nt. v nhnh ni gia cc nt. d nhng khng lo nn mt vn^ kn no. Cc nhnh ca cy c gi l n knk c, cc nhnh cn li ca mch khng thuc cy c gi l b cy.

Cc yii t nu trn c s dng c bit thun li khi cn phn tch tnh ton mch bng s .

NVi ta cn hiu din mch gn hn bng Iit s gm nhiu kii c nhng ng lin h vi nhau. Mi khi bao gm mt nhdin cc phn t lin kt vi nhau cin^ t.hc hin Iit nhim v k thu t c th c ch r (nhing khng ch ra c th cch thc lin kt bn trong khi). ! cch biu din mch bng s kki r t gn, qua d dng hinh dung tng qut hot, ng ca ton b h thng mch in t.

1.2. TTN TC V TN HIU

Tin tc v tn hiu l hai khi nim c bn ca k thu t in t tin hc, l i tng m cc h thng mch in t c chc nng nh mt cng c vt cht k thu t nhm to ra, gia cng x li hay ni chung nhm chuyn i gia cc dng nng lng gii quyt mt mc t iu k thu t nh t nh no .

. 2.. Tin lc c hiu l ni dung cha ng bn t rong mt s kin, mt bin c hay mt qu tr nh no (gi l ngun tin). Ti'ong hot, ng a dng ca con ngi, t lu hinh thnh nhu cu trao i tin tc theo hai chiu : v khng gian bin c xy ra ti ni A th cn nhanh chng c bit nhng ni ngoi A v v thi gian ; bin c xy ra vo lc cn c lu gi li c th bit vo lc + T vi kh nng T oo nhu cu ny c tha mn v pht trin di nhiu hnh thc v bng mi phng t in vt cht ph hp vi tr nh pht tr in ca x hi (k hiu, ting ni, ch vit hav bng cc phng tin ti t in khc nhau). Gn y, do s pht

trien v tn b nhanh chng ca kl thut in t, nhu cu ny ngy cng f iti(ia man su sc' t rong iu kin ca mt hng- n thng tin rn Xi hi hin 'i.

Tnh cht quan rong nht ca tin tc l n mang ngha ,vr S/ thig k. th' hi(*n cc mt SUI :

a) Ni ciun^ flua trong nil s kin cng CC) nghia ln (ta ni s kin c lng tin t c' (,ao i khi I1) xy ra t'nt h nc, cn g t c (;h i. N ^ h a l lng tin c ln ti l vi hat ng hay fi IC vi xc suat xut hn ca s kin v c th diin^ xac SU i nir (O lng tin tc.

b) Mac' di nhn c ni dung" ca mt s kin no , ti'ong hu ht nitrng hp. ngi ta chi khn^' nh ( tnh th' chan, xc thc ca n vi nit tin c-y no . Mr chic chn cng cao khi 'ng mt. ni dung c lp li (v c bni nhiu lan. nha l tin tc cn c tnh chr tvmg binh hng k ph thuc vo Iic* hn o;in (*ua ngun tin, ca i t i i n^ (knh) truyn tin v c vo ni nhn rin, vo t t r kh nan^ ^v sai nhm c Th ca mt h thng thng tin. Ngi ta C- th dng Entropy nh gi lng Un thng qua C(! gi tri ('ntropy 'if'Hg r ca ngun tin. knh t.i'uyon tin v nci nhn tin.

O Tin tc khng t nhin sinh ra hoc m t i in *hi l lt biu hin ca ccqu trnh chuyn ha nang lng hay qu tr inh trao i nng ln^ gia hai dng vt cht v trng. Phn ln cc CU trinh ny l ning t.nh ngii nhin tun thoo cc quy lut phn h ca Ihuy't xc. sut thnng k. Tuy nhin c th thy rng nu mt h thng c nang lng n nh, Iic t r t t cao thi cng kh thu thp c tin t.c t n v ngc li.

C s ton hc nh gi nh lng cc nhn xt trn c tr inh by trong cc gio tr inh chuvf^n ngnh v li thuyot. thng tin [ 10,] ] ] .

/ . 2 . 2 . T i n i u l k h i n i m m t c c b u k i n v t c i a t i n l . c . C c b i u

hin ny a dng v thng c phn chia thnh hai nhm : c bn cht in tv khng r bn cht in t. l \ iy nhin, dng cui cng thng gp l.rong cc h thng in t. the hin qua ihn^ s trng thi cli(n p hay dng in, l c bi cht in t.

C th coi tn hiu ni chung (d di dng no) l mt, lng vt l bin thin theo thi gian v biu din n di dng mt hm s hay th theo thi gian l thch hp hn c.

Nu biu thc theo thi gian ca mt tn hiu l s(t) tha mn iu kin :

s(t) = s(t. + T) 1 - 10)

Vi mi t v y T l mt hng s th st) c gi l mt tn hiu tun hon theo thi gian. Gi tr nh nh t trong tp {T} tha mn (1-10) gi l ch k ca s(t). Nu khng tn ti mt gi tr hu hn ca T tha mn I -IO) th ta c s(t) l mt tn hiu khng tun hon.

Dao ng hnh sin (h.1.2) l dng c trng nht ca cc tn hiu tun hon, c biu thc dng :

s(t.) = Acos(jt - ( f ) (1-11) n h 1 . 2 : ' t i h i t h i l t s i n V / i c c

h u / n s d c n c n \ A , ,

\'on\ 1 -1 1 t A, (!.>.

Ci Di dng ca tin hiu l t s giia cc gi tr ln nht v nh nht cia cng sut. tc thi ca tn hiu. Nu t nh theo n v logarit (clexibel), di ng c nh ngha l :

min {s(t) mins(t )

thng 30 ny c trng cho khong cng d hay khong ln ca tn hiu tc cng ln mch hoc h thng in t.

d Thnh ph n mt ckieii v xoay chit ca tn hiii,

Mt tin hiu s(t) lun c th phn tch thnh hai thnh phn nit chiu v xoay chiu rfao cho :

s(t.) = s- + (1-18)

vi s l thnh phn bin t.hin theo thi gian ca st) v c gi tr t rung bnh theo

thi gian bng 0 v l Ihnh phn c nh theo thi gian (thnh phn 1 chiu)

Theo cc h thc (1-13) v (1-18) c :

c : s- = s(t) s(t)

v s- = s(t) - s(t) = 0 ( 1- 2 0 )

e) Cc thnh p h n chn v Ic ca tn hiu.

Mt tin hiu s(t) ninfi; lun c th phn t rh cch khc thnh hai thnh phn chn v l c xf nh nh sau :

s^,|,(t) = s^.|,(-t) = | [ s ( t ) + s ( - t ) ]

S| ,(t) = -S|^.(-t) = [ s ( t ) - s ( - t ) ]

(1-2 1)

t suy ra

s^.|,(t) + S| .,(t) = s(t)

v s,.|,(t) = s(t) ; = 0 ( 1- 22)

f) Thnh p h a n thc v o ca tn hiu hay biu din phc ca mt t n hiu

Mt tn hiu s(t) bt k c th biu din tng qut di dng mt s phc :

s(t) = Res(t) + jlnis(t) (1-23)

y Re s(t) l phn thc v Im s(t) l phn o ca s(t).

Theo nh ngha, lng lin hp phc ca s(t) l :

s*(t) = Reset) - j lms() (1-24)

.12

(1-25)

Khi cc thnh phn thc Vi\ o ca s(t) l.hno (1-23) v (1-241 c xc nh bi :

R e s m = r si) + S{t )

Ims(t) = i Ls{t) -

Trng hp ring, nu s(t) = ta c cng thc Ole :

= COS + snaI (1-26)

, " y' cori jsinrl

Suy ra cch biu din cc hm iu ha di dng mt. hm s ni nli sau

COSc(1-27)

Cc* cch biu din (126) v (1-27) c bit thun li cho vic t nh ton cc hm lng gic khi chuyen chng v dng tnh ton CC hm s ni. Vi d, xt biu thc i vi hm iu ha s(t) = Asin(/-'t + fp) (1-1 l , p dng h thc (1-27), co' th vit li s(t) di dng

s(t) = A.Im

= Ini I

= IniAe' '"

Vi A = Ae' l bin phc ca s(t). Gi tr mod un ca A bn" :

(1-28)

= Ai COS (f + sin^y^) = A

tc l chnh bng bin ca s(t) biu din dng biu thc l l l ) \ Nu st.) cho di dng biu thc hm COS (1-11) thi khi d ta c cc kt qu tng t he thc (1-28) :

s(t ) = ARe +' '1 J

= Re Ae^' . e"'"!

- Re I A . o' O (1-29)

Ta c th tm mi lien h tng qut _ia hai cch bpu din phc ca s(t) nh coi s(t) nh 1 vect biu din trn mt phng phc ta Decc c chiu di A (gi l modun ca s(t) v gc nghing^vi trc honh l (p (argumen) ca s(t). T h thc (1-23) vi quy tc tam gic lng SU} ra cch t nh modun A v gc pha (f cia s(t) theo Re s(t) v Im s(t) nh sau :

Hnh .-

13

Nu s(t) = a + j b th A = / a + b

v

Gxpjiyi + tp

i^(t) Aj^ l = -./;) 'A, . 01

trong r Aj. ln lt l mdun v argumen ca Sj{t) v

(u;i I rnh tch tin tc khi li tin ly li ni dung tin tc tn s thp ti thit thu gi l qu tr inh

f) C kh nng o nhiu thng s (nhiu knh,) hay o xa nh kt, hp Ihit bi dovi mt h thng thng tin triiyn d liu, o t ng nh mt chng tr nh vac sn o iu khin bng //p)...

.3.3. H i (fu chnh

H c nhim v theo di khng ch' n)t hoc vi thng s ca lt q u tr nh sao cho (hng s ny phi c gi tr nm trong lit gii han fi nh trc (lOU' ngoigii hn ny) tc l c nhim v n nh thng s U n^) mi tr s hay mtdi tr s cho trc.

1. S cu trc :

) i

c n khnr i! c h e

^ icn i d u v;-K

Ch hi krl [

S o n I;m h n

s n n hch

liOu c l u i n

K h o i c h a i h a n h Khiicch ;.nsat Icch Al --= u.x - u.h

N I 1.7 : S(f (' k ; (UiU h ( il n ^ (lic it c l n i n i i i (t.

2. Cc c im ch yu :a) L h dng cu trc kn : thng tin truyn thoo hai hng nh cc mch phn hi.

b) Thng s cn o v khng ch c theo di lin tc v duy tr lc hoc gii hn nh sn.

V d : T' (cn theo di khng t:h) c bin i Irk: tin thnh Ux sau , ssnh Ux v ch c pht hin ra du v in ca sai lch (ch tng ng vi nucchun Tch c nh sn n i tng cn c khng ch ).

Sau khi c khuch i lng sai lch Au = ~ c a ti khi ckphnh iu khin tn g hoc gim theo yu cu ty du v ln ca Au. S c3 kh nng :

Khi Au = 0, ta cd (^ = u . ) i tng ang t rng thi mong mun,nhnh thng tin ngc khng hot ng

Khi Au > 0 (^ > > Tj.|, h iu chnh ln gim

- Khi Au < 0 h iu chinh lm tng qu tr nh iu chinh chngng khi Au = 0.

c) mn (chnh xc) khi iu chnh ph thuc vo

chnh xc ca qu tr nh bin i t thnh

phn di ca phn t so snh ( nh ca Au)

D chnh xc ca qu tr nh bin i Tj thnh

Tnh cht qun t nh ca h.

d) Cd th iu chnh lin tc theo thi gian (analog) hay gin on theo thi gian min sao t c gi tr trung bnh mong i.

16

IMnng' php digital cho php, tit kim nng lng ca h v ghp ni vi h thn^' l ng t.nh ton.

t'i 'h rang thng thng n u chn mt. ngng U(h. ta nhn c k't (|u l h du khin ('() hnh ng hay khng ty th ' 0 Ux ang ln hn hay nh hn Uch (v (lo o' hani s vt ] cn theo di ang ln hn hay nh hn gia tr nging nh s,in l trM. Khi chn t hai mc ngng Uch! v UcUl h s hnh ng mi khi l \ nnn lol vo troig khong hai gi tr ngng hoc ngc . iu nv mang V nghia lic l hn ca Iit h t ng iii chinh cn ting hp vi mt mc ngng h mang- ngha dng iu khin trng thi ihnh vi) ca i tng.

2- KT01A 17

Chng 2

K THUT T N G T

2.1 - CHT BN DN DIN - P?N T MT MT GIP P -N

2.7 ./. C h i hn dn tvn ch i v ch i hn (ln tap ch i

a - Cu trc v/ng nng ng ca ckf ran t nh thTa bit cu trc nng lng ca mt nguyn t, ng c lp c dng l cc mc

ri rc. Khi a cc nguyn t i gn nhau, do tng tc. cc mc ny b SIIV bin thnh nhng di giii nhiu mc st nhau c gi l cc ving niig lng. y l dng cu trc nng lng in hnh ca vt rn tinh th \2].

'Piv theo tnh trng cc mc nng lng trong mt vng c b in t chin h hay khng, ngi ta phn bit 3 oi vng nng lng khc nhau :

Vng ha tr (hay cn gi l vng y), trong t t c cc nic nng lngu b chim ch, khng cn t rng thi (nicl nng lng t do.

Vng dn (vng trng), trong cc mc nng lng u cn h t rng hay chb chim ch mt phn.

Vng cm, t rong khng tn ti cc mc nang lng no in t c th' chim ch hay xc sut, tm ht ti y bng 0 .

Ty theo v tr tng i gia 3 loi vng k trn, xt theo t nh cht dn cin fia mnh, cc cht rn cu trc tinh th c chia thnh 3 loi xt o K) th hin t r n hnh 2 .1.

l / /7C ^

)b )

/ / / / / / / / / / X'

V///

n h 2 . : H n o V! ran then c u in k ' v i; n m : \; :u ) ( ' h C i k h i / : ' > 2 c i ' : h ) ( h J i h ( l n d i c n : , ^ 2 f V : c i ( ' K J I ( n i i O i i .h i

Chng ta bit, mun to dng in trong vt rn cn hai qu tr nh ng thi :qu tr nh to ra h t dn t do nh c kch thch nng lng v qu tr nh chuynng c hng ca cc h t dn in ny di tc dng ca trng. Di y ta xt ti cch dn in ca cht bn dn nguyn cht (bn dn thun) v cht bn dn tp

18 2-KTO-B

t !i il in iin kh;t nlau t:h yu lin (iian ti (]ii tr inh sinh U;ui) *' hat t (l) n.mg linh th'.

) - Ch b( (i hn)

ai 1hal an dn thiian ci'n hinh l ('r(nianiuiii K(.'i v Siiriun (Si) to' cu ti'cvun: nan^;' ln^' hinh 2.11' vi K., " (),72V v thuc nho'm hnhang tun hon MikI(I(''[i . M hinh ('u trc mang tinh th' (1 t'hiou) (n chung (()

h i n i i 2 . 2 : \ v i h ; \ n c h t l c c l i n k 't ^ h p i c n h a t r v n h n g o i . ( 5

0 K chn l rac rh t ('ac'h (in. Khi dic lt ngun nan^ in^ ngoi kich thiVh-x ; \ Ia hin tin^ ; i o n lua cae n < ; u y n t l i i t Mai^^ v s i n h n ^ (I ht d n ( cio :dien t h\ khi ln k('\ h('*p di tr thnh hat l do v c('* lai 1 lif-n kt li khiiy't( l o trnnj :; ) . 'IVn t h viUX n a n ^ l n ^ h i n h 2.2h. dic'U n y t n ^ n ^ v i s c h u y n i(;n t t 1 lc nang ln^ tron^ vung h()'a tri l*n 1 mc trnng vn^ dan li 1iuii' U co MrnX) trong vn^ hoa tr. (-;U cp hi dn t do nv. di tac dng ra1 ti-ni n^oi hay mt (racliM nn^ c kh nang dch chuyi t'f) hn^ tron^'nii^ t i h h l o ln CII^ (in trong Cht han dn thu an .

K('I C|u l : li Mun tao ht dn t df) tron^ t'ha h;n dn thvin cn v nangI ' C 1 ; : k i c h t h i c h i i K ki

l, r:r: i

T .r

...

. r

2 e

'/ z:Zy/ -

. i 'I I

.* I

t f:*1 i-/

s'1 l u h 2 .2 ; (i) C n i rm /ntit/^ Ii /th i h r m i n ( i i r u c t th i ' i i hiii //// ih i i n -V/

h ) f ) i l i v h : : ( ) i l i i t l i t l t - ^ i i i s i n h r i ! - ^ c p h i t ( l ( h i I f l / .

2) Dng in t rong cht hn dan thiin gm hai t.hnh phn tng ng nhau doqua tr inh pht sinh l.ng c-p hat. dn to ra (n, = P | ) [2, 8].

c ~ ( 'hat hn cin tp cht OI n

Ngi ta t.in hnh pha thm cc nguyn t thuc nhrm 5 bii4 Mendeleep vomng tnh th cht bn cln nguyn cht nh cc cng ngh c bit, vi nng khong l o " n nguyn l/cnv\ Khi cc nguyn t. tp cht tha mt int vnh ngoi, lin kt yu vi h t nhn, d dng b ion ha nh 1 ngun nng lng yu to nn 1 cp ion dng tp cht - in t t do. iu kin binh thng (25''C) ton h cc Iifuyn t tp cht a b ion ha. Ngoi ra hin tng pht sinh ht. ging nh c ch ca cht bn dan thun van xy ra nh c m t trn hinh 2 .-3a. vi mc ycu hn. TTi'n th vng nng lng. cc Uc nng lng tp cht loi ny (gi l t.p cht loi n hay loi cho in t - Donor) phn b' bn trong vjig cm. nain st y vng dn (khong cch c vi C' eV).

O

K't qu l trong m ng t inh th tn ti nhiu ion dng ca lp cht bt n^ v dng in t rong cht hn cln loi n gnni hai thnh phn khng bng nhau lo ra :

;H n 2 . .< : f ) f ( t \ ; ( r f ^ ^ V) C ( f ( / / { S I / I l i a i l i i i r u i : l i h a n l n i t p l i i :

j} liii n hi hnu )

in t c gi l loi ht dn a s c nng l l trng - loi thiu s t nng (chnh nhau nhiu cp ; 1|

d - Cl. bi dn tp cht loi p

Nu tin hnh pha tp cht thuc nhm 3 bng tun hon Moncleloep vo mng tinh th cht bn dn thun ta c cht bn dn tp cht loi p vi c im chyii l nguyn t tp cht thiu mt in t vnh ngoi nn 1 lin kt hca .r ([h(*pi) b khuyt, ta gi l l trng lin kt, c kh nang nhn in t, khi nguyn t tp cht b ion ha s sinh ra ng thi 1 cp : inn m tp cht - l trng t do. Mc nng lng tap cht loi p nm t rong vng ci st inh vn^ ha tr. l inh 2.3b cho php gii thch cAch sinh ht dn ca cht bn dn loi ny. Vcy trong mng tinhi.h ch- hn cl tp cht, loi p tn ti nhiu ion i tp chi ('() tnh (h;it nh x lng vng v dng n trong cht bn dn loi p gm hai thnh phn khng tng ng nhau : l t rng c gi l cc ht. dn a s, in t ht. thiu s, vi cc nng tng n g l Pp v np (Pp Tip).

c ~ Vai hin tig vt l thng gp

( ach sinh h t dn v to thnh dng in trong cht bn dn thng lin quantrc tipp t.i cc hin tng vt l sau ;

in tng ioi IO nguyn t (ca cht bn dn thun hay ca cht bn dn lp cht) l hin tng gn lin vi qu tr nh to ht dn t do hay chuyn di nic nng lng ca cc ht. R rng s' h t sinh ra bng s mc nng lng b chim trong vng dn hay s mc b trng i t rong vng hda tr. Kt qu ca vt. l thng k lng t cho php t nh nng cc loi h t ny da vo hm thng k Fermi-Dirac [2 , 8 ] :

n = / N(E)F(E)dE ; p = / N(E)F(E)dE (2-1) 'c 'min

vi n, p l nng in t trong vng dn v l t rng trong vng ha tr.

E , l mc nng lng ca y vng dn,

20

Fj . l mc nng lng ca inh vng hda tr,

[p trng thi nang lng cao nht c in t,

l t rng thi nng lng thp nh t ca trng,

N( . j l hm mt t rng thi ht theo nang lng,

F(|,J l hm phn b thong k ht theo nng lng.

Thoo o' ngi ta xc nh c :

Kc - E. Ej.. - E,n = N,,exp ( ----- ) ; p = xp ( ------------------ ) *2-2)

vi N ., N , l mt t rng thi hiu dng trong cc vng tng ng, E|,- l mc th ha hoc' ti Formi.

Kt (lu phn tch cho php c cc kl lun, ch vu sau ;

() t r n ^ thi cn hng, tch s nng hai loi h t dn l mt hng s (trong b t k C'h;. b n d n lo i n o )

Pn = = N .N,,0xp ( - ) = const (2-3)

ngha l vic tng nng 1 loi h t ny lun km theo vic gim nng tng ng t*a lo;.ii ht k ia .

Ti'ong ('ht hn dn loi n c > > i >>Pn tlo d s' in t t do un bngs ln

('c thnn^^ so v r,, quyt nh ti c;u' linh cht tan s6 (tac ng nhanh 1 ra cnc ciui; - (U \vAn (.ln,

- ( t y c ( a c < t n M t i ) a l u l ( n t r o i , ( c n t n o L g

Di i ( ; i ' c l u n f i i a i t r n g . h ; i l d n t (]() 1h u y ' n d i ^ n h l i n ^ c ^ : t o e

tai } nf*n 1 d n o - c i n d n ^ t r i ) v i v n t r t r u n ^ ^ t i n h t i (- v j (Uf n j ; d p,tu a t r n g ' :

Vih = //K Suy ra v,h, = -//,1 (2-i

'\i'on^ o' //,,, //|, l cc h rf li gi l cl linh dng ca cc hl (ln tifn-- ln;^(vi 1 'ht han dan tp chl ch to l CV c = ;],S()() c n r Vs : // = ISOO c u r Vst Si I- = i ;3()0 r m - V.S ; = SOOcn r Vs ) .

' r o ', m t d n f t r i g m h a i i h n h J i h a n :

11 111 II = C , n . V| |if

vi q l in tch cc hat (2 - 7 hioi q p . V||,p

lin>' c l o n ^ t 1 t ( . n p h a n I .. 1 .. + 1' i'M Itiun

l i = ) K ( n + p / / | , i 2 - 8 i

- Chi/vc (ng lnrch tn ca (c ht (n

I)( ('.() s chnh lch v nng thfC) khn^' C*U' hit cln tlii* hin ('huynng khufh tn t ip co' nng CVO ti lp C( nng th'p. Mt cln^ khuch lan th(*) phng ^ini ci nng c dng' : fJSl

cl n cl n= ->1 n( - I = q n

- q D , _ .2-101

vi ) v D l cc h s 6 ti l gi l h s khurh ln ca cc' ht. tn^(])j = 32 cm-Zs ; = 12 cmVs)

Ngi ta chng minh c cc t nh cht sau :

KT# 0 = ' - i = U .,// (h th( E in s t ( in)

q ' 'Trong IJ'|- l th nhit

linh 2.4 biii dion m hnh l tn^! hoa nt lit ^h p - n khi chia c in ap n^oi t vo, Vi gi thit nhi phng, cc ngiiyn t tp cht b ion h a h o n t o n (n,-j = N | ^ ; P p -

Cc hin tng xv ra ti ni lip xc - th' 1 t toi t t nh sau ;

Do c s chih lch ln v nng (n,, Hj, v P n ^ ti vung tp xc (() hi(ti tng khuch tn cc ht a s (ua ni t ip g ip , x u t h in 1 d n g in khufh t;n | J hng t p sang n. Tai1 vin^' ln cn ,, hai bn mt. tip xc, xu;'U hin nit lp in .ch kli do ion t;ip cht to ra. trong ngh-o ht dn a s v co in tr n (hn nhiu cp s o v i C K' v n g c n l i K c o n g

th(i xu hin 1 in trng ni b hng t ving N (p ion dng Npi

san^ vng p (lp ion ni N, ) gi l i('n trng ti'p xc (h. 2.4c.

Ngi la ndi i xut hin ] hng ro in th' hay mt hiu th tip xc Uix- B day lp ngho n ph thuc vo nng tp tht, nu N,\ - N|) th 1,1 i xng qua mt tip xc : lon ~ Kip ; thng N,\ N n nn 1,U1 hii v phn Ih yu nm bn loi bn dn pha tp cht t hn (c in t r sut. cao hn). Din trng E(X cn tr chuyn ng ca ng khuch tn v gy ra chuyn ng gia tc (tri) ca cc ht. thiu s ({ua min tip XIC, c chiu nt^c li vi dng khuc'ch tn. Qu trinh ny ti'p din s dn ti 1 trng thi cn bang ng : ki Iir v khng c dng in qua tip xc p-n . Hiu th tip xc c gi tr xc p, c xc nh bi

n h ; \ f p - n k i c h ira (7/(iOn irina:,

d M h in h c i i tri i h C i i : h) r i u h f('fi;

( v i o ih it y c ff i'f) ff /\;[} :

(h U i u i l ic Cp xc lid v itii^ ro h c U ii n i cf} vc :

C} K l i i (ny c d in t hn ii)n.

[2, 8]

KT Pp KT n = - ln ( ) = ln ( )

q 'v Pn / q \ n,, /(2- 11)

Vi nhng iu kin tiu chu?, nhit phng, Uix c gi tr khong 0,3V vi loi tip xc p - n lm t Ge v 0,6V vi loi lm t Si, ph thuc vo t s nng ht dn cng loi, vo nhit vi h s nhit m i-2mV/K).

b - Mt ghp p - i khi c din trng tgoi

'IVng thi cn bng ng nu trn s b ph v khi t ti tip xc p - n mt in trng ngoi. C hai trng hp xy ra (h. 2.5a v b) :

23

+_ p / ' / p

____.................................... J

1 ^------- 1-------------4- ^ -

ti / / 1

'l / / I

4-

K

^)

\ n h 2 5 M i i;hcp p - n kh i p h n cc ihtin a) v p h n cc ngirc (b).

Khi in trng ngoi (Eni:) chiu vi Etx (tc l c cc t nh dng tti p, m ti n). Khi do Erv- ch yu t ln vng ngho v xp chng vi Etx nn cng trng tng cng ti vng 1,, gim i do d lm tng chuyn ng khuch tn Iki T ngi ta gi l hin tng phun ht a s' qua min tip xc p -n khi n c ni. Dng in tri do Ext gy ra gn nh gim khng ng k do nng ht thiu s nh. Trng hp ny ng vi hinh 2.5a gi l phn cc thun cho tip xc p-n. Khi d b rng vng ngho gim i so vi I(>.

Khi Eng cng chiu vi E|X ngun ngoi cr cc dng t ti n, m t ti p), dotc dng xp chng in trng t.i vng ngho, dng ki gim ti khng, dng Iir c tng cht t v nhanh n mt g tr ho hH gi l dng in ngc bo ha catip xc p-n. B rng vng ngho tng ln so vi t rng thi cn bng. Ngi ta gi l s phn cvc ngc cho tip xc p-n.

Kt qu l mt ghp p -n khi ttrong 1 in trng ngoi c tnh cht van : dn in khng i xng theo 2 chiu. Ngi ta gi l hiu ng chinh lu ca tip xc p -n : theo chiu phn cc thun (U,\K > 0 ), dng c gi tr ln to bi dng ht a s phun qua tip gip p -n m, theo chiu phn cc ngc (Usk < 0 ) dng c gi tr nh hn vi cp do ht thiu s tri qua tip gip p -n kha. y l kt qu trc tip ca hiu ng iu bin in tr ca lp ngho ca mt ghp p~n di tc ng ca trng ngoi.

c ~ c tuyn Von-Ampe v cc tham s c bn c it bn dn

it bn dn c cu to l mt chuyn tip p - n vi hai in cc ni ra pha min p gi l ant, pha min

24

. ( /T7 / )i

Sz

th 2.6 : Dc luyn V on-A m pe ca i t hn dn.

n Xi la catt. Ni tip it bn dn vi 1 ngun in p ngoi qua 1 in tr hn rh dng, bin i cng v chiu ca in p ngoi, ngi ta thu c c tuyn Von-Ampe ca it c dng hnh 2.6. v l 1 ng cong c dng phc tp, chia lm 3 vng r r t : Vng (1) ng vi trng hp phn cc thun, vng 2) tng ng vi trng hp phn cc ngc v vng (3) c gi ] vng nh thng tip xc p-n . Qua vic phn tch c t nh Von-Arnpe gia l thuyt v thc t ngi ta rt ccc kt un ch yu sau :

- Trong vng (1) v (2) phng trnh m t ng cong cd dng xeni [8])

I a = I,(T)[ - x p { ) - 1 (2 - 12)

^po I^pPn .trong o T ) = q . s . ( + L )

gi l dng in ngc bo ha c gi tr gn nh khng ph thuc vo U^J^, ch ph thuc vo nng ht th iu s lc cn bng, vo di v h s khuch tn tc l vo bn cht cu to cht bn dn tp cht loi n v p v do ph thuc vo nhit.

U | = gi l th nhit ; T = 300"K vi q = 1,6.10' c, k = L38.10'"- J/K

U | c gi xp x 25,5mV ; m = (1 -- 2) l h s hiu chinh gia l thuyt v thct.

- Ti vng m (phn cc thun) : u - v c ph thuc vo nhit nn dng ng cong ph thuc vo nhit vi h s nhit c xc nh bi o hni ring

theo nhit .

____, mV,')T

ngha l khi gi cho dng in thun qua van khng i, in p thun gim t l theo nhi t vi tc -2mV/K.

- Ti vng kha (phn cc ngc) gi tr dng bo ha nh (10 A/cm^ vi Si v 10'^* A/cm vi Ge v ph thuc mnh vo nhit vi mc +10% gi t r / ' K : AI (AT = 10 K) tc l dng in ngc tng gp i khi gia s nhit tng lo"C.

- Cc kt lun va nu i vi v ch r hot ng ca it bn dn phthuc mnh vo nhit v trong thc t cc mch in t c s dng ti it bn dn hoc tranzito sau ny, ngi ta cn cd nhiu bin php nghim ngt duy tr s n nh ca chng khi lm vic, chng (b) li cc nguyn nhn k trn do nhit gy ra.

- Ti vng nh th n g (khi < 0 v cd tr s ln) dng in ngc tngt ngt troig khi in p gia ant v ka t t khng tng. Tnh cht van ca it khi b ph hoi. Tn ti hai dng nh thng chnh :

nh thng v nhi t do tip xc p - n b nung nng cc b, v va chm ca ht thiu s c gia tc trong t rng mnh. iu ny dn ti qu tr nh siih ht t (ion ha nguyn t cht bn dn t.hun, c t nh cht thc l) lm nhit ni tip xc t ip tc tng... dng in ngc tng t bin v mt ghp p -n b ph hng.

25

nh thng v in do hai hiu ng : ion hra do va chm (gia h t th iu sc gia tc t rong trng mnh c lo'^v/cni vi nguvn t ca cht bn dn thunthng xy ra CC mt ghp p -n rng (hiu ng Zener) v hiu ng xuyn hm (l\ inen) xy ra cc tip xc p -n hp do pha tp cht vi nng cao lin quan ti hin tng nhv mc trc tip ca in t ha tr bn bn dn p xuyn qua ro th tip xc sang vng dn bn bn dn n,

Khi phn tch hot ng ca it t rong cc mch in c th, ngi ta thng s dng cc i lng (tham s) c trng cho nd. C hai nhm tham s chnh vi mt it bn dn l nho'ni cc tham s gii hn c t rng cho ch lm vic gii hn ca it v nhm cc tham s nh mc c t rng cho ch lm vic thng thng.

- Cc tham s gii hn l :

in p ngc cc i ' it cn th hin t nh cht van (cha b nh thng) : thng gi tr chn khong 807f gi tr in p nh thng j),

Dng cho php cc i qua van lc m :

Cng sut tiu hao cc i cho php trn van cha b hng v nhit ; p

Tn s gii hn ca in p (ng in) t ln van nd cn c t nh ch t van

- Cc tham s nh mc ch yu l :

in tr 1 chiu ca it

U a k t I a

= U ' U "( L

in tr vi phn (xoay chiu) ca it :

AK u-l-rd = = 7r r~FT (2-14)

)Ia ( I a + Is)

u-rvi nhnh thun do ln nn gi tr I'j nh v gim nhanh theo mc tng

/\

ca ; vi nhnh ngc ^ -~ ln v t ph thuc vo dng gi tr v

cng chnh lch nhiu th t nh cht van cng th hin r.

in dung tip gip p - n : lp in tch khi tng ng nh 1 t in gil in dung ca m t ghp p - n : Cpp = C^J +

trong l thnh phn in dung chi ph thuc vo in p ngc (vi phnchc pF) v l thnh phn ch ph thuc vo in p thun (vi pF).

nhng tn s' im vic cao, ngi ta phi ti nh hng ca Cpp ti cc t nh cht ca mch in. c bit khi s dng it ch kha in t ng m vi nhp cao, it cn mt thi gian qu hi phc li t nh cht van lc chuynt m sang kha. Din p m van U ) l gi tr in p thun t ln van tng ng dng thun t c gi tr

Ngi t a phn loi cc it bn dn theo nhiu quan im khc nhau :

Theo c im cu to c loi it tip im, it tip mt, loi v t liu sdng : Ge hay Si.

Theo tn s gii hn c loi it t n s cao, it t n s thp.

26

Theo cng sut p.^ . c loi it cng sut ln, cng sut t rung bnh hoc cng sut nh (I^ I < 300 niA)

Thoo nguyn l hot ng hay phm vi ng dng c cc loi it chinh lu, it n nh in p (it Zener), it bin dung (Varicap), it s dng hiu ng xuyn hm (it Tunen)...

Chi t it hn, c th xem thm trong cc ti liu chuyn ngnh v dng c bn dn in [1, 8],

Khi xt it trong mch thic t, ngi ta thng s dng s tng ng ca it tng ng vi 2 t rng hp n v kha ca n (xem h.2.7)

./7fC

+

t ta c =dih

/r

7 C

UHiIC

-

0 ) V

Vh 2 .7 : S ( t ( ih ( m ^ c i t h n lt l c m

a - B chnh ii cng sut nhS dng t inh cht van ca it bn dn, cc mch chinh lu in hnh n h t u-ng

suat nh , c cho trn hnh 2 .8a, b, c, d.

n gin cho vic phn tich hot ng v rt ra cc kt lun chnh vi ccmch trn, chng ta xt vi trng hp ti ca mch chinh lu l in tr thun , saud c lu cc c im khi t,i c tnh cht in dung hay in cm v vi gi thitcc van it l l tng, in p vo c dng hnh sin ph hp vi thc t in pmng 110V/220V xoay chiu, 50 Hz.

- Mch c nh I hai na chu k : Nh bin p ngun, in p mng a ti scp c bin i thnh hai in p hnh sin U;> I v >-) ngic pha nhau t rn thcp. Tng ng vi na chu k dTig (Uij > 0, U-,^ < 0) D| m D-, kha, ''rvn Rdng nhn c c dng 1 chiu l in p na hnh sin do U^I qua D m to ra.Khi in p vo i du na chu k m) U-. < 0, > 0) D kha t m v trnRj nhn c dng do . to ra, (h.2.9).

Gi tr t rung bnh ca in p t rn ti c xc nh theo h thc (1.13) :

(2-15)

vi Ut l gi tr hiu dng ca in ap trn 1 cun ca th cp bin p.

Gi tr t rung bnh ca dng t rn ti i vi trng hp ti thun tr

I, = ~ (2- lG)R.

Khi dng qua cc clit D| v Dl

_ ,^al ^a2 ~ 2 (2-17)

v dng cc i i qua it l

= 7 1 , I , = f I, (2-18)a m a x

nh gi bng phng ca in p trn ti sau khi chinh lu, thng s dng h s p mch (gn

/ / / i / : Cii d h diCt p c a h c h n h l u 2 t ic h u k

(J) ) n p l c p b ) ) i i p ircti l i .

sng), c nh ngha i vi thnh phn sng bc n ;

.nmqn = (2-19)

Trong d l bin sng c tn s n.t ; l thnh phn in p 1 chiutrn ti.

u Imu. vi m l s pha chnh lu

q, = 0,67 (vi mch hai na chu k ni = 2).

28

in p ngc cc i t vo van kha hng tng in p cc i trn 2 cun th cp ca bin p:

= 2\T2 U, = 3,14 u (2 - 2 0 )

Khi cn chn van D|, c in p ngc cho php

ungccl > u Hicmax Khi dng ti l t lc c (ng t nt trn hinh 2.8a) ch xc lp, do

hin tng np v phng in ca t c mch lc o' lm vic ch khng lintc nh trng hp vi ti in tr. Trn hinh 2.9b vi trng hp ti in dung, ta thy r khc vi trng hp ti in tr c ny mi van chi lm vic t rong khong thi gian O -4- (vi van n) v ~ (i vi van O) nh hn na chu ki v thng mch np cho t t ngun . v 2 ]

IVong khong thi gian cn li, cc van u kha (do in p trn t np nhn gi tr tc thi ca in p pha tng ng u . 1 v T ), Lc t c phdng inv cung cp in p ra t,rn R.

Cc tham s chnh ca mch trong trng hp ny c thay i, khi

u = 1,41 . (2-21)

v qj ^ 0,02

(khi chn hng s thi gian mch phng ca t r = R..C ln) cn u khngi so vi trc y.

Nu xt mch hnh 2.8a vi tng na cun th cp bin p ngun lm vic V! 1 van tng ng v mch ti ta cd 2 mch chinh lu Iit na chu k l dng s n gin nht, ca CC mch chnh lu. Da vo cc kt qu phn tch trn, d dng suy ra cc tham s ca mch ny tuy nhin chng chi c s dng khi cc yu cu v cht lng ngun (hiu sut nng lng, chi tiu bng phang' ca U...) i hi thp.

- Mch chinh Ui c

Mch in nguyn l ca b chinh lu cu cho tr hinh 2 .8b, trong cu gni 4 van it c k hiu thu gn ; nu v y cu chinh lu ta c hnh 2 ,10.

Trong tng na chu k ca in p th cp , mt cp van c ant dng n h t v ka tt m n h t m, cho dn^ mt chiu ra R, cp van cn li kha v chu 1 in p ngc cc i bngbin V d ng vi na chu kl t\ u\'cn li h chnh hat cu /ni pha.dng ca -,, cp van DjD^ m, -.4 kha. R rng in p ngc cc i t ln van lc kha c gi tr bng mt na so vi trng hp b chinh lu hai na chu ki xt trn, y l u im quan trng nht ca s cu. Ngoi ra, kt cu th cp ca bin p ngun n gin hn. Cc thaii s chnh ca mch l :

in p 1 chiu lc h mch Rj.

= \2 U;; - 2U,) (2 - 22)vi U|) l in p thun t rn cc van in.

+ ^I 0

29

in p 1 chiu lc cd ti R, :

u,,oo = (1 - f ^ l ) (2-23)

vi R l ni tr tng ng ca ngun xoay chiu

T7 [ 7 - 1 cc gi tr U:)It l in p v dng in cun th cp bin p.

l in tr tng ng ca ti Ry = ^ra

Cng sut danh nh ca bin p ngun

Pb. = 1-21,, ( U , , ^ + 2U|,) (2-24)

in p ngc cc i trn van kha :

Ungcn = ^ ^ 2 = ^2-25)

Khi c ti in dung, mch lm vic ch xung lin quan ti thi gian phng ca t c lc cc van u kha v thi gian np lc mt cp van m gig nh phn tch vi mch chnh lu hai na chu k. Lc , dng in xung qua cp van m np cho t c l :

-

b - Cc mch g h imMt ng dng in hnh khc ca it bn dn l s dng trong mch ghim (mch

hn ch bin ).

+ T

^ - /T7 E m x

1 1

/ X / / / / / 0 / / /k i

n h 2.7/ ; ('n ii h n che n i licp.Mch hn ehe irCn mc (a) v d'n ih ihi i ian minh h (h) :M c h h / e h e d t r i m c E c ) T) c n i h i h ^ i a n m i h h ( c ) .

H n h 2.11 l cc mch hn ch ni tip. (it hn ch nic ni tip vi mch ti).

Xt trong t r ng hp n gin khi l mt in p hnh sin khng c thnhphn 1 chiu v gi th i t it l l tng (ngng m kha xy ra ti gi tr in pgia 2 cc ca nd bng khng U j = 0).

Khi ^ 0 it m v in p ra bng :

R _ _ Rl h RngUral = R + Rth + Rng

. Uv +R + Rth + Rng

. E (2-30)

vi R,( l gi tr trung bnh ca in tr thun it, l in tr trong ca ngun vo

Khi Uj_ < 0 it kha in p ra bng :

R __ -i- ncc ^ -ngUra2 = R + Rngc + Rna

. Uv +ng

Rngc + Rng R + Rngc + R h l >

. E (2-31)ng

vi Rngcl gi tr t r u n g bnh ca in tr ngc it.

31

Nu thc hin iu kin R ^Rn.^c ^ng

R R0 v = 1

do ,ai - U,,,:

PU kin Uj = 0 xy ra khi = E nn ngng hn ch ca mch bng ETc l vi mch hn ch trn la) thc hin ciu kin :

(Khi u,, ^ E , ,, < 0 cd u ,,,2 = E Khi U, < E , U , > 0 cd = IJ,,

v vi mch hn ch di (ci c :

'Khi U , & E , u, |Khi L\, < E , u | i < 0 , = E

> 0 . IJ,, = , ,

Khi thay i gi tr E, ngng han ch s thay i trong 1 di rng t - vmax< E < vniax vi vm.iN hin ca in p vo.

Ti*ng hp ring khi chn E = 0 ta c mch hn ch mc 0 (mch ghim ly 1 cc tnh ca t n hiu vo hav mch chinh lu na chu k xt trc).

Cng c th mc it song song vi mch ra nh hnh 2.12, lc ta c mchhn ch kiu song song.

; R, ^ c

khi u , 5 E , ^, > 0 , = Ekhi u < E , u| , < 0 , U, -

T iu kin : R|| ^ R,

Vi mch hnh 2.12a

Vi mch hnh 2,12b khi II,, s E , U I < 0 , U,. , = u,

ikh i l < V.

Lu rng nu n ngng m ca it thc th (loi Si c + 0 ,6V v loi Ge c + 0,3V) th ngng hn ch' ca cc mch trn b thay i i 1 gi tr tng ng vi cc mc ny.

c - On dnli din p bng dit Zener

it n p lm vic nh hiu ng nh thng Zener v hiu ng nh thng thc l ca chuyn tip p -n khi phn cc ngic. Trong cc it. thng thng hin tng nh thng ny s lm hng it, nhng trong cc it n nh, do c ch to c bit v khi lm vic mch ngoi c in tr hn ch dng ngc (khng cho php n tng qu dng ngc cho php) nn it lun lm vic ch nh thng nhng khng hng. Khc vi it thng dng, cc it n nh cng tc ch phn cc ngc. Nhng th a m s k thu t ca it Zener l :

0

0 -

o

11y-

p

- Din p n nh Uy (in p Zener) l in p ngc t ln it lm pht sinh 1ii hin tng nh thng, Trn thc t i vi mi it n p chi cr lt, khong r t hp ni n cr th n nh c, Khong ny b gii hn mt m t bi khong c tuyn ca it t phm vi dng bo ha sang phm vi nh thng lm dng tng t ngt, mt khc bi cng sut tiu hao cho php. Hay dng cc i cho php.

- Din tr ng ca it Zener c nh ngha l dc c tuyn tnh ca it ti im lm vic;

- d . /d l , (2-32)

Cn c vo (2 -32 ) c th thy rng dc ca c tuyn phn nh thng c tc dng quyt nh i cht, lng n nh ca it. Khi in tr ng bng khng (lc phn c tuyn nh thng song song vi trc tung) th s n nh in p t ti mc l tng.

Nh hnh 2.13a, thc hin chc nng n nh ngiri ta thng mc ni tip vi it Zener lt in tr v tc dng n nh c chng minh bng th t rn hnh2.13h.

C th thit lp quan h hm s gia in tr ng v in p n nh ca it. V d i vi it Zener Si, cng sut t iu hao 0,5W c dng th nh hinh 2.13c. T th ny thy in tr ng cc t iu khi in p vo khong 6 n 8V. V trong khong in p ny xut hin ng thi hin tng nh thng Zener v nh thng Ihc l lm cho dng ngc tng n t ngt.

- in Ir t nh R, c t inh bng ti s gia in 4p t vo v dng in i qua it.

/~

R, - , / , (2 -3 3 )

Dng in v in p k trn c xc nh t im cng tc ca it (h.2.13bj. in tr t nh ph thuc r t nhiu vo dng chy qua it.

- H s n nh c nh ngha bng t s' gia cc bin i tng i ca dng in qua it v in p ri t rn it do dng

gy ra :

l nh 2.7. ; a) n cni (tin p hfi (it /.rncr h) rtn lch (lo ih dc lnh n cinh c) S ph mc ca din tr (ti vo

in i (lnh.

(2-34)

Chng ta thy h s ny chnh bng t s gia in tr tnh v in tr ng ti im cng tc ca it.

D t h s n nh cao, vi mt s bin i dng in qua it cho trc, in p ri, t rn it (do dng ny gy ra) phi bin i nh nht. Cc it n dnh

3- KT0T-A .S3

Si thiing cd Z ^ 100. Trd khng ra ciia mach dn dinh cng l mt thong so ch you dnh gi chat lung ca mach :

Rr:i = dy AU, j l gia so ca dien p ra, gay ra bdi gia so AI,.., ca dng ti.

R rang t{ so ve phAi cng nh thi cht lung mach 6n dinh cng cao, vi the ccmach n dinh dng dit Zener cd dien tr ra cng nh cng tot (dieu ny ph hpvi vai tr mot nguon dien ap li tifng).

He so nhiet dp ca dien p n dinh he so ny cho biet s\i bien doi tUng doica din p dn dinh khi nhiet d thay doi l C ;

=(1 / Uj(du^ / dt) I.. = ('i)nsi (2-35)

He so ny xc dinh bdi he so nhiet dp ca dien p dnh thng chuyen tiep p-n . Sii phu thupc ca dien p dn dinh vo nhiet dp cd dang

U, = U, [1 + -, (T - T,,)]

Trong dd : l din p dn dinh ca dit Zener nhiet dpHe so nhiet dp

(2-36)

cd gi tri am neu hien tiing dnh thng ch yeu do hieu ng Zener gay ra. Nd cd gi tri diidng neu hien tUng dnh thng ch yeu do hien tiidng thc l gay ra. He s6 nhidt dung ca dit Zener cd the b t r cho he so nhiet dp am ca dit chinh lUu d

f f / 1 S///C r*A f

P fB,af

len erIr

Uhih 2.14 : To h(rp b nhici d ^o/n 2 idt m c noi fiep nhaii. Hkih 2.15

nhiet dp thng thiidng v he so nhiet ca c to hdp cd the dat den 0,0005%/^^C (h.2.16).

Cn ch y l he so nhie t dp ca dien p dn d inh tai 1 gi tri di^n p no dd trong khong t 5 den 7V, bng khng. S di nhii vay l vi t rong khong nhie t d ny tn tai c hai hien tUng dnh thng l Zener v thc l v he s nhiet ca hai hieu ng ny lai ngUc du cho nen cd ch chng triet tieu ln nhau. Dy l mt dc diem r t dng quy, chi xu t hien tai diem cng tc ca tng dit Zener trong khong t 5 den 7V. Tren hinh 2.15 tr inh by dc tuyen ca 3 dit do d hai nhiet dp khc nhau. N hng vng trn dnh d'u diem cng tc ca dit tai dd he so nhiet bng khng.

2.2. PHN T HAI MT GHEP P - N

N^u trn cng mt de bn dn ln lt tao ra hai tiep gip cng nghe p - n gn nhau thi t a dUc mt dung cu bn dn 3 cc gpi l tranz ito bipolar, cd kh nng khuech dai t in hieu dien. Nguyen li lm vic ca tranz ito dua t ren dac t inh dien ca t n g tiep gip p - n v tc dung tUng h6 gia chng.

34 3 r.lT.b

2.2-/ . ( a u ao, nutyn it l vict\ dc luyen v iam s ca run tito bipolar

a) Cu tao : T*anzito c cu to gni cc min bn dn p v n xon k nhau, ty theo trinh t sp xp CC min p v n m ta c hai loi cu trc in hnh l pnpy npn nh trn hnh 2.16. D cu to ra cc cu trc ny ngii ta p dng nhng phng php cng

/o' Fo-7 p

Mt/r i

//^ . . y . ^ 1.'. M//'/

f/n/fT

: 7 ^ - ^ ,

/ l / f 7C/^c^

? /ec/r

Af/7ir

frr?f/7 ' o /

V \ \ ' \ V/7

. N \ N ' . /M//7

/r;//i/ o

^ C/ec/I

M/rr [ t / / C / '

----------oSc?r

)

o-------

\ \ \ \ \ 1

' / / . \

N - X O

>

c

4

i h 2 . ( ) : \ f l i i n l ! i n r i r f i \ ^ l d r k i i i i a V i i i i i i p n p ( ) I'f) Hyt i h )

ngh khc nhau nh phng php hp kim, phng php khuch tn, phng php epitaxi...

Min bn dn thv nht ca tranzito l liin eniit vi c im l c nng tp cht ln nht, in cc ni vi min ny gi l cc emit. Min th hai l min baz vi nng tp cht nh nht v dy ca n nh c //m, in cc ni vi min ny gi l cc baz. Min cn li l min coloct. vi nng t.p cht trung bnh v in cc tng ng coloct. Tip gip p-n gia niin emit v baz gil tip gip emit (J| ), tip gip pn gia min baz v min colect l tip gip colect J(0. V k hiu tranzito cn ch l mi tn t gia cc emit v baz c chiu t bn dn p sang bn dn n. Vmt cu trc, c th coi tranzito nh 2 it mc inhau nh hnh 2.17. (?U ny hon ton khng c ngha l c mc 2 it nh hnh 2-17 l c th thc hin c chc nng ca tranzit.0. Bi vi khi khng c tc dng tng h ln nhau ca 2 tip P 1. Hiu ng tranzito ch xy ra khi khong cchgia 2 tip gip nh hn nhiu so vi di khuch tn ca ht dn).

b) Ngiiyn l lm vic : D tranzi to lm vic, ngi ta phi a in p 1 chiuti cc in cc ca n, gi l phn cc cho tranzito. i vi ch khuch i thJ| phn cc thun v phn cc ngc nh hlnh 2-18.

>1

' i h l . ' ^ : P n h t c h K ( n r u H z i n

t h n h h i i i l i o l v t i c i I t r t i i ^ l

% }

Vmh 1 8 : S o p h n c c ca iran io //V ) rt) )np b ) J tc h i i c h i.

35

phn tch nguyn l lm vic ta ly tranzito pnp lm v d. Do J. phn cc thun cc h t a s (l trng) t min E phun qua J|. to nn dng emit (I| ). Chng ti vng baz tr thnh h t thiu s v tip tc khuch tn su vo vng baz hng ti J('. 'Trn ng khuch tn mt phn nh b ti hp vi ht a s ca baz to nn dng in cc baz (I). Do cu to min baz mng ni gn nh ton b cc ht khuch tn ti c b ca v b trng gia tc (do Jj;' phn cc ngtc) cun qua ti c min colect to nn dng in colect (I( ). Qua vic phn tch t rn r t ra c h thc c bn v cc dng in trong tranzi to (h thc gn ng do b qua dng ngc ca J(') :

I,-: = Ib 2.37 >

nh gi mc hao ht dng khuch tn trong vng baz ngi ta nh ngha h s truyn t dng in a ca tranzito.

a =Ic

II.(2-38)

h s a xc nh cht lng ca tranzito v c gi tr cng gn 1 vi cc tranzito loi tt.

nh gi tc dng iu khin ca dng in I| ti dng colect I(', ngi ta nh ngha h s khuch i dng in ^ ca tranzito.

(2-39)

thng c gi tr t rong khong vi chc n vi t rm. T cc biu thc (2-37), (2 38), (2-39) c th suy ra vi h thc hay c s dng i vi tranz i to :

v

I i i = 1b (1 + ) (2-40)

(2-41).

c) Cch mc t ranzito v tham s ch t n hiu nh

Khi s dng v nguyn tc c th ly 2 t rong s 3 cc ca tranzi to l u vo v cc th 3 cn li cng vi mt cc u vo lm u ra. Nh vy c t t c 6 cch mc mch khc nhau. N hng d mc th no cng cn c mt cc chung cho c u vo v u ra. Trong s 6 cch mc y ch cd 3 cch l tranzito c th khuch i cng su t l cch mc chung emit (EC), chung baz (BC), chung colect (CC) nh hnh 2.19. Ba cch mc cn li khng c ng dng t rong thc t.

T o-

IE

/7k

c nh 2.19 : Phng' php m c iranzito trong hc t.

T tri sang phi : Chung emita, chung haza, chung colecta.

i

36

T cch mc c dng t rong thc t ca tranzito v nit s c th coi tranzito l mt phn t 4 cc gn tuyn t nh c 2 u vo v 2 u ra (h.2 .20).

C th vit ra 6 cp phng trnh m t quan h gia u vo v u ra ca mng 4 cc trong dng in v in p l nhng bin s c lp. Nhng trong thc t t nh ton thng dng nht l 3 cp phng trnh tuyn t nh sau :

Cp phng tr inh tr khng c c khi coi cc in p l hm, cc dng in l bin c dng sau :

nh 2.20 : Irunziio nh mi mi; bn cc.

U| - f(I| , I2) - r, I . I) + r j 2 . I2 -

^2 = f(ll ) I2 ) ~ ^ 2 1 1 * 22 2 =

ii ri2 I

>"21

Cp phng trnh dn np c c khi coi cc dng in l hm ca cc bin in p :

I , = f ( , , U 2 ) = g . U i + g , 2

I 2 = f ( U | , U i ) = 2 1 U | + g 2 2

Cp phng t r nh hn hp ;

U-U.

ii i

21 22

u ,u .

U i = f ( I i , 2 ) ' h u h i 2 ^ f l l

2 = f(Il , 2 ) ~ h 2 1 h 2 2l

2\ /

' 2 = f(Il , 2) h21 h22 2 \ / \ /

trong d Tjj, gj v hjj tng ng l cc tham s tr khng, dn np v hn hp catranzito.

Bng cch ly vi phn ton phn cc h phng trnh trn, ta s xc nh c cc tham s vi phn tng ng ca tranzito. v d :

ri2 =iU2dh I] = consl

gi l in tr ra vi phn (2-42)

g2l =dhdUi U2 - const

= s c gi l h dn truyn t (2-43)

n i =dUj 911 I2 = const

hii l in tr vo vi phn (2-44)

h2i =dhdh 2 = const

l h s khuch i dng in vi phn (2-45)

Khi xc nh c tuyn t nh (ch cha c t n hiu a ti) ca tranzito, dng h phng t r nh hn hp l thun t in v khi dd d dng xc nh cc tham s ca h phng tr nh ny.

d) c tuyn t nh da vo cc h phng tr nh nu t rn cd th a ra cc tuyn t nh ca tranzito khi coi mt i lng l hm 1 bin cn i lng th 3 coi nh mt tham s. Trong trng hp tng qut c 4 h c tuyn t nh :

c tuyn vo

c tuyn phn hi

u , = f(I,)

Ui = f(2)

U-> = const

(2-46)

37

Dc tuyn truy-n t

Dc tu vn ra

I, = 1(1,)

1, = f(i^)

l ^ - C iu is l

1, = c o n s i

'y th(u rch lic tranzito m CC quan h ny c tn gi c th dng in v ir-n p iduic nhau, v d vi kiu mc EC c tuyn vo l quan h

hay dc tuyn ra l quan h I(- = f(Uc.)I. = CDHSl

lnx ' 2 . 1) di y cho cc phng trnh ca h c tm-n tng ng suy ra r h phng trinh hn hp trong c trng hp inc nuich BC, KC v c c .

Bnq 2 .L Quan h hm xc nh h c tuvn tnh ca tranzito

Tng qut. J3C EC c c

= l J 1 )l ) . C( Misl 1. 1', u,,.

U|,(. = f(In)

| = [UJ.I lj Cinsi l i ' 1,i'= fCUci;' , U h(. = f< u , , 'li

1, = f(I|) u , - L'unsi 1, = illc = f( I , .

= nUr)11 consi

I, = UJ,, . Ic = f

.)

C'd th xy dng s tng ng xoay chiu tn hiu nh ca tranzito theo h phng trnh tham s hn hp

AU = H A I + h p A l

AI-, = h-,|Al| + ,AIj\

dng nh trn hinh 2 .2 1 .

(2 -47)

//

0 4 / AU^

H t h 2 . 2 : S ( l i ltrni i i '^ ftni^ 4 c c n y n l n l ( l a lCi) i h a n s .

Ch : i vi cc s EC, BC, c c cc i lng AIj, A U , AU-> tng ng vi cc dng vo (ra), in p vo (ra) ca tng cch mc. Ngoi ra cn c th biuth s tng ng ca tranzi to theo cc tham s vt l. V d i vi kiu mcBC c s hnh 2.22

y : - rj; l in tr vi phn ca tip gip emit v phn cht bn dn lm cc E.

- rn in tr khi ca vng baz.- I'('(l) in tr vi phn ca tip gip colect. Cc(i) in dung tip gip colect.- al\-: ngun dng tng ng ca cc emit a ti colect.

38

Mi lin h gia cc tham s ca hai cch biu din t rn nh sau : Khi AL-, vi mch u vo t,a c : A U = Al| [T| + (1 - a) r|]

AUihay hii = = [n; + (1 - a)rn]

= 0

re

CJB)-------- 1 I ^ L

I 1---------o L

> -

B

ih 2.22 : S( '() n (tn^ tham s vt ca rtizio on, S ( / l m c BC.

Vi mch u ra : AI-, = a . A do a = h 2 i khi AI = 0.

Dng mch raAU.

AI, =

h 22 =

A2

^C(H)do

1

vTB

rc:(B)

rc(B)

A = A h . r n nn ta c h i 2 =A2 = AI2 . rc'(B)

2.2.2. Cc dang mc mach c han ca ranzito

a - Mch chung emito (EC)

Trong cch mc EC, in p vo c mc gia cc baz v cc emit, cn in p ra ly t cc colect v cc emit. Dng vo, in p vo v dng in ra c o bng cc miliampe k v vn k' mc nh hnh 2.23. T mch hnh 2.23, cd th v c cc h c tuyn t nh quan trng nh t ca mch EC ;

/ < /

0 70-

nh 2.23 : S o tranzito m c chung cmit dng d xc nh cc h c tuyn.

nh 2,24 : H d c tuyn vo ca tranzito m c chung cmii vi cc gi tr khc nhau.

\)

xc nh c tuyn vo, cn gi nguyn in p U('., thay i tr s in p | , ghi cc tr s | tng Lng sau o' dng th quan h ny, s thu c kt qu nh hinh 2.24. Thay di ( n 1 gi tr c nh khc v lm li tng t s c ng cong th hai. Tip tc lm nh vv s c mt h c tuyn vo ca (ranzito mc chung emit.

T hinh 2.24, c nhn xt c tuyn vo ca tranzito mc chung eniit ging nh c tuyn ca chuyn tip p -n phn cc thun, v dng I| trong trng hp ny lmt phn ca dng tng | chy qua chuyn tip emit phn cc thun (h.2.23). n gvi nit gi tr ( .; nht nh dng cng nh khi ( |. cng ln v khi tng U c , tc l tng ( y cc gi tr in p l gi tr tuy t i) lm cho m in dintch khng gian ca chuyn tip coloct rng ra ch yu v pha min baz pha tpyu. in p Uc cng ln Ihi ti l ht dn n colect cng ln, s ht dn b tihp trong m in baz v n cc baz to thnh dng h;iz cng t, do dng baz nh i.

v c tuyn ra ca tranzito mc CE, cn gi dng I| mt tr s c nh no , thay i in p C|. v ghi li gi tr tng ng ca dng kt qu v c ng cong s ph thuc ca !( vo (-|. ng vi | cho trc. Thay i I| n cc gi tr c nh khc v lm tng t nh trn s c mt h c tuyn biu th quan h gia in p ra ( . vi dng khi coi dng I| l tham s nh hnh 2.25. T h c tuyn ny c nhn xt sau ; ti min khuch i dc ca c tuyn kh ln v trong cch mc ny dng khng gi c nh. Khi tng U( . rng hiu

dng nin baz hp li lm choJ c f j^ /r i /y /7 ^ /c / / / > / / ! c /^u^er?rc

/ u / 7 4 0 Z 0 - / -J -4 - 5 - s

ht dn n colect nhiu hn do dng tng ln. Khi U(-|, gim xung 0 th cng gim xung 0 (cc c tuyn u qua gc ta ). S d nh vy v in p ghitrn trc honh l U(-. = U('| ++ U|. nh vy ti ii un ca c tuyn, (| gim xung 0 , tip tc gim U('. s lm cho chuyn tip colect phn cc thun . in p phn cc ny y n h n g ht dn th iu s to thnh dng colect quay tr li min baz, kt qukhi U( .; = 0 th cng bng 0.Ngc li nu tng (|, n quln th dng I(' s tng ln t

ngt (ng t on t rn hnh 2.25), l min nh thng tip xc (it) ca tranzito. (Tng t nh c tuyn ngc ca it, khi U(|, tng qu ln tc l inp phn cc ngc ln ti mt gi tr no , ti chuyn tip colect s xy rahin tng nh thng do hiu ng thc l v hiu ng Zener lm dng tng t ngt). Bi v khi t ranz i to lm vic in p U( . ln cn c bin php hn ch dng

phng tranzito b ph hy bi dng qu ln.c tuyn truyn t biu th mi quan h gia dng ra (I) v dng vo khi

U(|, c nh. c tuyn ny c th nhn c bng cch gi nguyn in p thay i dng baz ghi li cc gi tr tng ng !( t rn trc ta , thay i cc gi tr ca U(;. lm tng t nh t rn c h c tuyn truyn t, cng c th suyra h c tuyn ny t c tuyn ra (h. 2.25). Cch lm nh sau : Ti v tr U q:

lnli 2 2 5 ; D c uycn ra v d c luyen iruyti di ca irruiio nc chum; Cii.

40

ch) trc trn c tuyn ra v ng song song vi trc tung, ng ny ct h c tuyn ra nhng im khc nhau. Tng ng vi cc giao im ny tm c gi tr

. T'n h ta If-, I| c th v c nhng im tha mn cp tr s va tim c, ni cc im ny vi nhau s c c tuyn truyn t cn tm.

b - Mch chung bazo

Tranzito ni mach theo kiu chung baz l cc baz dng chung cho c u vo v u ra. Tn hiu vo c t gia hai cc emit v baz, cn tn hiu ra ly t cc colect v baz. o in p v dng in u ra v u vo t xc nh cc h c tuyn t nh c bn ca tranzito mc chung baz (BC) ngi ta mc nhng von k v miiampe k nh hnh 2.26.

rv L

() rc

n h 2 .2 : S

Khi din p ngoai gim don 0. bn thn chuyen tiep colectd vn con din theticp xvic, chnh dien the tiep xc colecta da cuon nhufng hat dn tu baza sang colecta lm cho dong tiep tuc chy. De lni ding han thi chuyen tiep colecta phi diacphn CAC thun vi gi Iri nh nht la bng din the tiep xc, khi y din the tren chuyen tiep colecta se bng hoc duang ln, lm cho cc hat dn t baza khng the ang diac col(*cta (1 = 0).

^a7/r7 \4 /77/VffdC /a^er?

U - " > , 7

I

- 1- - / V '8

/ / 7 / 7 4u

UcJ- =V/77/?

I J r r 7 / i I

I

I

- 4 - 5 - 6 V

i iu \h J.JS : i ) c f i i v rn ri. iranzi l t n \ c chum ;f f n h 2 . i y ; f ) c inven i r u y c n lI i d t f / c \uv ra ir

(tac inven ra ca iran:itn rnuc BC.

Min dc tuyen t rong do' chuyen t iep colecta phan cUc thun goi la min bao hoa.

Nu tn g dien p nguac U(-|^ den mot gi tr i nh t dinh nao do (gpi la din p dnh thng) dng t n g ln dgt ngpt cd the dn den lm hng tranzito. Hien t i ang dnh thng ny do mpt t rong hai nguyn nhn ; Hoc la do hiu ling thc l hoac hieu ng Zener nhi tricng hdp dit, hoac do hien tupng xuyn thng (do dien p ngidc

16n lm m in dien tch khng gian ca chuyen tiep colecta md rng ra ti nuc tiep xc vi min din t ch khng gian chuyen tiep emita, ket qu lm dng tng ln dt ngot).

Dac tuyn t ruyn d a t ch ro quan he hm so gia dng ra v dong vo = fdj.;) khi din p ra gi c6 dinh. D ve dac tuyen ny cd the lm bng 2 cch : hoc bng thuc nghim p dung sa d (h.2.25), gi nguyn dien p thay doi dng vo I|.,ghi lai cc ket qu t i dng n g dng I(', sau do biu din cc ket qu thu diac t ren toa do - I. s duac dac tuyen t ruyn dat. Thay doi gi tri c dinh s duacho dac tuyen t ruyn d a t nhi hinh 2.29. Hoac bng cch suy ra ti dac tuyen ra : ti diem cho trUc t rn dc tuyen t a k ding song song vi t ruc tung, ding nys ct hp dac tuyen ra tai cc dim ng vi If. khc nhau. T cc giao diem ny cd the t im duac t rn t ruc tu n g cc gi tr i tuan g ng. Can c vo cc cap gi tri

ny cd the ve dac tuyen truyn dat ng vi mt dien p cho trUc, lm tuang tu vi cc gi tri khc nhau s duac hp dac tuyen truyn dat nhu hinh 2.29.

c - Mach chung colecto (CC)

Mach chung colecta cd dang hinh 2.30, cUc colecta duac dng chung cho du vo v du ra.

D do din p vo, dng vo, dng r a qua do xc dinh cc dac tuy'n t inh ca bn cua m ach CC d n g cc von ke v miliampe ke duac mc nhu hinh 2.30.

42

/?7\----

W' ( ) \ \

T" J, '_____ - - _L - ^ - i -y

n I60 \

4-

^0

0

u =4C

' . i i h 2..' ii) : S ( i \ ' , v c i ! i / i i , ' h' l t c l u y i ' n l i n i

C(J r u n z i i n k irtJi:H(> / Ni c ( ' ( ' .

Dc tUYn vo ca mch chung colect (CC) | = f(U(-|^) khi in p ra U(-. khng i c dng nh hinh 2.31 n c dng khc hn so vi cc c tuvn vo ca hai cch mc mch EC v BC xt trc y, D(5 l vi t rong kiu mc mch ny in p vo ph thuc r t nhitu vo in p ra (- I (khi lm vic ch khucch i in p U|^|. i vi tranzito silic lun gi khong 0,7V, cn tranzito Gecmani vo khong 0,3V trong khi d in p U(-. bin i trong khong rng). Vi d t rn hnh 2.31 hy xt trng hp U| ( = 2V ti I| = 100//ACH = Uci : - U ri, = 2V - 0,7V = 1,3V

Khi in p vo tng in p^, gim lm cho | cng gim.

c tuyn ra ca tranzito nic c c khi dng vo 1| khng i. c tuyn h gia dng ra |. v dng vo khi coi = I|. cho nn c tuyn ra v coect) tng t nh t rng hp mc

i n l 2.M : i o (ic n i vrn \-(}(> c iJ rranzilo f f i j c C(

n - J ' - i / 7 7 ^ - ^J ] c fu y {7 J J ^C / y /T r n\ /ruyr/ / ^ k _ - /7/7'-^

Ig

- "- - - 4 - 6 4 0 ------

i h 2..'f2 : l d c m vcn ra v h d c uycn iruyCn (l ca iranziio m c

m t quan h gia dng I|. v in p U ( - . truyn t t rong t rng hp ny m t quan in p ( . khng i. Trong thc t c th

c tuyn t ruyn t ( trng hp mc chung chung emit (h.2.32).

2.2.3. phn cc v n nh nhit i m cng lc ca iranzi to

a - Nguyn tc chung p h n cc tranzito

Mun tranzito lm vic nh nit phn t tch cc th cc th am s ca tranz i to phi tha mn iu kin thch hp. Nhng th am s ny ca t ranz i to nh mc trc bit, ph thuc r t nhiu vo in p phn cc cc chuyn tip colect v emit. Ndimt cch khc cc gi tr tham s ph thuc vo im cng lc ca tranzi to. Mtcch tng qut, d tranzito c mc mch theo kiu no, m un nd lm vic ch khuch i cn c cc iu kin sau ;

- Chuyn t ip emit - baz lun phn cc thun.

- Chuyn t ip colect - baz lun phn cc ngc.

43

C th minh ha iu ny qua v d xt tranzito, loi pnp(h.2 33).Nu gi | . , |^, Uc ln lt l in th' ca cc emit, baz, colect, cn c vo cc iu kin phn cc k trn thi gia cc in th ny phi tha in iu kin :

K > u > u,- (2-48)

~ i ^ -

- o

C

l n h 2 . .U : / ) / t 7 p r iir^ p h n c c ircJ/i2 oI l i n h : j \ \ u v c n i p n c c l m ^ ( n i i r u n z i h t . f n c H C .

Hy xt iu kin phn cc cho tng loi mch.

- T mch chung baz hnh 2.34 vi chiu mi tn l hng dng ca in pv dng in, c th xc nh c cc t inh ca in p v dng in cc cc khitranzito mc CB nh sau :

I.H = K - Ui > 0 h, >0UcB = Uc - Uii < Ic < 0 (2-49)

Cn c vo iu kin (2-48) in p m, dng cng m c ngha l hng thc t ca in p v dng in ny ngc vi hng mi tn trn hnh 2.34.

- T mch chung eiiit hnh 2.35, l lun tng t nh trn, cd th xc nh ccc tnh ca in p v dng in cc cc nh sau :

U hi-: - ii - |.; < 0

Uci-: = U( - 1-; < 0I b < 0

I ( ' < 0 (2-50)

/ . c

0--------------- ------0

l th ; f)in p ') (dti phn cc tranzili) m c EC.

l nh 2. ; Din j7 r dti; din phn cc iranzito m c CC.

- Vi mch chung colect hnh 2.36, cn c vo chiu quy nh t rn s v iu kin 2 -48 c th vit :

u c = u , - U e > 0 I , < 0

U ^ , | , = U | . ; - U e < 0 Ir . < 0 (2-51)

41

)6 i vi tr;inzil.o npn iu kin phn cc n lm vic ch khuch i l

U|. < | < LV (2-52)

T bt ng thc (2-52) c th thy rng hng diig in v .in p thc t trong tranzito npn ngc vi tranzito pnp.

b Dng ti tnh v. dim cng c tinh)Lfng ti t nh c v trn c tuvn ra tnh ca tranzito nghin cu dng

in v in p khi n mc trong mch c th no (khi c ti). im cng tc (hay cn gi l im tnh, im phn cc) l im nm trn ng ti tnh xc nh dng in v in p trn tranzito khi khng cd tn hiu t vo, ngha l xc nh iu kin phn cc tnh cho tranzito.

hiu r v ng ti tinh v ini cng tc tinh, ta xl trng hp tranzito lo:. npn in;'ic chung oniit nh hnh 2,37. Ping trinh quan h dng v p mchC() dng

Uc,.; = Ef.(. - L.R, 2-r33)

Nu nh in p phn cc U lm cho tranzito kha, khi v 1 . = 0 vu,. = E,.,,. - (0.R,) ^ E r r = 20V.u ^ . . - :.,-,.Nh vy cin a5 ta ( I . - 0, U.. = 20V.) l im A trn c tU3'n ra. Gi thit rng . ; tng m cho tranzito m v I,, = 0,5niA lAi y IJ -|, = 20V - 0,5niA. .lOkQ = 20V - V 15V, trn c tuyn ra l im B c ta (,5mA ; 15V) bng cch tng |.;, lm tng t nh trn c th v c v d cc dim ng vi ta

/T 7 /

sau

im c ng vi . = ImA ;('].; = lOVDiii ng vi = l ,5mAU (,, = 5Vim ng vi = 2mA ;U(>|; = o v Ni cc im trn y

vi nhau ta s c mt ng thng d l ng ti t nh vi Rj = lOkQ.

C th v c bng cch chn 2 im c bit, im ct trc tungE(U^.|, = 0 ; = 2niA) v im ct tiTic honh A (U .|. = = 20V ; = OV).

Qua nhng im phn tch trn thy rng ng ti chnh th bin thin ca dng 1 , theo in p U ..; ng vi in tr ti v in p ngun nh t nh. lYong 3 gi tr l v U(.|. chi cn bit mt ri cn c vo tng gi tr ti xc nh hai gi tr cn li. Cn nhn mnh l ng ti v trng hp trn ch ng trong trng hp E . , = 20V v R( = lOkQ. Khi thay i cc iu kin ny phi v ng ti khc.

ni 2.J7 : V i/iffV/zi; i lnh ) S ( / (i t i c h ch n i s ; CKI c l i ; h ) f ) c l i y c r a l n h v c i i ^ l i l n h .

45

Khi th i t k mch, im cng tc t nh l im c chn ti*n ng ti t nh. Nh ti'n ni, im nay xc ih gi tr dng v in .p .|. khi khng c tn hiu t vo. Khi c t n hiu t vo, dng I bin i theo s bin i ca bin tn hiu, dn ti dng bin i, kt qu l in p ra t rn ti bin i ging nh quy lut bin i ca t n hiu u vo.

Vi s nguvn li nh h nh 2.37a t rn ng ti tnh lOkQ gi th i t chn im cng tc t nh Q n h hnh2.38. n g vi icM Q ny I| =20//A ; L. - Im A : . . , = 10V.

/ /

Khi I

nh 2..^H : Chn d i tn cni, lc linh.

xviii X t n g t 20//A n 40//A, t rn hnh 2.38 thy cd gi tr bng l ,9 5 n iA v u,,. = ^.,. - == 20V - l ,95mA.10kQ = 0,5V. C th thy r n g khi A1| - + 20//A dnti AU ^ , = -9,5V. Khi I| g i m t 20//A xu n g 0 th 1 . g im x u ng ch cn 0 ,0 5 m A v u^.. = 20V - (0 ,05m A.10kQ) = 19,5V, tc l khi I g im i mt lng l Al = 20iiA lm cho u^, t n g ln mt lng Alj\. = + 9,5V.

Tm li, nu chn im cng tc t nh Q nh trn th u ra ca mch c thnhn c s bin i cc i in p A .. = 9,5V. Nu chn im cng tc t nh khc. V d Q ti c = 0,525 mA ; = 14,75V. Tnh ton tng t nhtrn ta cd A = 10,//A v AU^. = 4,75V. Ngha l bin bin i cc i cain p ra m bo khng mo d n g lc ny ch l 4,75V.

Nh vy vic chn im cng tc t nh t rn hoc di im Q s dn ti bin thincc i ca in p r a t r n t i (m bo khng mo dng) u nh hn 9,5V, hay c bin in p ra cc i, khng lm Iio dng t n hiu, im cng tc t nh phi chn gia ng ti t nh. C ng cn ni thm l khi in p ra khng yu cunghim n g t v mo th im cng tc t nh c th chn nhng im thch hpt r n ng ti.

c - n n h d m cng tc t nh khi nhi t d thay di

Tranzito l mt linh kin r t nhy cm vi nhi t v vy t rong nhng s tay hng dn s dng ngi t a th n g cho di nhi t lm vic cc i ca tranzito. Ngoi gii hn nhit k t r n t r anz i to s b hng hoc khng lm vic. Ngay c t rong khong nhi t cho php t r anz i to lm vic bnh thng th s bin th in nhit cng n h hng n th a m s ca tranzi to. Hai i lng nhy cm vi nhit n h t l in p em i t -baz P v dng ngc (xem phn 2 .1). v d i vi t ranz i to silic, h s nh i t ca U |. (A U ./A T) l ~2,2mVAC, cn i vi tranzi to gecmani l - l , 8mV/^C. i vi ni chung khi nh i t tng ln 10' c gi tr dng ngc ny t n g ln hai ln.

46

Khi tranzi to lm vic, dng nguc chy qua chuyn tip ny nh bit rtnhy cm vi nhit , khi nhit tng s ph t x cp in t, J t rng tng, dng

tng, t quan h gia v nu phn trc :

I,. = I| + (o +

C th thy rng , , t ang lm cho ij,. t n g (d cho gi thit r ng v a khng i). Dng tng ngha l ut cc ht dn qua chuyn t ip coect t n g ln m cho s va chm gia cc ha vi mng tinh th tng . Nhit t an g lm cho t n g chu k li lp ]i nh trn lm dng . v nhi t ca t ranz i to t n g mi. ?Iin tng ny gi l hiu ng qu nhit. Hiu ng qu nhit a ti : Lm thay i im cng tc t nh v nu khng c bin php hn ch thi s tng nhit c th lm hng tranzito. S thay i nhit cng lni cho U ^ . ihav i v do lm thay i dng 1 , dn ti thav i im cng tc tnh. Trong nhng iu kin thng thng nh hng ca dng n 1 . nhiu hn so vi ^ / . Bi vy khi ni nh hng ca nhi t n im cng tc thng ch quan tm n dng Nh vy s n nh nhi t y hm chi s thay i dng . khi dng thay i c t h nh ngha h sn nh nhit ca tranzito nh sau :

(2-54)

^IVong : = h.,, + (1 + h.],.) (2-55)T nh ngha ny thy rng s cng nh thl t nh n nh nhi t cng cao, t rong

trng hp l tng s = 0 , ( trong thc t khng c s n nh nh i t tuy t i). xc nh h s n nh nhit s vi mt s t ranz i to cho trc, gi th i t do

nhi t thay i, dng bin i mt lng l bin i mt lng l Algv bin i mt lng l AI ,

Qua mt s bin i t biu thc (2-55) ta c :

s = - 2-56)

Khi bit cc gia s dng in cn c vo (2 -56) c th t nh c h s n nh nhit. Biu thc (2-56) l biu thc tng qu t t nh h s n nh nhi t chung cho cc loi mc mch.

d - Phn cc tranzito bng dng c dinh Nu tranzito c mc nh hlnh

2.39, dng t ngun mt chiu I-------------- J ------------------------------------------------0cung cp cho tranzito s khng i, bi vy ngi ta gi diu kin phn cc ny l phn cc bng dng khng i. C th c hai cch to ra dng c nh, trng hp th nh t nh hnh 2.39a dng mt ngun mt chiu DngI| c c nh bng v Rj.T hnh 2.39a t nh c ;

n h 2 , ?y ; M ch phn c c dng khn di. a) Mch m Hun : b Mch hai n^n.

^cc Up(2-57)

47

Ti*ng hp th hai nh hnh 2.39b ngi ta dng hai ngun mt chiu Hai mch ny hon ton tng ng nhau. Nu E . . = V thi hlnh 2.39b c th thay bng hnh 2.39a.

Cn c vo s nguyn l hnh 2,39a. c th suy ra nhng biu thc cho vic tnh ton th i t k mch phn cc dng c nh p dng nh lut Kickhp (KirchhoT) cho vng mch baz v ch rng y V = E .,, c th vit

(2-58)

Khi lm vic chuyn tip emit lun phn cc thun cho nn | | thng rt nh (t 0.2V n 0,7V) v trong biu thc (2-58) c th b qua. nh vy c th vit :

Ecx -

v Iu ==EccR

(2-59)

(2-GO)

trong mch colect c th vit :

Ec

\ lc mc tranzito nh hlnh 2.40 s tha mn iu kin trn, Cch phn cc tranz itonh vy gi l phn cc bng colect. Nh thy trn s , in tr R c ni trctip gia cc colect v cc baz. S khc nhau c bn gia mch phn cc bng in p phn hi v bng dng phn cc c nh l : t rong mch phn cc bng in p phn hi bao hm c ch dng I : cm bin theo in p (hoc dng in) mch ra, cn trong mch phn cc dng c nh th khng cd iu ny. im cng tc t nh c xc nh nh sau ;

T hnh 2.40, quan h in p t rong mch ra cd dng :

E , , = (I , + I) R, + ,,.; (2-64)

cn quan h in p trong mch baz c th vit dng :

Ecc = dc + Ir) Rt + Ib-Ri + ii: (2-65)

Nu coi nh, c th b qua th

Ecc = (Ic + Ib) R + I rRb (2 - 66)

T 2 -64 VP 2 -66 c th suy ra :

c. - nRn (2-67)

Thay vo biu thc (2-66) ta t m c

Ecc = (h2ic + 1) Ib.Ri + IiRb (2 - 68)

r t rar :

EccIb = TT-----. , p (2-69)(h2ic + l)Ri + Ri

Sau t nh dng coect ng vi im cng tc t nh Q

IcO ^ ^ 21o-Ibq (2-70)

V in p gia colect v emit ng vi im cng tc t nh Q cn c vo (267) t nh c :

d - o = I bo .Rb (2-71)

Nu bit h2iL. ca tranzito c th p dng biu thc (2-70) v (2-71) t nh c iu kin phn cc t nh tranzito.

By gi hy xc nh c t nh n nh nhi t ca mch phn cc dng in pphn hi.

T biu thc (2-66), t m c :

Ecc Rir a

Ly vi phn biu ' thc (2-72) theo c :

(2-73)dlR R,

dlc R| + Rt

Thay biu thc (2-73) vo (2-56), c :

4-KT-A 49

Ct th bin i (2-74) v dng thun li cho vic t nh ton hn.

O^Ilc 1)(^B ^ K-l)(h2,^.+ 1)R, + R,5

(2-75)

/8

T biu thc (2-75) co nhn xt rng h s n nh nhit s trong mch phn cc bng in p phn hi khng c nh m ph thuc vo gi tr cc in tr R v R(. Trong t rng hp R R j th s gn ti 1 n v, iu ny ndi ln r n g d cd mnh R: th h s n nh nhit s khng gim xung nh hn 1.

in p phn hi m qua in tr R t rong mch phn cc lm tng n nh nhit ng thi li lm gim h s khuch i tn hiu xoay chiu (xem mc 2.3). Nh t rn ndi t ng t nh n nh nhit , phi gim in tr R|^ n h ng khi h s khuch i ca mch cng gim i, y cd mu thun gia n nh nhit ca mch v h s khuch i.

C mt cch cho php t c n nh nhit cao m khng phi t r gi v h s khuch i d l cch mc mch nh hnh 2.41. in tr R| t rong t rng hp ny c chia lm hai phn Rj v R-,, im ni 2 in tr ny c ni vi t qua t c.Di vi in p v dng 1 chiu th t c coi nh h mch do khng ih hng g n ch 1 chiu. Ngc li vi t n hiu xoay chiu th t c coi nh ngn mch xng t khng cho phn hi tr li u vo.

Qua phn tch trn thy rng mch phn cc in p phn hi c n nh nhit tt hn mch phn cc dng c nh, tuy nhin hai mch phn cc ny khng th t ng n nh nhit ln cao v im cng tc t nh v n nh nhit ca mch ph thuc ln nhau, d chnh l mt nhc im l kh khn cho vn thit k loi mch ny.

g. Phn cc tranzito bng dng emit (t pin cc)

Mch phn cc tranzito bng dng emit cd dng nh hnh 2.42. in t r Rp Ro to thnh mt b phn p c nh to Uj t vo baz tranz i to t in p ngun

in tr R.; mc ni tip vi cc emit ca tranz i to c in p ri t r n nd l Uj. = IeR:- Mt khc cn c vo hnh 2.42 cd U; = Uj3 - Ur

n l 2.4 : Ph(/f}i php loi ir phn hi xoax chicu trom^ mch phn

cc hni (in p phn hi.

vy Ie - (U b - U,,,.)/R,, (2-76)

(2-77)Nu tha mn iu kin Uf3 ^ .- th If;; U/R.;

v r t n nh. t in cho vic phn tch tip theo cd th v s tng ng ca hnh 2.42 nh hnh 2.43 bng cch p dng nh l Tevenin t rong :

=

R | . R2

R j + R2

^ 2 ^cc R + R2

(2-78)

(2-79)

Vn cn ch y l phi chn R v R2 th no m bo cho 3 n nh T hnh 2.42 thy r cn chn R v R2 sao cho R3 khng ln hn nhiu so

50 4- KTI-B

R.

- - L

4 / ^

~ T ~ ~ ^

^ t h

u,C .c c

_ t__ -1H i n h 2 -4 2 : S / cio n iiyn l f f iuch p i i i in l c h m ;

d m ; c n i i i f i p i c c ) ./ n h 2 .4 . . S (T> (hrn l n C tn c h

h n h 2.42.

vi R , nu khng th s phn cc ca mch li tng t nh t rng hp phn cc dng c nh. c | n nh cn chn R| v R-, chn cng nh cng tt, nhng m bo cho in tr vo ca mch ln th R| v R-, li chn cng ln cngtt. dung ha gia hai yu cu mu thun ny trong thc t thng chi R = R|..

Cn c vo s tng ng (h.2.43) phn tch mch phn cc dng eniit.Tng in p ri trn mch baz bng

u, + |,.; + (I,, + |)R|.; (2-80)Tlong d tha^ I|, - + I| nu nh bit c th bin i (2-80) thnh

| [ + I ) R .] + U . + lo 1) R (2-81)u , =

TiUc khi phn tch hy ch l in p U|^| . trong trng hp phn cc ny khng ""the b qua nh nhng trng hp khc. Trong qu tr nh lm vic chuyn tip emit ' l u n phn cc thun cho nn tng i p mt. chiu u vo ca mch ny l U|.

Trong hu h t cc t rng hp nh hn nhiu ln. Trc y c th b qua |^|, vi n qu nh so vi nhng t rong trng hp ny U| . c ln vo c U^ cho nn khng th b qua c. S hng cui cng trong (2-81) cha thng c b qua vi t rong thc t dng ngc ny r t nh vi tranzi to silic dng ny ch c vi nano ampe)-

Cng t s tng ng hnh 2.43 c in p gia emit v t bng .. Rj.. Dng emit I|. = + l = (h2t> + 1)I| (b qua dng ngc Nh vy in pgia emit v t c th vit U . = R.. i lng (h 2 [> + 1) l i lngkhng th nguyn nn n c th lin h vi l to thnh dng (h2i , + 1) hoc lin hp vi R|. to thnh in tr + 1)R . hoc lin hp vi R. to thanh intr + 1)R .. Nu quan nim nh vy th c th ni rng in p gia emit v t l in p do dng (h2j - + 1)I ri t rn in tr R, hay do dng ri t rn in tr + 1)R|..

Nu th n h phn in p gy ra bi trong biu thc (2-81) c th b qua th biu thc ny c th minh ha bng s tng ng hnh 2.44. y, in tr R. t rong n hnh eiiit bin thnh in tr (h2i . + 1)R|. t rong mch baz. Mt cch tng qut, b t ki mt in khng no t rong mch emit u c th bin i sang mch baz bng cch nhn nd vi (h-,,. + 1).

T hnh 2.44 v biu thc (2-81) c th t m c dng baz ti im phn cc.

51

I,,i - u,.

'J Rh + ( h , + 1)R~

T t nh ra c

li

(2-82)

(2-83)

- I -

88T-

T s tng ng hnh 2.44 t rong mch coect c th vit ;

E , , - I,R + |._ + l.R. (2-84)Nu thay I[, = + l (2-85)

vo (2-84) s c :

E .,. = I ,(R, + R|.) + U(.|. + I|,R|.

n l 2 . 4 4 : St r i n { f i \ i i h f f f i ^ f h hZif i sir ( hinh 2.4.

(2 - 86 )

Bit rng thng ln hn I |5 r t nhiu ln cho nn y c th b qua thnh phn in p do I| gy ra t rn R.. Nh vy (2- 86) c vit thnh :

E , , - (R, + R ,) I, + ci. (2-87)Biu thc (2-87) chnh l biu thc ng ti t nh ca mch phn cc bng dng

emit. Nu dng v ^,.Q l dng in v in p ng vi im cng tc t nh th c th vit li (2-87) thnh dng

^cc (2 - 88)

Cn c vo biu thc (2-88) c th t nh c iu kin phn cc t nh ca tranzito khi bit h s khuch i h 2iL v loi tranzito.

Sau y xt n nh nhit ca mch phn cc bng dng eniit, c th vit li (2-80) dng :

U h U||, - I|(Rh + R ,)

Do

I c =

In

R,.

U b - U b i-, Rg + R[/

R,L

R | J + R i .(2-89)

Ly o hm ring biu thc ny theo Ij, v mt ln na ch rng |^|. khng i s c :

liII-,

R. 1R| + R|,

(2-90)

Theo nh ngha ca h s n nh nhit th t rong trng hp ny :

s = h2ic+ 1 (2-91)

T (2-91) thy r n g h s n nh nhit t in ti cc t iu ( n nh cao nht) khi l2 co' gi tr nh nht. iu y c ngha l cho mch n nh, phi th i t k sao cho R. c gi tr cng ln cng tt, v gi tr R j cng nh cng tt. H s k2 khng bao gi nh hn 1, gi tr ny ch dn ti 1 (ng vi trng hp R. r t ln v Rg r t nh) t suy ra rng h s n nh s chi c th gim nh ti gii hn

52

n h J .4 > : DII l p h n m c h d s^n h i i p I XDu y c h c c n l i i R ,:

l i Ni ^ i u n l i h n / i K/ : h ) ; \ \ i n m a c h / n i p l n .

l 1. Mt nhn xt quan trng na l h s n nh s khng ph thuc vo Rj nghal khng ph thuc vo im cng lc.

0 trn ndi ti vn nng cao n nh nhit ca loi mch ny bng cch tng R|, v gim R . Bn cht ca s n nh nhit trong loi mch ny chni l dng phn hi m qua in Ir R| , Tng R . c ngha l tang phn hi m do lmgim tn hiu khuch i xoay chiu ca Iich. D khc phc mu thiin ny thc tc th dng hai loi mch mc nh hnh 2.45a, b. Dng kiu mch ny c th loi tr hoc gim nh tc dng phn hi m i vi tn hiu xoay chiu (xem phn 2.3), do khng lm gim h s khuch i tn hiu xoay chiu ca mch. Gi tr C|,phn mch y phi chn ln sao cho i vi tn hiu xoay chiu th tr khngca nd gn nh bng 0 , ngc li i vi dng mt chiu th coi nh h mch.

Thc to thvng gp trng hp phi thit k mch phn cc khi bit cc iu kinphn cc cng nh h s khuch i ca tranzito.

nhng phn trn mi chi xt nh hng ca nhit, n dng Sau y strnh by nh hng cia nhit n dng U i v h s khuch i i vi chai loi tranzito, lm t silic v gecmani, khi nhit tng gim, cn litng. nh hng ca nhit n cc tham s ca tranzi to silic cng tc trong khong nhit t - 65 C n + 175c cn tranzito th t - 63c i +75 *c. S khc nhau na l tr s v |^|, ca tranzito silic v tranzito gecmani bin thin ngc nhau khi nhit thay i. Bng (2-4.) lit k nhng gi tr in hnh ca . v 2 [.ca tranz ito silic v gecmani nhng nhit khc nhau.

Bng 2 -4

Gi tr in hnh ca mt. s tham s chu nh hng nhiu ca nhit

V'I l iu l m I r a n / i i o Icof A ) U b-:(V) h : i c

Si K)-'- 0 .8 20 - 6 . 5

ClL' 1 0 ^ 0 .4 15 - 6 . 5

Si 10 0 .6 50 + 25

( c l 0^2 50 + 25

Si 30 0 .2 5 100 + 175

( e 30 0.51 95i+ 75

53

'

T bng 2-4 c nhn xt : nhit phng i vi tranzito silic chi c nano ampe, cho nn nu nr thay i th cng khng gy nh hng ng k n I_. v nh

hng ca nhit n im cng tc t inh ca tranzito ch yu thng qua |^|.. khc phc nh hng ny trn thc t thng mc ni tip emit mt it silic phn cc thun c chiu ngc vi chuyn tip emit nh hnh 2.46. Bng cch mc nh vy c th thy rng s thay i in p thun trn 2 cc it c th b tr s bin i | , ca tranzi to do nhit gy ra. it b nhi t s ny lun c phn cc thun bi ngiin E)^ cho nn in tr thun ca n r t nh. S ny hon ton tng ng vi s phn cc bng dng emit xt phn trn. i v t ranz i to gecmani th ngc li, ti nhit phng kh ln cho nn khi nhi t thay i nh hng ca dng n thams ca t ranzito chim u th. n nh nhit, cho s , ngi thit k phi ch ch yu n vic gim h s n nh nhit s.

Qua bng (2-4) t rn y cd th thy rng h s khuch i dng ph thucrt nhiu vo nhi t . Hn na ngav cng mt nhit , tranzito c cng loi k hiu c ch to nh nhau) nhng h s h^ . ca tng chic cd th hn km nhau vi ba ln. Nh bit h s nh hng nhiu n im cng tc t nh catranzito. Bi vy n nh im cng tc t nh, ngi thit k phi ch n s thay i h s c th co' ca loi tranzito dng t rong mch in. nh lngs ph thuc ca vo gi th i t rng cc gi tr ca v Rj bit h skhuch i dng ca tranzito bin thin t n b qua (gi I .J l dngng vi trng hp h s khuch i v 1 -, ng vi h-, 1,.2) t nh c :

iitih2.4() : s

Nu gi s, l n nh nhit khiI c . S .

c , hzicl (h21el + 1)

= ^2\c^ (2-95) c th vit thnh

(2-97)

Trong AY^ = ~ thng gi l sai lch ca

Biu thc (2-97) cho thy s b i n 'doi dng colect ph thuc trc tip vo sai lch h s khuch i h-) . k trn. Ngoi ra biu thc ny cn cho php ngi thit k tnh c gi tr ca in tr cn thit gi cho dng I,,, bin i trong mt phm vi nh t nh khi thay i.

2.2.4. Tranzito r n g (FET)

Khc vi t ranzito lng cc xt p hn trn m c im ch yu l dng in trong chng do c hai loi ht dn (in t v l trng t do) to nn, qua mt h thng gm hai mt ghp p -n rt gn nhau iu khin thch hp, t ranzito trng (cn gi l tranzito n cc FET) hot ng da t rn nguyn l hiu ng trng, iu khin dn in ca n t inh th bn dn nh tc dng ca 1 in trng ngoi. Dng in trong FET ch do mt loi h t dn to ra. Cng ngh bn dn, vi in t cng tin b, F E T cng t r nhiu u im quan trng trn hai m t x l gia cng tn hiu vi tin cy cao v mc tiu hao n n g lng cc b. Phn ny s tr nh by trm t t nhng c im quan trng nht ca FE T v cu to, nguyn l hot ng v cc tham s c t r ng i vi hai nhm chng loi ; FET c cc ca l tip gip p -n (J"ET) v F E T c cc ca cch li (MOSFET hay IGFET).

o ~ Tranzito trng c cc ca tip gip (JFET)~ Cu to va k hiu quy c :

( f )

D

5-)

D

( + )'

/(err?

s

n h 2.47: ) c u lo JFET v cch phn cc hng inrn^ ri^oi h) K hiu quy c vi Jt'E' vi hi loi knh dn n v knh dn p.

Hnh 2.47a a ra mt cu trc J F E T kiu knh n ; trn t inh th bn dn Si -n ngi ta to xung quanh n 1 lp bn dn p (c tp cht nng cao hn so vi ) v a ra 3 in cc l cc ngun s (Source), cc mng D (Drein) v cc ca G (Gate). Nh vy hnh thnh 1 knh dn in loi n ni gia hai cc D v s, cch li vi cc ca G (dng lm in cc iu khin) bi 1 lp tip xc p -n bao quanh knh

55

dn. Hon ton tng t, nu xut pht t bn dn loi p, ta c loi J F E T kni p vi cc k hiu quy c phn bit cho t rn hinh 2.47b,

Nguyn li ot dng : phn cc JFET. ngi ta dng hai ngun in pngoi l > 0 v < 0 nh hnh v (vi loi knh p, cc chiu in p phncc s ngc li, sao cho tip gip p -n bao quanh knh dn lun c phn cc ngc). Do tc dng ca cc in trng ny, trn knh dn xut hin 1 dng in (l dng in t vi knh n) hng t cc D ti cc s gi l dng in cc m n g Dig I|;j co' ln ty thuc vo cc gi tr U).^ v ( v dn in ca knh ph thuc manh vo c hai in trng ny. Nu xt r ing s ph thuc ca I) vo tng in p khi gi cho in p cn li khng i coi l mt tham s) ta nhn c hai quan h hm quan trng nht ca JF E T l :

1d -

Id = f2((is) u I)S

= const

= const

Biu din fj ng vi vi gi tr khng i ca ta thu c h c tuyn ra ca JFET.

ng biu din 2 s gi tr khng i ca cho ta h c tuyntruyn t ca JFET. Dng in hnh ca cc h c tuyn ny c cho trn hnh 2.48 a v b.

^

G/am /c/7

" - /

r/ ----------- -----------

Vng ngoi i m B gi l vng nh thng, khi U-S tr kh ln, tngt hin do tip gip p - n b nh thng thc l xy ra ti khu vc gn cc D doin p ngc t ln tip gip p -n ti vng ny l ln nht.

Qua d th c tuyn ra, ta r t ra my nhn xt sau :

Khi t tr s ni dn, im un A xc nh ranh gii hai vng tuyn tnhv bo ha dch dn v pha gc ta . Honh im A (ng vi tr s ha t nh ca (S 1 in p gi l in p bo ha cc m ng (cngi in p th t knh). Khi |(-^ | tng, gim.

Tng t vi im B : ng vi cc gi tr m hn, vic nh thng tipgip p -n xy ra sm hn, vi nhng gi tr nh hn.

Dc tuyn truyn t ca J F E T (h.2.48b) ging ht c tuyn anot- li ca n 5 cc chn khng, xut, ph t t 1 gi tr U q s j ti ^ = 0 , gi l in p kha (cnk hiu l p). ln U q (, bng ng vi ng = 0 trn h c tuynra. Khi tng I ) tng gn nh t l do dn in ca knh tng theo mc gim phn cc-ngc ca tip gip p-n . Lc (-^ = 0, ^ = I|)(J ^tnh cc m ng khi khng c in p cc ca. Khi c U qj < 0, I

Gi tr | 3(, l dng < I (j v c xc

nh bi [4]

ulo = [)() 1 - -,

('iS

Uoso(2-98a)

C th gii thch tm t t cc c tuyn ca J F E T bng gin cu to hnh 2.49 trong 3 trng hp khc nhau ng vi cc gi tr ca v |)^.

f 7 7 X / y / . y , y

> v x z z Z

n h 2.49 : Gii thch V I c iiyn ca JFET.

Khi c gi tr ni tng dn v = 0, b rng vng ngho ca chuyn tip p - n rng dn ra, ch yu v pha knh dn n v tp cht pha yu hn nhiu so vi vng p, lm knh dn b th t li u dc theo phng DS (h. 2.49a). Ngc li khi cho = 0 v tn g dn gi tr ca in p m ng ngun J)^, knh b co li khngu v c hnh phu, pha cc D th t mnh hn do phn b trng dc theo knh t D tci s, cho ti lc = l^^o knh b th t li ti im A. Sau , t n g lm in: tht A dch dn v pha cc s (h.2.49b). Qu tr nh t rn s xy ra sm hn khi c t i m < 0 nh hnh 2.49c lm gi tr in p th t knh gim nh. R rng d in in ca knh dn ph thuc c hai in p v cn sau khi c hintng tht knh, dng cc m ng do cc h t dn (in t) phun t knh qua tip gip

57

p-n ti cc mng ph thuc yu vo )^ v ph thuc ch yu vo tc dng iu kh in ca ti ch u y n t ip p - n phn cc ngc, qu a ti dng in ccm n g

- Cc tham s ch yu ca J F E T gm hai nhm :

Tham s gii hn gm c :

Dng cc m ng cc i cho php dng in ng vi im B t rn ctuyn ra (ng ng vi gi tr U qs - i khong ^ 50mA ;

in p m ng - ngui cc i cho php v in p ca ngun

U|:)Smax = |^/(l ,2 1,5) (c vi chc Vn)

y U l in p m ng ngun ng vi im B.

in p kha (hay u ^ ) (bng gi tr ng vi ng (i;^ = 0)

Tham s lm vic gm c :

in tr t rong hay in tr vi phn u ra T = u ~ const (c 0,5

MQ) r th hin dc ca c tuyn ra t rong vng bo ha.

H dn ca c tuyn truyn t :

)I,s - D TT = const

IXS

cho bit tc dng iu khin ca in p cc ca ti dng cc mng, gi tr in hnh vi J F E T hin nay l s = (7 10)niAA^-

Cn ch gi tr h dn s t cc i s - lc gi tr in p ln cn im 0 (xem d n g c tuyn t r u y n t ca J F E T hnh 2.48b) v c t nh bis - 2 I,),,/U(.,so [6].

in tr vi phn u vo ;

do tip gip p - n quyt nh, cd gi tr khong

t n s lm vic cao, ngi t a cn quan tm ti in dung gi cc ccv (c pF).

b - T ram ito trng c cc ca cch li (MOSFET)

- Cu to v k hiu quy c :

c im cu to ca MOSFET cd hai loi c bn c th hin t rn hnh 2.50 a v b.

Ki hiu quy c ca MOSFET t rong cc mch in t c cho t rn hnh 2.51 a, b, c v d.

Trn nn l n t inh th bn dn tp cht loi p (Si-p), ngi t a pha tp cht bng phng php cng ngh c bit (plana, Epitaxi hay khuch tn ion) to ra2 vng bn dn loi n+ (nng pha tp cao hn so vi ) v ly ra hai in ccl D v s. Hai vng ny c ni thng vi nhau nh mt knh dn in loi n c th hnh thnh ngay trong qu tr nh ch to (loi knh t sn hnh 2.50a) hay ch

58

t ) G (f)

.//.J / / /

S i.p

d) Cc

fr)

h ) \ Cc /

nh 2.>0 : ( ii lo C M O S i l i f a) I.Oi kt'nh di Sih : h) .oi kt'iih fni n\;.

hinh thnh sau khi c 1 in trng ngoi (lc lm vic