1 Lesson04:Bpolar Juncton ransstors
2 Chapter Goals Explore the physcal structure of bpolar transstor tudy termnal characterstcs of BJ. Explore dfferences between npn and pnp transstors. Deelop the ransport Model for bpolar deces. Defne four operaton regons of the BJ. Explore model smplfcatons for the forward acte regon. Understand the orgn and modelng of the Early effect. Present a PPCE model for the bpolar transstor. Dscuss bpolar current sources and the current mrror.
3 Physcal tructure he BJ conssts of 3 alternatng layers of n- and p-type semconductor called emtter (E), base (B) and collector (C). he majorty of current enters collector, crosses the base regon and exts through the emtter. A small current also enters the base termnal, crosses the baseemtter juncton and exts through the emtter. Carrer transport n the acte base regon drectly beneath the healy doped (n + ) emtter domnates the - characterstcs of the BJ.
4 ransport Model for the npn ransstor he narrow wdth of the base regon causes a couplng between the two back to back pn junctons. he emtter njects electrons nto base regon; almost all of them trael across narrow base and are remoed by collector. Base-emtter oltage BE and base-collector oltage BC determne the currents n the transstor and are sad to be poste when they forward-bas ther respecte pn junctons. he termnal currents are the collector current( C ), the base current ( B ) and the emtter current ( E ). he prmary dfference between the BJ and the E s that B s sgnfcant, whle G = 0.
5 npn ransstor: orward Characterstcs orward transport current s s saturaton current 1 BE C 9 A A 10 = k/q =0.025 at room temperature Base current s gen by 1 BE B s forward current gan Emtter current s gen by 1 BE B C E n ths forward acte operaton regon, B C E C
6 npn ransstor: eerse Characterstcs eerse transport current s 1 BC E 1 BC B 0 20 Emtter current s gen by 1 BC C s reerse current gan Base current s gen by Base currents n forward and reerse modes are dfferent due to asymmetrc dopng leels n the emtter and collector regons.
7 npn ransstor: Complete ransport Model Equatons for Any Bas 1 BC BC BE C 1 BE BC BE E 1 1 BC BE B he frst term n both the emtter and collector current ressons ges the current transported completely across the base regon. ymmetry exsts between base-emtter and base-collector oltages n establshng the domnant current n the bpolar transstor.
8 pnp ransstor: Operaton he oltages EB and CB are poste when they forward bas ther respecte pn junctons. Collector current and base current ext the transstor termnals and emtter current enters the dece.
9 pnp ransstor: orward Characterstcs orward transport current s: 1 EB C Base current s gen by: 1 EB B Emtter current s gen by: EB B C E
10 pnp ransstor: eerse Characterstcs eerse transport current s: 1 CB E Base current s gen by: 1 CB B Emtter current s gen by: CB C
11 pnp ransstor: Complete ransport Model Equatons for Any Bas 1 CB CB EB C 1 EB CB EB E 1 1 CB EB B
12 Crcut epresentaton for ransport Models n the npn transstor (ressons analogous for the pnp transstors), total current traersng the base s modeled by a current source gen by: BC BE 1 1 BC BE B Dode currents correspond drectly to the 2 components of base current.
13 Operaton egons of the Bpolar ransstor Base-emtter juncton orward Bas eerse Bas Base-collector juncton eerse Bas orward Bas orward acte regon aturaton regon (Normal acte regon) (Not same as E (Good Amplfer) saturaton regon) (Closed swtch) Cutoff regon (Open swtch) eerse-acte regon (nerse acte regon) (Poor amplfer)
14 - Characterstcs Bpolar ransstor: Common-Emtter Output Characterstcs or B =0, the transstor s cutoff. f B >0, C also ncreases. or CE > BE, the npn transstor s n the forward acte regon, C = B s ndependent of CE.. or CE < BE, the transstor s n saturaton. or CE < 0, the roles of collector and emtter are reersed.
15 - Characterstcs of Bpolar ransstor: Common-Emtter ransfer Characterstc hs characterstc defnes the relaton between collector current and base-emtter oltage of the transstor. t s almost dentcal to the transfer characterstc of a pn juncton dode. ettng BC =0 n the collector-current resson: C 1 BE
16 Juncton Breakdown oltages f reerse oltage across ether of the two pn junctons n the transstor s too large, the correspondng dode wll break down. he emtter s the most healy doped regon, and the collector s the most lghtly doped regon. Due to these dopng dfferences, the base-emtter dode has a relately low breakdown oltage (3 to 10 ). he collector-base dode s typcally desgned to break down at much larger oltages. ransstors must therefore be selected n accordance wth the possble reerse oltages n crcut.
17 mplfed orward-acte egon Model n the forward-acte regon, the base-emtter juncton s forward-based and the base-collector juncton s reerse-based. BE > 0, BC < 0 f we assume that 4k 4k BE q BC q then the transport model termnal current equatons smplfy to: BE C BE E BE B BE BE BE C C E E B ( 1) B he BJ s often consdered a current-controlled current source, although fundamental forward acte behaor suggests a oltage-controlled current source.
18 mplfed Crcut Model for orward- Acte egon Current n the base-emtter dode s amplfed by the common-emtter current gan and appears at the collector he base and collector currents are onentally related to the base-emtter oltage. he base-emtter dode s often replaced by a constant oltage drop model ( BE = 0.7 ), snce t s forward-based n the forward-acte regon.
19 mplfed orward-acte egon Model (Analyss Example) Problem: nd Q-pont Gen data: = 50, BC = B - C = -9 Assumptons: orward-acte regon of operaton, BE = 0.7 Analyss: BE 8200 E ( EE ) ma E 8200 E 1.02 ma 19.8A B 1 51 C B ma CE ( EE ) CC CE Note : E here.
20 Basng for BJ he goal of basng s to establsh a known Q-pont, whch n turn establshes the ntal operatng regon of transstor. n BJ crcuts, the Q-pont s represented by ( CE, C ) for the npn transstor or ( EC, C ) for the pnp transstor. n general, durng crcut analyss, we use a smplfed mathematcal relatonshps dered for the specfed operaton regon of the transstor. he practcal basng crcuts used wth BJs are: he our-esstor Bas network he wo-esstor Bas network
21 our-esstor Bas Network for BJ EQ EQ CC EQ B BE E EQ E , ,000( 1) B B B A 201A C B ( 1) 204A E B BE Loop CE Loop CE CC C C E E E CC C C 4.32 Q-pont s (4.32, 201 A)
22 our-esstor Bas Network for BJ (Check Analyss) All calculated currents > 0, BC = BE - CE = = Hence, the base-collector juncton s reerse-based and the assumpton of forward-acte regon operaton s correct. he load-lne for the crcut s: CE CC C C 12 38,200 C he two ponts needed to plot the load lne are (0, 12 ) and (314 A, 0). he resultng load lne s plotted on the common-emtter output characterstcs for B = 2.7 A. he ntersecton of the correspondng characterstc wth the load lne determnes the Q-pont.
24 our-esstor Bas Network for BJ: Desgn Objectes rom the BE loop analyss, we know that B EQ BE EQ ( 1) E EQ BE for ( 1) ( 1) EQ E E hs wll mply that B << 2 so that 1 = 2 to good approxmaton n the base oltage dder. hen the base current doesn t dsturb the oltage dder acton, and the Q-pont wll be approxmately ndependent of base oltage dder current. Also, EQ s desgned to be large enough that small aratons n the assumed alue of BE won t hae a sgnfcant effect on B. Base oltage dder current s lmted by choosng /5 2 C hs ensures that power dsspaton n base bas resstors s < 17 % of the total quescent power consumed by the crcut, whle 2 >> B.
25 our-esstor Bas Network for BJ: Desgn Gudelnes Choose 2 = C /5. hs means that ( ) = 5 CC / C. Let C C = E E = ( CC - CE )/2. hen C = ( CC - CE )/2 C ; E = C f EQ <<( +1) E, then E E = EQ - BE. hen ( CC - CE )/2 = EQ - BE, or EQ = ( CC - CE + BE )/2. nce EQ = CC 1 /( ) and ( ) = 5 CC / C, 1 2 CC CE BE 5 CC 5 2 CC CE BE 2 CC C 2 C hen 2 = 5 CC / C - 1. Check that EQ <<( +1) E. f not, adjust bullets 1 and 2 aboe. Note: n the LabEW bas crcut desgn (NPNBas.), bullet 1 s called the Base Margn and bullet 2 s called the C-E (oltage) Drops.
26 Problem Bas Crcut Desgn
29 wo-esstor Bas Network for BJ: Example Problem: nd the Q-pont for the pnp transstor n the 2-resstor bas crcut shown below. Gen data: = 50, CC = 9 Assumptons: orward-acte regon operaton wth EB = 0.7 Analyss: 9 EB 18,000 B 1000 ( C B ) 9 EB 18,000 B 1000 (51) B A B 69, ma C B EC ( C B ) 2.87 Q-pont s : (6.01 ma, 2.87 )
30 PNP ransstor wtch Crcut Desgn
31 Emtter Current for PNP wtch Desgn
32 BJ PPCE Model Besdes the capactances whch are assocated wth the physcal structure, addtonal model components are: dode current, capactance C J, related to the large area pn juncton that solates the collector from the substrate and one transstor from the next. B s the resstance between external base contact and ntrnsc base regon. Collector current must pass through C on ts way to the acte regon of the collector-base juncton. E models any extrnsc emtter resstance n the dece.
33 BJ PPCE Model -- ypcal alues aturaton Current = 3 e-17 A orward current gan = 100 eerse current gan = 0.5 orward Early oltage = 75 Base resstance = 250 Collector esstance = 50 Emtter esstance = 1 orward transt tme = 0.15 ns eerse transt tme = 15 ns
34 Mnorty Carrer ransport n Base egon Wth a narrow base regon, mnorty carrer densty decreases lnearly across the base, and the aturaton Current (NPN) s: qad n n bo W qad B AB B N AB = the dopng concentraton n the base n 2 = the ntrnsc carrer concentraton (10 10 /cm 3 ) n bo = n 2 / N AB D n = the dffusty = (k/q) n aturaton current for the PNP transstor s: where B DB Due to the hgher moblty () of electrons compared to holes, the npn transstor conducts hgher current than the pnp for equalent dopng and appled oltages. N n n W 2 qad p p bo W qad N n 2 p W B
35 Dffuson Capactance or BE and hence C to change, charge stored n the base regon must also change. Dffuson capactance n parallel wth the forward-based base-emtter dode produces a good model for the change n charge wth BE. C D dq d BE Q po nt 1 qan W bo B 2 BE nce transport current normally represents collector current n the forward-acte regon, C C D
36 Early Effect and Early oltage As reerse-bas across the collector-base juncton ncreases, the wdth of the collector-base depleton layer ncreases and the effecte wdth of base decreases. hs s called base-wdth modulaton. n a practcal BJ, the output characterstcs hae a poste slope n the forward-acte regon, so that collector current s not ndependent of CE. Early effect: When the output characterstcs are extrapolated back to where the C cures ntersect at common pont, CE = - A (Early oltage), whch les between 15 and 150. mplfed.a.. equatons, whch nclude the Early effect, are: O 1 CE A B O BE BE C 1 CE A B
37 BJ Current Mrror he collector termnal of a BJ n the forward-acte regon mmcs the behaor of a current source. Output current s ndependent of CC as long as CC 0.8. hs puts the BJ n the forward-acte regon, snce BC Q 1 and Q 2 are assumed to be a matched par wth dentcal, O, and A,. E BB BE C1 B1 B 2
38 BJ Current Mrror (contnued) Wth an nfnte O and A (deal dece), the mrror rato s unty. nte current gan and Early oltage ntroduce a msmatch between the output and reference currents of the mrror. BE O A CE BE E O A BE A CE E A CE BE C M O E 1 CE 2 A 1 BE A 2 O s the " Mrror ato".
39 BJ Current Mrror: Example Problem: nd output current for gen current mrror Gen data: O = 75, A = 50 Assumptons: orward-acte operaton regon, BE = 0.7 Analyss: BB BE A E 56 k 1 12 M (202 A) 75 O E A
41 BE = e-01 C2 = e-04 C21 = e-04
42 BJ Current Mrror: Alterng the Mrror ato A E O where A O s the saturaton current of a BJ wth one unt of emtter area: A E =1(A). he actual dmensons of A are technologydependent. he Mrror ato of a BJ current mrror can be changed by smply changng the relate szes of the emtters n the transstors. or the deal case, the Mrror ato s determned only by the rato of the two emtter areas. O n. E 1 1 BE A CE 2 A 2 O n A E A E 2 1
43 BJ Current Mrror: Output esstance A current source usng BJs doesn t hae an output current that s completely ndependent of the termnal oltage across t, due to the fnte alue of Early oltage. he current source seems to hae a resste component n seres wth t. 1 CE 2 ce2 A O C2 E 1 BE 2 A O E 1 CE o A 1 BE 2 A O o o o Q pt 1 C2 A CE 1 A O o s defned as the small sgnal output resstance of the current mrror.