Whits, EE 322 Ltur 16 Pag 1 of 8 Ltur 16: Bipolar Juntion Transistors. Larg Signal Modls. Transistors prform ky funtions in most ltroni iruits. This is rtainly tru in RF iruits, inluding th NorCal 40A. Thr ar two asi familis of transistors: (1) Bipolar juntion transistors (BJTs) and (2) Fild fft transistors (FETs). n th NorCal 40A w will us th following transistors: npn BJT. Q1 is an xampl. pnp BJT. Q4 is an xampl. Juntion FET (JFET). Q5 is an xampl. Ths transistors will usd to prform a plthora of funtions in th NorCal 40A transivr inluding: Eltroni swithing (Q1 and Q4). Timvarying voltag amplifirs: o Small signal amplifir (Q6), o Powr amplifir (Q7). Buffr amplifir (Q5). Voltag ontrolld varial rsistan (Q2 and Q3). Osillator (Q8). W will first onsidr th opration of th BJT. At th nd of th nxt haptr in th txt, w will onsidr th JFET. 2006 Kith W. Whits
Whits, EE 322 Ltur 16 Pag 2 of 8 Bipolar Juntion Transistors (BJTs) Th BJT an drawn in simplifid skths as npn BJT pnp BJT pn jt pn jt pn jt pn jt ()olltor n p n ()mittr ()mittr p n p ()olltor ()as ()as BJTs atually look a it diffrnt than this whn manufaturd. Most importantly, thy ar usually not symmtri dvis. Thr ar four asi mods of opration for a BJT dpnding on th stats of th two pn juntions of th transistor: Mod EmittrBas Jt. ColltorBas Jt. Cutoff Rvrs Rvrs Ativ Forward Rvrs Saturation Forward Forward Rvrs Ativ Rvrs Forward
Whits, EE 322 Ltur 16 Pag 3 of 8 Rvrs and Forward Biasd Juntion Thr ar two important stats for a pn juntion, as disussd arlir in Ltur 3. Ths ar th rvrsd iasd and forward iasd stats. (1) Rvrsd iasd stat: E attry p E n width of dpltion rgion inrass V Th ltri fild produd y th attry E attry adds to th ltri fild of th spa harg E in th dpltion rgion. This inrass th width of th dpltion rgion. Littl urrnt flows (only th drift urrnt s ) unlss th juntion raks down. This ours whn E attry is strong nough to strip ltrons from th ovalnt onds of th atoms, whih ar thn swpt aross th juntion.
Whits, EE 322 Ltur 16 Pag 4 of 8 (2) Forward iasd stat: E attry minority arrir p E n minority arrir width of dpltion rgion drass V Whn V is larg nough so that E attry > E, thn (i) hols ar swpt from th p to n rgions, and (ii) ltrons ar swpt from th n to p rgions. W now hav urrnt! Bipolar Juntion Transistor (BJT) As shown arlir, th BJT is formd from two aktoak pn juntions: npn BJT CBJ EBJ n p n Masuring th d rsistan from to would indiat a vry larg valu sin w hav two aktoak pn juntions. This
Whits, EE 322 Ltur 16 Pag 5 of 8 would our rgardlss of th polarity of th masurmnt sin on of th juntions will rvrsd iasd. Howvr, w an osrv an intrsting fft whn thr is a urrnt into th as and a voltag applid to th olltor (so that it is iasd in th ativ rgion): V Rvrsd iasd Forward iasd B V n p n diffusing E olltd injtd B Th EBJ is forward iasd and ltrons ar injtd into th p rgion just as with a forward iasd diod. Howvr, th as is vry thin and many diffusing ltrons mak it to th CBJ without romining with hols in th as. Hr at this sond juntion, th strong ltri fild (of th rvrsdiasd CBJ) swps th ngativly hargd ltrons aross th juntion into th olltor whr thy ontriut to. Th proportion of ltrons from th mittr that mak it to th olltor is alld th olltor ffiiny, α
Whits, EE 322 Ltur 16 Pag 6 of 8 α or = α (8.1) α typially rangs from 0.98 to 0.99. This rlationship (8.1) is ONLY tru if th dvi is oprating in th ativ mod. From KCL for th BJT = or = = ( 1 α ) (8.2) With α 1, thn will gnrally muh, muh smallr than (again, in th ativ mod). Th ratio of th olltor urrnt to th as urrnt is alld th urrnt gain, β β or = β (8.3) This rlationship (8.3) is ONLY tru if th dvi is oprating in th ativ mod. Dividing (8.1) y (8.2) w find that α α = or β = (8.4) 1 α 1 α With α 0.99, thn β 100 200 in th ativ mod. (Not that β is oftn alld h f in data shts.) n th ativ mod, w an think of th BJT as urrnt as ontrolling th olltor urrnt (sin = β ). Baus β is a larg numr in th ativ rgion, thn a small hang
Whits, EE 322 Ltur 16 Pag 7 of 8 produs a muh largr hang. W v mad an amplifir whr th as is th input and th olltor is th output! Larg Signal BJT Modls Basd on our disussions in this ltur, w an now onstrut a larg signal modl of th npn BJT as (Fig. 8.3a): β W v modld only th EBJ with a diod. W an also inlud th fft of th CBJ as shown in Fig. 8.3() of th txt. n that as, w us th rvrs ativ urrnt gain βr β. Ths β s ar not qual sin BJTs ar gnrally not manufaturd symmtrially: n p n Th modl in 8.3() is alld th ErsMoll modl and is usd in SPCE.
Whits, EE 322 Ltur 16 Pag 8 of 8 r β f β r r = β β r f r r = β β f r r r = β ( 1 β ) f f r r BJT Rgions of Opration Thr ar four distint rgions of opration of a BJT, as mntiond at th ginning of this ltur. Ths rgions ar dfind y th stat of th EBJ and CBJ as listd in Tal 8.1: Rgion V V V Forward Rvrsd Ativ?? β iasd iasd Saturation Forward Forward 0.2 < β ( on ) iasd iasd 0.3 V Cutoff Rvrsd Rvrsd?? 0 ( off ) iasd iasd Rvrs Ativ Rvrs iasd Forward iasd?? ( 1 β ) ( 1 β ) = β f r r = n th nxt ltur, w will onsidr th saturation and utoff rgions of opration. Ths ar th on and off stats of th BJT whn it is usd as an ltroni swith. r