Consider the circuit below. We have seen this one already. As before, assume that the BJT is on and in forward active operation.

Transcription

1 Saturatio Cosider the circuit below. We have see this oe already. As before, assume that the BJT is o ad i forward active operatio. VCC 0 V VBB ib RC 0 k! RB 3V 47 k! vbe ic vce βf 00. ( )( µ µ ). (. )(!! ) Red alert! Red Alert! Somethig is seriously wrog here. VCE is extremely egative, which is ot at all compatible with the B-C juctio beig reverse-biased. Furthermore, there is o way that we could get 39 V across ay two poits i the circuit whe there is oly oe 0-V source ad oe 3-V source. The problem lies i the assumptio that the base-collector is reversebiased. The collector curret that we calculate is much too big. EE 230 saturatio

2 Saturatio Whe both juctios are forward-biased, the trasistor is said to be i saturatio. The B-E juctio is still forward-biased, ad electros ad are still ijected across it. The electros will still cross the base ad travel ito the collector, as we saw i the forward-active case. But ow, the base-collector is also forward-biased. Holes are beig ijected from base to collector. Also electros are ijected from collector to base. These electros will cross the base ad travel ito the emitter. v BE v BC p electros electros emitter base collector I saturatio, there are lots of carriers flowig across the two juctios. There is more hole curret (i.e. more base curret). The two electro currets flow i opposite directios ad ted to cacel each other. EE 230 saturatio 2

3 ib ipip2 ie ip i i2 p 2 electros base collector electros ijected emitter to base: i ISN exp ijected base to emitter: ip ISP exp electros ijected collector to base: i2 ISN exp ijected base to collector: ip2 ISP exp EE 230 ic ip2 i i2 2 electros emitter ISN exp ISP exp ISN2 exp ISP2 exp saturatio 3

4 ib ipip2 ie ip i i2 p 2 electros ib ISP exp ic ISN exp ie ISN exp 2 electros emitter base ic ip2 i i2 collector ISP2 exp ISN2 exp ISN2 exp ISP2 exp ISP exp Wow - big mess. Note that ib has icreased (extra ijected out of base) ad ic has decreased (electro currets are partially cacelig), so ic < βf ib. We do t eve have that simplificatio i this case! EE 230 saturatio 4

5 With some kowledge from a future class (EE 332) ad a approximatio or two, we ca simplify the euatios slightly. First, ICBS (it ca be show ad will be i EE 332) that for all BJTs, ISN ISN2. Secodly, we ca always express the base-collector voltage i terms of the base-emitter ad collector-emitter voltages: vce. Usig these two bit of ifo, the collector curret euatio ca be writte: ic ISN exp exp vce ISP2 exp Thirdly, we might make the assumptio that the hole curret ijected from base to collector (represeted by the terms with the ISP2 coefficiet) is small i the base ad collector curret euatios. (This approximatio may be iffy sice ISP2 >> ISP for most BJTs. However, let s go with it for ow.) With this, the base ad collector currets ca be writte: EE 230 ib ISP exp ic ISN exp Better, but still ot easy. exp vce β F ib exp vce saturatio 5

6 A stadard way of presetig a BJT i-v relatioship is to fix the base curret at some value (same as fixig V BE ) ad the measurig (ad plottig) the collector curret while varyig the collector-emitter voltage, V CE. i C forward active i B 40 µa (v BE V) i B v be v ce saturatio For the trasistor: I SN 5x0 4 A, I SP 5x0 6 A, I SP2 x0 4 A, EE 230 saturatio 6

7 i C - v CE family of curves for p trasistor. EE 230 saturatio 7

8 The euatios for saturatio look very complicated. How do we cope? As always, we make a simplifyig approximatio. Look more closely at the saturatio regio, plottig i C /i B vs. v CE. (I this way, we get the same curve for all values of B-E bias.) 20 I C /I B β F Saturatio oly occurs whe v CE < 0.3 V. I C /I B saturatio forward active A simple approximatio the is to set v CE 0.2 V i saturatio The cofirmatio that the BJT is i saturatio is that i C /i B < β F V CE (V) EE 230 saturatio 8

9 Summary of p BJT operatio. forward active saturatio off V BE 0.7 V V BE 0.7 V I C I B I E 0 I C β F I B V CE 0.2 V check V BE < 0 I E I B I C I E I B I C check V BC < 0 check: V CE > 0.2 V check I C < β F I B. Note that we have ot metioed the case where the base-emitter is reverse biased ad the base-collector is forward biased. This is called reverse active. Geerally, BJTs are ot operated i reverse-active mode. If your trasistor eds up i RA mode, it is probably because you made a mistake. EE 230 saturatio 9

10 Re-cosider the circuit from the first slide. We kow that it is ot i forward active, so it must be i saturatio. Start by calculatig i b the calculatio is uchaged, because our assumptios about the base-emitter juctio are the same for saturatio as for forward active. v BE i B V BB R B 3V 0.7V 47 kω 49 μa V BB 3 V i b R B 47 k! R C 0 k! v be V CC 0 V i c v ce β F 00 Usig the approximatio for saturatio: v CE 0.2 V. The, v CE i C V CC R C 0 V 0.2 V 0 kω 0.98 ma Because of the bad iitial guess, we kow that the BJT must be i saturatio. But we ca still apply the cofirmatio check: i C i B 0.98 ma ma 20 < β F The calculatio is uite simple. The approximatio for saturatio helps us avoid a pile of computatioal messiess. EE 230 saturatio 0