Unformatted text preview: harges induced in the base are the electrons in the depletion region, the other half being imaged in the collector. i.e.,
QBimg I c xc 2 sat Jason Woo QBimg VBE qI C xc = kT 2 sat
EE121B Spring 2010 49 Similarly,
C b c =
Q B is the depletion capacitance of the base-collector junction VBC i rb c = bc vbc is due to the reversed biased base-collector junction current Also gm vb e = ic ic 1 gm = * ib rbe * with vbe = ib rbe Recall ic = * ib VA ro = IC VCB is the Early effect factor: = 1 + VA Also:
Jason Woo and VA is the Early voltage
50 EE121B Spring 2010 Finally, if we ac ground the collector terminal (tie to the ac emitter). ic gm rbe h fe = = ib qI c qI c xc 1 + j rbe C bedep + + C b c B + kT kT 2 sat That is, the cut-off frequency for current gain, fT, ic (T ) T =1 fT = with ib (T ) 2 can be derived to be: rbe xc 1 C bedep + C b c + B + EC = 2 fT gm rb e 2 sat ( ) In the case where there is other parasitic capacitance, Cp, between the base-collector and the base-emitter junctions xc 1 kT C b edep + C bc + C p + B + = 2 f...
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