R. T. HoweEECS 105 Spring 2005 Lecture 29Dept. of EECSUniversity of California, BerkeleyLecture 29• Last time:– Transistor action, large-signal operation• Today :– Ebers-Moll model– Small-signal model of the npn bipolar transistorR. T. HoweEECS 105 Spring 2005 Lecture 29Dept. of EECSUniversity of California, BerkeleyTransconductance (cont.)• Forward-active large-signal current:)1(/ACEVvSCVveIithBE+=• Differentiating and evaluating at Q = (VBE, VCE )R. T. HoweEECS 105 Spring 2005 Lecture 29Dept. of EECSUniversity of California, BerkeleyComparison with MOSFET gm• Bipolar transistor:• MOSFET:• Typical bias point: drain/coll. current = 100 µA;Select (W/L) = 8/1, µnCox= 100 µA/V2R. T. HoweEECS 105 Spring 2005 Lecture 29Dept. of EECSUniversity of California, BerkeleyWhat about the Base Current?Unlike MOSFET, there is a DC current into thebase terminal of a bipolar transistor:())1(/thCEVVFSFCBVVeIIIthBE+==ββTo find the change in base current due to changein base-emitter voltage:=∂∂∂∂=∂∂QBECQCBQBEBviiiviR. T. HoweEECS 105 Spring 2005 Lecture 29Dept. of EECSUniversity of California, BerkeleySmall-Signal Current Gain βoR. T. HoweEECS 105 Spring 2005 Lecture 29Dept. of EECSUniversity of California, BerkeleyInput Resistance rπ()QBEBvir∂∂=−1πIn practice, the DC current gain βFand the small-signalcurrent gain βoare both highly variable (+/- 25%)Typical bias point: DC collector current = 100 µAR. T. HoweEECS 105 Spring 2005 Lecture 29Dept. of EECSUniversity of California, BerkeleyOutput Resistance roWhy does current increase slightly with increasing vCE?Model: math is a mess, so introduce the Early voltage)1(/ACEVvSCVveIithBE+=R. T. HoweEECS 105 Spring 2005 Lecture 29Dept. of EECSUniversity of California, BerkeleyGraphical Interpretation of roTypical value:R. T. HoweEECS 105 Spring 2005 Lecture 29Dept. of EECSUniversity of California, BerkeleyBJT Small-Signal ModelR. T. HoweEECS 105 Spring 2005 Lecture 29Dept. of EECSUniversity of California, BerkeleyBJT CapacitancesBase-charging capacitance Cb: due to minority carrier charge storage (mostly electrons in the base)FmbgCτ=Base-emitter depletion capacitance: CjE= 1.4 CjEoTotal B-E capacitance: Cπ= CjE+ CbR. T. HoweEECS 105 Spring 2005 Lecture 29Dept. of EECSUniversity of California, BerkeleyComplete Small-Signal ModelR. T. HoweEECS 105 Spring 2005 Lecture 29Dept. of EECSUniversity of California, BerkeleyIBM SiGe Heterojunction BJTR. T. HoweEECS 105 Spring 2005 Lecture 29Dept. of EECSUniversity of California, BerkeleySiGe BJT/CMOS vs. RF CMOSFrom “IBM and Cadence collaborate to accelerate silicon-accurate design of advanced RF integrated circuits,” IBM Microelectronics Division, March 11, 2005.R. T. HoweEECS 105 Spring 2005 Lecture 29Dept. of EECSUniversity of California, BerkeleyfTvs. ICFrom J. S. Dunn, et al,IBM J. Res. & Dev., 47, 101-138
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