Lecture 40Common-Emitter Voltage AmplifierBase-Collector Time ConstantSolving for RT?Dominant Pole of CE AmplifierMultistage Amplifier Frequency ResponseSystematic ApproachCascode Two-Port ModelFinding the Thèvenin ResistancesDominant PoleGain-Bandwidth ProductR. T. HoweEECS 105 Spring 2002 Lecture 40Dept. of EECSUniversity of California at BerkeleyLecture 40• Last time:– Bias and output swing for BiCMOS voltage amp– Start open-circuit time constant analysis(back to Chapter 10)• Today :– Applications of open-circuit time constant analysis: CE amplifier and cascode amplifierR. T. HoweEECS 105 Spring 2002 Lecture 40Dept. of EECSUniversity of California at BerkeleyCommon-Emitter Voltage AmplifierTime constant for base-emitter capacitance Cπ:R. T. HoweEECS 105 Spring 2002 Lecture 40Dept. of EECSUniversity of California at BerkeleyBase-Collector Time ConstantMust apply a test source (can’t see RTµby inspection):R. T. HoweEECS 105 Spring 2002 Lecture 40Dept. of EECSUniversity of California at BerkeleySolving for RTµ()intStRirRiv′−=−=ππ||Find vπ:()()outinmtoutmtoutooRRgiRvgiRiv′′+=′−=′−= 1πFind vo:())(1intoutinmtotRiRRgivvv−−′′+=−=πFind vt:Solve for Thèvenin resistance:R. T. HoweEECS 105 Spring 2002 Lecture 40Dept. of EECSUniversity of California at BerkeleyDominant Pole of CE AmplifierEstimate dominant pole as inverse of sum of OCTCs:()11][1−′′+′+′+′=+≈µπµπττωCRRgRRCRoutinmoutininCCIdentical to the “exact” analysis in Chapter 10Why bother with the OCTC technique … add effect of CcsR. T. HoweEECS 105 Spring 2002 Lecture 40Dept. of EECSUniversity of California at BerkeleyMultistage Amplifier Frequency ResponseCS*-CB cascodeR. T. HoweEECS 105 Spring 2002 Lecture 40Dept. of EECSUniversity of California at BerkeleySystematic Approach1. Construct two-port small-signal models for each stage2. Add the capacitors for each device across the appropriatenodes in the two-port models. Make sure how the gate,drain, and source (or base, collector, and emitter) terminalsof each device fit onto the two-port models!3. (Optional) Use Miller’s Theorem to transform capacitorsacross amplifiers into effective capacitances to ground(note that we ignore the “output Miller” in this course)R. T. HoweEECS 105 Spring 2002 Lecture 40Dept. of EECSUniversity of California at BerkeleyCascode Two-Port ModelAdd capacitors: Cgs1, Cgd1, Cπ2, Cµ2…what about Cdb1, Ccs2?R. T. HoweEECS 105 Spring 2002 Lecture 40Dept. of EECSUniversity of California at BerkeleyFinding the Thèvenin ResistancesCgs1:STCRRgs=1Cgd1:=′′+′+′=outinmoutinTCRRgRRRgd11=2πTCRCπ2:=2µTCRCµ2:R. T. HoweEECS 105 Spring 2002 Lecture 40Dept. of EECSUniversity of California at BerkeleyDominant PoleApplying the theorem:()222121111)/1(/1µπωCRCgCggRCRLmgdmmSgsS++++≈−Find approximate voltage transfer function:()LocoomoomRRsoutvoRrrgrrgvvALS||||/1222111,β+−==R. T. HoweEECS 105 Spring 2002 Lecture 40Dept. of EECSUniversity of California at BerkeleyGain-Bandwidth ProductMetric for amplifier performance: note that()1*=ωjAvwhen 1*ωωvoA=()222121111//1µπωCRgCCggRCRRgALmgdmmSgsSLmvo++++=Special case: small
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