R. T. HoweEECS 105 Spring 2005 Lecture 31Dept. of EECSUniversity of California, BerkeleyLecture 31• Last time:– Bipolar amplifier two-port models• Today :– BiCMOS voltage amplifier: example of “dissection” technique for a complicated circuitR. T. HoweEECS 105 Spring 2005 Lecture 31Dept. of EECSUniversity of California, BerkeleyMulti-Stage Voltage AmplifierR. T. HoweEECS 105 Spring 2005 Lecture 31Dept. of EECSUniversity of California, BerkeleyCutting Through the ComplexityTwo Approaches:1. Eliminate “background” transistors to reduceclutter2. Identify the “signal path” between the inputand outputR. T. HoweEECS 105 Spring 2005 Lecture 31Dept. of EECSUniversity of California, BerkeleyFirst Approach: Find I & V SourcesR. T. HoweEECS 105 Spring 2005 Lecture 31Dept. of EECSUniversity of California, BerkeleyWhat’s Left? Voltage at base ofQ2is set by totempoleR. T. HoweEECS 105 Spring 2005 Lecture 31Dept. of EECSUniversity of California, BerkeleySecond Approach: Find Signal PathR. T. HoweEECS 105 Spring 2005 Lecture 31Dept. of EECSUniversity of California, BerkeleyIdentifying the StagesFirst stage (or two stages): CS/CB cascodeSecond stage (or two stages): CD/CC voltage bufferWhy does this make sense for a voltage amplifier?R. T. HoweEECS 105 Spring 2005 Lecture 31Dept. of EECSUniversity of California, BerkeleyFind Key Two-Port ParametersOutput resistance of cascode:(){}2222/,||1(||SmoocCBCSoutRrgrrRπ+=()6661SmoupocRgrRr+==R. T. HoweEECS 105 Spring 2005 Lecture 31Dept. of EECSUniversity of California, BerkeleyTwo-Port Parameters (Cont.)R. T. HoweEECS 105 Spring 2005 Lecture 31Dept. of EECSUniversity of California, BerkeleyOutput Resistance and Voltage GainSource resistance of the CC stage is the output resistanceof the CD stage (small)434,4,1111mommoCCSmCCoutoutgggRgRR ≈+=+==ββOpen-circuit voltage gain Av(last two stages have nearly unity gain): ()()766211||omooomvrgrrgA+−=βR. T. HoweEECS 105 Spring 2005 Lecture 31Dept. of EECSUniversity of California, BerkeleyOutput Swing: VOUT,MINMinimum output voltage: M10, M3, and Q2are “suspects”M10goes into triode when VOUT= 0.5 VM3goes into triode when VSD3= 0.5 V ÆVOUT= 0.5 V – 0.7 V = -0.2 VQ2goes into saturation when VCE2= 0.1 Vor VBC2= 0.6 VVOUT= VB2– VBC2+ VSG3– VBE4= 2 V – 0.6 V + 1.5 V – 0.7 VVOUT= 2.2 VR. T. HoweEECS 105 Spring 2005 Lecture 31Dept. of EECSUniversity of California, BerkeleyOutput Swing: VOUT,MAXMaximum output voltage: Q4, M5, and M6are “suspects”Q4 goes into saturation when VCE4= 0.1 V Æ VOUT= 4. 9 VM5goes triode when VSD5= 0.5 V Æ VOUT= 3.8 VM6goes triode when VSD6= 0.5 V ÆVOUT= VS6–0.5 V + VSG3– VBE4= 3.5 – 0.5 + 1.5 – 0.7 V = 3.8 VR. T. HoweEECS 105 Spring 2005 Lecture 31Dept. of EECSUniversity of California, BerkeleyInsight into the Frequency ResponseR. T. HoweEECS 105 Spring 2005 Lecture 31Dept. of EECSUniversity of California, BerkeleyQualitative InsightCould always do “brute force” open-circuit time constantsCS*-CB is a wideband stage … so is the CD-CC bufferLook for large RTxCxproducts: high-impedance nodesare likely candidatesR. T. HoweEECS 105 Spring 2005 Lecture 31Dept. of EECSUniversity of California, BerkeleyNode XCS*-CB is a wideband stage … so is the CD-CC buffer“High impedance node” is node X … look at RTxCxCapacitance:Cx = Cgd6+ Cdb6+ Cµ2+Ccs2+ Cgd3+ CM3Miller for CDstage (M3)R. T. HoweEECS 105 Spring 2005 Lecture 31Dept. of EECSUniversity of California, BerkeleyFinding the Miller Capacitance CM3Gain across Cgs3: 333/13LmLvCRgRAgs+= CD X Cgs3 RL3 RL3= Rin4 =R. T. HoweEECS 105 Spring 2005 Lecture 31Dept. of EECSUniversity of California, BerkeleyDominant Pole of Voltage AmplifierThévenin resistance for CX:CDinCBoutinoutTxRRRRR,,32||||==Thévenin resistance for CX:ooomoSmoocTxrrgrRrgrrRβπ27662222||)1())||(1(||+≅+=Dominant Pole of Voltage AmplifierThévenin resistance for CX:Dominant pole:xTxCR≈−11ωR. T. HoweEECS 105 Spring 2005 Lecture 31Dept. of EECSUniversity of California, BerkeleyMagnitude Bode PlotLow-frequency voltage gain was found in Lecture 38:()()766211||omooomvrgrrgA+−=β(neglect loading at output (RL>> Rout) ω|Av(jω)| ω1 Gain-Bandwidth product = unity gain
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