Lecture 17: Common Source/Gate/Drain Amplifiers

1 Department of EECSU niversity of California, BerkeleyEECS 105 Fall 2003, Lecture 17 Lecture 17: Common Source/Gate/Drain AmplifiersProf. NiknejadDepartment of EECSU niversity of California, BerkeleyEECS 105 Fall 2003, Lecture 17 Prof. A. NiknejadLecture Outline MOS Common Source Amp Current Source Active Load Common Gate Amp Common Drain AmpDepartment of EECSU niversity of California, BerkeleyEECS 105 Fall 2003, Lecture 17 Prof. A. NiknejadCommon-Source AmplifierIsolate DC levelDepartment of EECSU niversity of California, BerkeleyEECS 105 Fall 2003, Lecture 17 Prof. A. NiknejadLoad-Line Analysis to find QQDDDoutRDVVIR =110kslope=0V10kDI=5V10kDI=Department of EECSU niversity of California, BerkeleyEECS 105 Fall 2003, Lecture 17 Prof. A. NiknejadSmall-Signal AnalysisinR= Department of EECSU niversity of California, BerkeleyEECS 105 Fall 2003, Lecture 17 Prof. A. NiknejadsvsRinRoutRLRminGvinv+ Two-Port Parameters:Find Rin, Rout, GminR= mmGg=||outoDRrR=Generic transconductance AmpDepartment of EECSU niversity of California, BerkeleyEECS 105 Fall 2003, Lecture 17 Prof.

2 A. NiknejadTwo-Port CS ModelReattach source and load one-ports:Department of EECSU niversity of California, BerkeleyEECS 105 Fall 2003, Lecture 17 Prof. A. NiknejadMaximize Gain of CS Amp Increase the gm(more current) Increase RD(free? Don t need to dissipate extra power) Limit: Must keep the device in saturation For a fixed current, the load resistor can only be chosen so large To have good swing we d also like to avoid getting to close to saturation||vmD oAgRr= ,DSDDDDDS satVV IRV= >Department of EECSU niversity of California, BerkeleyEECS 105 Fall 2003, Lecture 17 Prof. A. NiknejadCurrent Source Supply Solution: Use a current source! Current independent of voltage for ideal source Department of EECSU niversity of California, BerkeleyEECS 105 Fall 2003, Lecture 17 Prof. A. NiknejadCS Amp with Current Source SupplyDepartment of EECSU niversity of California, BerkeleyEECS 105 Fall 2003, Lecture 17 Prof. A. NiknejadLoad Line for DC BiasingBoth the I-source and the transistor are idealized for DC bias analysisDepartment of EECSU niversity of California, BerkeleyEECS 105 Fall 2003, Lecture 17 Prof.

3 A. NiknejadTwo-Port ParametersFrom currentsource supplyinR= ||outoocRrr=mmGg=Department of EECSU niversity of California, BerkeleyEECS 105 Fall 2003, Lecture 17 Prof. A. NiknejadP-Channel CS AmplifierDC bias: VSG= VDD VBIAS sets drain current IDp= ISUPD epartment of EECSU niversity of California, BerkeleyEECS 105 Fall 2003, Lecture 17 Prof. A. NiknejadTwo-Port Model ParametersSmall-signal model for PMOS and for rest of circuitDepartment of EECSU niversity of California, BerkeleyEECS 105 Fall 2003, Lecture 17 Prof. A. NiknejadCommon Gate AmplifierDC bias:SUPBIASDSII I==Department of EECSU niversity of California, BerkeleyEECS 105 Fall 2003, Lecture 17 Prof. A. NiknejadCG as a Current Amplifier: Find Aioutdtiii== 1iA= Department of EECSU niversity of California, BerkeleyEECS 105 Fall 2003, Lecture 17 Prof. A. NiknejadCG Input ResistanceAt input:Output voltage: touttmgsmbtovvigvgvr = ++ (||)(||)out docL tocLvir R ir R= =gstvv= ()||tocLttmtmbtovrRiigvgvr =+ + Department of EECSU niversity of California, BerkeleyEECS 105 Fall 2003, Lecture 17 Prof.

4 A. We have this messy result But we don t need that much precision. Let s start approximating:11||1mmbtoocLintoggirrRRvr ++==+1mmboggr+>>||ocL LrRR 0 LoRr 1inmmbRgg=+Department of EECSU niversity of California, BerkeleyEECS 105 Fall 2003, Lecture 17 Prof. A. NiknejadCG Output Resistance()0sstmgsmbsSovvvgvg vRr + =11tsmmbSoovvggRrr +++= Department of EECSU niversity of California, BerkeleyEECS 105 Fall 2003, Lecture 17 Prof. A. NiknejadCG Output ResistanceSubstituting vs= itRS11ttSmmbSooviRggRrr ++ + = The output resistance is (vt/ it)|| roc||1ooutocSm omb oSrRrRgrgrR =+++ Department of EECSU niversity of California, BerkeleyEECS 105 Fall 2003, Lecture 17 Prof. A. NiknejadApproximating the CG RoutThe exact result is complicated, so let s try tomake it simpler:Sgm 500 Sgmb 50 kro200][||SSombSomoocoutRRrgRrgrrR+++=][ ||SSomoocoutRRrgrrR++ Assuming the source resistance is less than ro,)]1([||][||SmoocSomoocoutRgrrRrgrrR+= + Department of EECSU niversity of California, BerkeleyEECS 105 Fall 2003, Lecture 17 Prof.

5 A. NiknejadCG Two-Port ModelFunction: a current buffer Low Input Impedance High Output ImpedanceDepartment of EECSU niversity of California, BerkeleyEECS 105 Fall 2003, Lecture 17 Prof. A. NiknejadCommon-Drain Amplifier21()2 DSoxGSTWIC VVL = 2 DSGSToxIVVWCL =+Weak IDSdependenceDepartment of EECSU niversity of California, BerkeleyEECS 105 Fall 2003, Lecture 17 Prof. A. NiknejadCD Voltage GainNote vgs= vt vout||outmgsmb outocovgvg vrr= ()||outmtoutmb outocovgvvgvrr= Department of EECSU niversity of California, BerkeleyEECS 105 Fall 2003, Lecture 17 Prof. A. NiknejadCD Voltage Gain (Cont.)KCL at source node:Voltage gain (for vSBnot zero):()||outmtoutmb outocovgvvgvrr= 1||mbmoutm tocoggv gvrr ++= 1||outminmbmocovgvggrr=++1outminmbmvgvgg +Department of EECSU niversity of California, BerkeleyEECS 105 Fall 2003, Lecture 17 Prof. A. NiknejadCD Output ResistanceSum currents at output (source) node:||||toutooctvRrri=tmtmbtigv g v=+1outmmbRgg +Department of EECSU niversity of California, BerkeleyEECS 105 Fall 2003, Lecture 17 Prof.

6 A. NiknejadCD Output Resistance (Cont.)ro|| roc is much larger than the inverses of the transconductances ignore1outmmbRgg +Function: a voltage buffer High Input Impedance Low Output ImpedanceDepartment of EECSU niversity of California, BerkeleyEECS 105 Fall 2003, Lecture 17 Prof. A. Nikneja