EE 240B discussion 3

EE240Bdiscussion3

EricChang

Outline

• HW1designsolution.• TransistorcharacterizationusingBAG.• Tipsoncascode design.

GeneralAmplifierDesignStrategy

• Gain/maximumBWindependentofsizeandloadcapacitance.

• BW/noiseimproveslinearitywithsize.• Strategy:designaunit-sizeamplifierfirstthatmeetsgainspecandhavesufficientmaximumBW.ThencomputeminimizesizethatmeetsBW/noiseconstraint.– Tominimizepower,pickthe“best”unit-sizeamplifier.

CSAmplifier

• Ignoreloadcapfirst,assume1finger.

• Whatistheminimumnumberofvariablesthatcompletelydeterminesamplifierperformance?

CSUnitAmplifierdesign

• Given𝑉"# and𝑉$#,allSSparametersandbiascurrentaredetermined.– LoadresistancealsodeterminedfromKCLand𝑉$$.

• Thus,given𝑉"# and𝑉$#,welookupSSparameters,and𝐴& = 𝑔)(𝑅,||𝑟/),𝑤)23 =[ 𝑅, 𝑟/ 𝐶66 78.

• If𝑤)23 > 𝑤:;,thenthesizeis𝑘 ==>/=@@ABCDAEA

78,and

biascurrentisjust𝑘 F 𝐼:H2I.

CSAmplifierflow

• Sweep𝑉"# and𝑉$#,foreachcombination,dothefollowing:– Compute𝐴& and𝑤)23.Ifeitherconstraintsfailed,goontonextcombination.

– Computeamplifiersize𝑘 andfinalbiascurrent.Thisisthesmallestcurrentwe’veseenrecordbiaspoint.

• Returnthebestoperatingpoint.

BAG:UniversalVerificationFramework

• Withschematicgenerators,itisnowpossibletodeveloptestbenches thatworkwitharbitrarycircuits.

• Complexmeasurementproceduresnowonlyneedtobeprogrammedonce,thencanbere-usedeverywhere.

• Transistorcharacterizationisthefirsttestbenchdevelopedinthisframework.

BAG’stransistorchar.algorithm1. Determinestherangeof𝑉"# thatcorrespondstoarange

ofuser-specifiedcurrentdensity(uA/finger)a) Designergenerallyhaveabetterideaontheboundsofcurrent

densityinsteadof𝑉"# rangeb) Thisgivesusfiner𝑉"#stepsizeforthesamenumberofpoints.

2. SimulateYparameters(atagivenfrequency)acrossbiaspointsandextractsmall-signalparameters.

3. (Optional)Simulatenoise,thencalculate“effective𝛾”fromagivenintegrationinterval.a) Calculates𝛾 from𝑔) andtotalintegratednoiseassuming

whitenoisespectrum.b) worksbestfornarrow-bandnoisecharacterization(orwhen

flickernoiseisnotdominant).4. Resultssavetoadatabaseandavailableasinterpolated

functions.

Transistorcharacterizationspecificationfile

• Characterizationsettingsspecifiedinspecs_mos_char/*.yaml.

• dut_lib/dut_cell definesthetransistorbeingcharacterized.

• schematic_params sectiondefinestheschematicparametersofthetransistor.

• measurementssectionscontainscharacterizationoptions.

Exampleusage

• Seescripts_char/nch_char_90n.pyforhowtoruntransistorcharacterization.

• Seescripts_char/mos_query.py forhowtogetsmallsignalparametersgivenoperatingpoint.– HW1solutionhasexamplesofhowtousethistodesignyouramplifier.

• Seescripts_char/mos_plot.py forhowtogettheinterpolatedfunctions.

• SeeMOSDBDiscrete implementationformoredetails/functionalities.

Cascode designnotes• Whyiscascode designhard?

– Manypossiblecombinationofsizes/biasvoltages.

• Howyougeneratethebiasvoltagecanreducedegreesoffreedom.

• “Lazy”bias:justtietoVDD/VSS.– Onlypossiblewayinlow-voltage

processes.– Largeprocess/supplyvariations.

• Cascode currentmirrorbias.– Morestableacrossvariations.– Biasvoltagesfunctionofbiasnetwork

->simplifiesdesign.• Forhomework,yougetidealvoltage

source.– Lesscircuitstobuild,butharderto

design.

Cascode design:blindsweeping• Vb constant->drawablackboxandthinkofitasone“compositetransistor”.– AslongasthepoleatVm isnegligible,whichistrueformostopen-loopapplications.

• Canusethesametransistorcharacterizationroutinetocharacterizecascode transistors.– Havetosweep5+variables,butgivesthemostaccurate/optimaldesign.

– Seench_char_cas.py andmos_plot_cas.py forexamplesofhowtocharacterizeanduseMOSDBDiscrete withcascodetransistors.

Cascodedesign:𝑉∗/𝐴&M based• Notethat𝑉∗ = 2𝐼:H2I/𝑔) implies𝑉∗

ofthecompositetransistor=𝑉∗ ofthebottomtransistor.

• Specify𝑉:/O∗ ,𝑉O/P∗ /𝑉:/O∗ ,and𝐴&M,)HR ofbothtransistors.– 𝐴&M,)HR ensuresthatbothtransistorsare

in“saturation”,andknowingratioof𝑉∗makesdesigningbiasnetworkeasy.

• Firstfindlowest𝑉) suchthat𝑉:/O∗ and𝐴&M,)HR specscanbemet.– Thisgiveslargestheadroomfortop

transistor.• Thenfind𝑉S2I from𝑉O/P∗ .Checkthat

cascode transistormeet𝐴&M,)HR spec.– Ifnotthenthespecsareimpossibleto

meet.