1EE105 - Fall 2006Microelectronic Devices and CircuitsProf. Jan M. Rabaey (jan@eecs)Lecture 20: Cascode2Administrative Midterm next Th – Covering everything up to, but NOT INCLUDING multi-stage amplifiers Review session next week on Tu evening23Method of Open Circuit Time Constants (cntd) General two-pole transfer function:)1)(1()1()(2110ppzjjjAjAωωωωωωω+++=()22101)()(ω+ω+ω=ωjajajNAjA21111ppaω+ω=2121ppaωω=Sum of time constants4Higher-Order Time Constants Coefficient a1 a1= R011C1+ R022C2 Coefficient a2: a2= R211C1R022C2 =R011C1R122C2 This is exact! If ωp1<< ωp2: 111paω≈212paaω≈35Example CalculationRsRL+voutvsro||rocvgs+CgsCgdgmvgs6The Cascode ConfigurationDC bias: Two-port model: first stage has no current supply of its ownCommon source / common gatecascade is one version of a cascode(all have shared supplies)47Cascode Two-Port Model CS1* CG2 Output resistance of first stage = 1, oCSoutrR =Why is the cascode such an important configuration?()2121||oomocoutrrgrR +=1mmgG =∞=inR8Miller Capacitance of Input StageFind the Miller capacitance for Cgd1Input resistance to common-gatesecond stage is low Æ gain acrossCgd1 is small.59Two-Port Model with CapacitorsMiller capacitance:11)1(gdgdvCMCAC −=1111221(||) 1gdmvC m ommgAg rgg=− ≈− =−12MgdCC=10Other contributions611Improved Current SourcesGoal: increase rocApproach: look at amplifier (?) output resistance results… to see topologies that boost resistance12Cascode (or Stacked) Current SourceInsight: VGS2= constant ANDVDS2= constantSmall-Signal Resistance roc:713Drawback of Cascode I-SourceMinimum output voltage for all transistors saturated:2,44,4, GSSATDSSSATDSMINOUTVVVVV +=+= vOUT iOUT 14815Complete Amplifier Schematic16Complete Amplifier SchematicGoals: gm1= 1 mS,Rout=10 MΩ917Device SizesM1: select (W/L)1= 200/2 to meet specified gm1= 1 mSÆ find VBIAS= 1.2 VCascode current supply devices: select VSG= 1.5 V(W/L)4= (W/L)4B= (W/L)3= (W/L)3B = 64/2M2: select (W/L)2= 50/2 to meet specified Rout=10 MΩÆ find VGS2= 1.4 VMatch M2with diode-connected device M2B.Assuming perfect matching and zero input voltage,what is VOUT?18Output (Voltage) SwingMaximum VOUTMinimum VOUT1019Two-Port ModelFind output resistance Routλn= (1/20) V-1, λp= (1/50) V-1at L = 2 μm Æron= (100 μA / 20 V-1)-1 = 200 kΩ, rop= 500 kΩ()()()1223332221||11||omoSmoSmoocoutrgrRgrRgrrR ++=+=SVVAVVIgTnGSDmμ=−μ=−= 50014.1)100(22222SVVAVVIgTpSGDmμ=−μ=+−= 40015.1)100(2)(233320Some Interesting Questions
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