M niknejad and b boser 15 different device length

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Unformatted text preview: ∆VTH=0V R = 1.55 = m5 VDS 5 + 2Vdsat + 2∆VTH VDS 5 6 solve : ( 1 2 Vdsat = m5VDS 5 1 + 1 + m5 + m5 6 R= ( 1 1 2 m5 1 + 1 + m5 + m5 ∆VTH VDS 5 ∆VTH VDS 5 ) ) EECS 240 Lecture 7: Current Sources © 2006 A. M. Niknejad and B. Boser 15 Different Device Length High Swing Cascode Bias 3: Current Source and Cascode have different Channel Length 3V DC Analysis 3V I3 dc = 30uA DC1 Device V1 sweep from 0 to 3 (51 steps) 3V I2 dc = 30uA I1 dc = 30uA Vout V1 M8 10 / 1 M6 10 / 0.35 M4 10 / 0.35 M2 10 / 0.35 Vds1 M7 10 / 1 M=250m M3 10 / 1 M1 10 / 1 VSS Vss EECS 240 Lecture 7: Current Sources © 2006 A. M. Niknejad and B. Boser 16 High-Swing Bias 4 I D 7 = 1 µCox (W )7 (Vdsat ) 7 L 2 2 I D 6 = µCox (W )6 (Vdsat − 1 VDS 6 )VDS 6 7 2 L solve : [ VDS 6 = Vdsat 1 − 1 − m 7 with m= (W L )7 (W L )6 e.g. : (W L )7 = 1 / 16 (W L )6 = 1/8 for Vdsat = 4Vdsat 7 1 m = 0.5 then • • • • • VDS 6 = 0.3Vd 7 = 1.2Vd 1 M6 in triode Insensitive to body effect Current source and cascode device length may differ Need 3 reference sources (increased power dissipation) Large device ratios sat sat Ref: Carlos A. Laber, Chowdhury F. Rahim, Stephen F. Dreyer, Gregory T. Uehara, Peter Kwok, Paul R. Gray; Design considerations for a high-performance 3-µm CMOS analog standard-cell library, IEEE Journal of Solid-State Circuits, vol. 22, pp. 181 - 189, April 1987. EECS 240 Lecture 7: Current Sources © 2006 A. M. Niknejad and B. Boser 17 Gain Boosting • Use feedback to further increase Rout – No increase of Vmin (unlike double cascode) • Local feedback potential instability • Beware of doublets (slow settling) • Noise enhancement EECS 240 Lecture 7: Current Sources © 2006 A. M. Niknejad and B. Boser 18 Gain Boosting Analysis Note: C1 & C2 would not be present in an actual circuit (or smaller). They are added here to separate pole frequencies. EECS 240 Lecture 7: Current Sources © 2006 A. M. Niknejad and B. Boser 19 Zout • Ztotal = Zboost // ZCL • Doublets • Booster bandwidth tradeoff: slow settling - push doubled above closedloop ba...
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