M niknejad and b boser 7 output resistance eecs 240

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Unformatted text preview: or (and large Vmin, VDD) © 2006 A. M. Niknejad and B. Boser 6 Cascoding EECS 240 Lecture 7: Current Sources © 2006 A. M. Niknejad and B. Boser 7 Output Resistance EECS 240 Lecture 7: Current Sources © 2006 A. M. Niknejad and B. Boser 8 Rout = f(k) f(k) VDS 1 = kV1* How choose k? Issues: • Swing versus Ro • Large k useful only for large Vmin simultaneously • Note: small or no penalty for large k and small Vmin • typically choose k>1 EECS 240 Lecture 7: Current Sources © 2006 A. M. Niknejad and B. Boser 9 High-Swing Cascode Biasing • Need circuit for generating Vbias2 • Goal: Set Vbias such that VDS1 ≈ kV* – k > 1 (typical: 1 … 2) – Important for high Rout – No penalty for moderate Rout • Design for insensitivity to – Process variations (µ, Cox, VTH, γ, …) – Reference current Iref EECS 240 Lecture 7: Current Sources © 2006 A. M. Niknejad and B. Boser 10 High-Swing Bias 1 • M4 quarter size or less – M=1/5 for high Rout – Note: M ≠ k • • Sensitive to body-effect • Lcurrent-source = Lcascode • EECS 240 Lecture 7: Current Sources M5 sets VDS3 = VDS1: improves matching Simple © 2006 A. M. Niknejad and B. Boser 11 Rout EECS 240 Lecture 7: Current Sources © 2006 A. M. Niknejad and B. Boser 12 High-Swing Bias 2 • M5 … M10 replace quarter size device • All devices same size • Less sensitive to bodyeffect • Lcurrent-source = Lcascode EECS 240 Lecture 7: Current Sources © 2006 A. M. Niknejad and B. Boser 13 High-Swing Bias 3 • M5 in triode & smaller • All other devices same size • Sensitive to body-effect • Lcurrent-source = Lcascode EECS 240 Lecture 7: Current Sources © 2006 A. M. Niknejad and B. Boser 14 Device Sizing O bjective : Examples: VDS5 = R × Vdsat 1 Square - law : ( ) I D 5 = µCox m5 W VGS 5 − VTH 5 − VDS 5 VDS 5 L 2 I D 6 = 1 µCox W (VGS 6 − VTH 6 ) 2 L 2 with m5 = 1/3, ∆VTH=0V R=1 I D5 = I D6 VGS 5 = VGS 6 + VDS 5 VGS 6 = VTH 6 + Vdsat 6 with Vdsat = Vdsat 6 1 substitute (V ) sat 2 d6 ( ) m5 = 1/4,...
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This note was uploaded on 03/18/2014 for the course EECS 240 taught by Professor Boser during the Spring '04 term at Berkeley.

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