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Unformatted text preview: change V* = 2 ID / gm • “Short channel” devices: – All interpretations of V* are approximations – Except V* = 2 ID / gm (but V* ≠ Vdsat) EECS 240 Lecture 2: CMOS - passive devices © 2006 A. M. Niknejad and B. Boser 33 Design Example Example: Common-source amp av0 > 70, fu = 100MHz for CL = 5pF • av0 > 70 • L =0.35µm g m ≈ 2πf u C L = 3.14mS • High fT (small CGS): V* = 200mV g mV * = 314µA • ID = 2 EECS 240 Lecture 2: CMOS - passive devices © 2006 A. M. Niknejad and B. Boser 34 Device Sizing 141uA • Pick L • Pick V* • Determine gm 0.35µm 200mV 3.14mS • ID = 0.5 gm V* NMOS 314µA • W from graph (generate with SPICE) W = 10µm (314µA /141µA) = 22µm • Create such graphs for several device lengths for design reference EECS 240 Lecture 2: CMOS - passive devices © 2006 A. M. Niknejad and B. Boser 35 Common Source Sims • Amplifier gain > 70 • Amplifier unity gain frequency is “dead on” • Output range limited to 0.6 V – 1.5 V to maintain gain (about 0.45V swing) EECS 240 Lecture 2: CMOS - passive devices © 2006 A. M. Niknejad and B. Boser 36 Small Signal Design Summary • Determine gm (from design objectives) • Pick L – Short channel – Long channel • high fT high ro, av0 Pick V* = 2ID/gm – – – – Since V* is approximately the saturation voltage Small V* large signal swing High V* high fT Also affects noise (see later) • Determine ID (from gm and V*) • Determine W (SPICE / plot) • Accurate for short channel devices EECS 240 Lecture 2: CMOS - passive devices key for design © 2006 A. M. Niknejad and B. Boser 37 Device Sizing Chart Generate these curves for a variety of L’s and device flavors (NMOS, PMOS, thin oxide, thick oxide, different VT) EECS 240 Lecture 2: CMOS - passive devices © 2006 A. M. Niknejad and B. Boser 38 Device Parameter Summary Device Parameter V* Circuit Implications • • • • • Current efficiency, gm/ID Power dissipation (ID) Speed (gm) Cutoff frequency, fT phase margin, noise Headroom, VDS,min L • Cutoff frequency, fT phase margin, noise • Intrinsic transistor gain (av0) W • Obtain from L, ID • Self loading (CGS, CDB, …) EECS 240 Lecture 2: CMOS - passive devices © 2006 A. M. Niknejad and B. Boser 39...
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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 University of California, Berkeley.

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