Set 4
4. Active Loads and IC MOS Amplifiers Reading: Sedra & Smith: Chapter 7 (MOS portions)
ECE 102, Winter 2011, F. Najmabadi
Progress towards an IC relevant amplifier Resistors and capacitors take a lot of space on ICs: o Minimize (i.e., very few) R & C and small sizes (e.g., nF or smaller)
o Get rid of coupling capacitors by direct coupling between stages (makes biasing design complicated)
o Replace Rs with a current source o Still need to get rid of Cs! o What to do about RD?
Current mirrors are the principle method for biasing in ICs
Io =
Identical MOS: Same k’n and Vt Q1 is always in saturation VDS1 = VGS > VGS – Vt Q2 has to be in saturation for current mirror to work VDS2 > VGS – Vt
(W / L) 2 I ref (W / L)1
Small signal model of Current Mirrors Small signal response of an ideal current source is an open circuit! However, Current mirrors are NOT ideal current sources as we used λvDS
ECE 102, Winter 2011, F. Najmabadi
Progress towards an IC relevant amplifier Resistors and capacitors take a lot of space on ICs: o Minimize (i.e., very few) R & C and small sizes (e.g., nF or smaller)
o Get rid of coupling capacitors by direct coupling between stages (makes biasing design complicated)
o Replace Rs with a current source o Still need to get rid of Cs! o What to do about RD?
Current mirrors are the principle method for biasing in ICs
Io =
Identical MOS: Same k’n and Vt Q1 is always in saturation VDS1 = VGS > VGS – Vt Q2 has to be in saturation for current mirror to work VDS2 > VGS – Vt
(W / L) 2 I ref (W / L)1
Small signal model of Current Mirrors Small signal response of an ideal current source is an open circuit! However, Current mirrors are NOT ideal current sources as we used λvDS
