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发表于 2011-10-24 21:33:56
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The primary issue for a current source is its fidelity to the
master reference current. You have a Vds dependence
inherent to the MOSFET, from both the transition between
linear and saturation and from short channel effects.
Headroom goes more to the former. If the reference
device is a simple VDS=VGS MOS diode, then the current
source will dead-match only at VDS=VGS (Vbias), be
higher than reference at higher Vds (maybe not by
too much,
as long as you're not talking high accuracy requirements)
and lower at low Vds and much more so, because it is now
just a crudely controlled resistor. That's where your headroom
bind comes - gm*Rout just went in the tank, bias current
is below setpoint, all kinds of fun.
Cascodes enforce drain voltage matching on the lower,
"mirror" FETs and this is good. But once the guard FET loses
-its- VDS, that falls apart and the sensitivity comes right
back. |
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发表于 2011-10-22 22:44:21
