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current mirrors comparision

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Newbie level 6
May 26, 2006
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hi all

among the cascode and self regulated current mirror(in which the source voltage of the top transistor is amplified and supplied as the gate voltage to the same transistor), which one has better(less) compliance voltage and why?

thanks in advance!


I think the cascode need a bias voltage , so need additional current

if the cascode transistor doesn't change, then adding a regulator with gain A will always improve the output impedance with that same factor A...

i know the output impedance of the self regulated is more than the cascode current mirror..what i want to know is in which case the compliance voltage is less?compliance voltage i mean, the minimum voltage needed to keep the config in saturation mode and thereby make it work as current mirror..

anyways thanks for the reply!!

that's a strange definition, I thought you meant a voltage compliance -> a conductance. Well, for the cascode source, it's typically 2 Vdsat. For the regulated cascode, if you use a same type device, you'll end up with 2Vdsat + Vth; if you use a folded cascode for regulating, you'll still have 2Vdsat.

for the cascode current mirror, the compliance voltage is 2vdsat + vth!!!the bottom transistor requires vdsat+vth for it to be in saturation and the top transistor therefore requires vdsat+vdsat+vth.

can someone clearly explain the derivation of compliance voltage for the self regulated current mirror or point me to any online information ?


As you say
"for the cascode current mirror, the compliance voltage is 2vdsat + vth"

for the self regulated current mirror, the compliance voltage is 2vdsat + 2vth

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