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rty94

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current_mirror.PNG
Why we use transistors M1,M18 etc for CUR current like that instead of one transistor?
 

MahmoudHassan

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He wanted to multiply the current in M19 to be 5 times the current in the transistors chain
so what is done is using a unit transistor of 1.28u/300nm and in the chain a 5 transistors in series that's like multiplying L by 5
so in the reference mirror, the chan will have a size of 1.28u/(0.3u*5) and on the mirror branch the size will be 1.28u/0.3u
which 5 times higher
the idea of using a unit transistor is to simplify layout and matching techniques (common centroid and interdigitization)

I don't think this the best way to design the current mirror with different L sizes because this will affect the current due to the 2nd order effects
(Like Vth and channel length modulation)
 

dick_freebird

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Integer ratio matching is superior for litho effects.
The alternative would be to use a 1X reference
and a 5X output device, but maybe there was
an interest in keeping current density higher
(such as leakage floor or low current "gate kink"
observed to be a threat to fidelity). Particularly
when VT is not so high that you are on the
junction leakage floor by Vgs=0, there could be
a loss of ratio control at some corners if current
density is let to sit too low.
 

BigBoss

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W ratios of two branches are same but Effective L ratios 5 times smaller so this circuit multiplies the current by 5.
Why 5 MOS have been used ?? In order to improve the matching,...
 

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