curious wrote on Aug 6th, 2015, 12:29pm:Hi all,
If I have designed current mirrors with same ratio but the current that I am mirroring is small that it pushes the transistor close to subthreshold region.
Question is I draw the two mirroring transistors with multiple fingers, each finger will have a very less current, how will it affect?. For example let's say mirror current is 1000nA and there are 10 fingers, each finger will have 100nA. Does this affect the mirroring action?
Not really at those current levels - you have half the current but also half the width so it is basically a wash. At higher current levels the gate and source/drain resistances can be an issue.
Quote:I know that they are biased closed to the threshold voltage of transistor, the Vth-mismatch will affect it alot, apart from this is there any concern having a transistor operating at 100nA.
As Tako said, you have to consider leakage. Make the widths as small as possible.
If it is a static mirror then you should be able to take it out of subthreshold by making the device longer. 100nA isn't that low of a current for a properly sized device.