vivkr
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Hi Peter,
I think it is undesirable to place current mirrors in weak inversion. Let me list some reasons:
1. Matching is poor. Current mismatch in current mirrors will get worse with smaller (VGS-VT). In fact, it should normally be quite poor in weak inversion regime.
2. Noise from the current mirrors will shoot up. By placing transistors in weak inversion region, you are maximizing Gm/I for these devices. As a result, the noise contribution is significant, especially if the current mirror happens to be used in an opamp as an active load. Of course, then you would also have to worry about its speed.
3. The mirror will be very slow. It is worth remembering that even if you are simply using the mirror to feed in a DC current, you do expect the mirror output to stay undisturbed due to changing signals elsewhere or in the circuit which the mirror is feeding. Otherwise, performance of the whole circuit is affected. Fast circuits need fast-responding bias as well.
4. Finally, your simulations are only as good as your models. While some foundries specializing in analog processes may characterize the operation in weak inversion well enough, the same in not true for all. Then, the results from simulation are unreliable.
There ought to be a longer list of reasons, and I think other people would have more constructive inputs too.
Regards Vivek
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