Subthreshold biasing circuit

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Hello,

I have to design a biasing circuit for a Trasconductance amplifier operating in weak inversion region with a 1.25 V supply. I am using Cadence.

I've seen that the Threshold voltage depends slowly (tens of mV) by the dimensions of the transistors.

This causes large variations of the currents even if the aspect ratio is the same.

How can I solve this problem?

Thanks for your reply.

Lorenzo Manoni.
 

You can't get rid of that issue with subthreshold mosfets because VT has an exponential dependence. If you're looking for constant current, look into using BJTs since the built in potential doesn't significantly change over process. But a constant gm bias is typically better for amplifiers, and has a lot less variation--that's the way I would go about it.
 

what's the bias current range?
 

What's the bias current value and what dispersion are you obtaining?
 

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