Proper Biasing for cascode with cascode current source as load

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samihawasli

Joined Oct 19, 2020
10
Hello all,
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I'm fairly new to CMOS design so i had a question about proper biasing for a cascode circuit whose load is also a cascode current source. I want to use current mirrors to bias most the FETS in the design. Above is my initial circuit schematic. I am using current mirrors to bias all the FETS. In this case I get my reference current through my cascode and a pretty good gm. But, and i could be wrong here, i have to replace the bottom FET's current mirror with bias resistors in order to provide an AC input correctly
1620966391895.png
Above is the circuit with the added AC input. I choose Rbias1 and Rbias2 to achieve same gate voltage as the above example with the current mirror, ~472mV. However, in this case the current though the cascode is no were near the reference current, and my gm dropped substantially. Even when I run a parametric simulation adjusting Rbias2 to adjust the the bottom gate voltage, in no case can i achieve the same Id and gm as I did with the full current mirror model. Clearly I am missing something pretty basic here. Can anyone lighten me as to why this didn't' work?
 

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