Hello everyone! I'm trying to design a differential amplifier with current mirror biasing using the the gm/Id methodology. Hand calculations and the simulation results are not in agreement. (I've attached the relevant screenshots). I'm new to the method and I'm trying to learn by designing an actual circuit. I think one of the problems I'm having is with the calculated width W (from the gm/Id and Id/W relationship). The calculated W is 3.142e-5m and maximum W I've seen so far in the sm141064.ngspice file is 0.5um(I'm using the GF180mcu ngspice models). Please I don't mind having a call for a tutorial on this design methodology. I'm an undergraduate I really need to learn these things. Thank you.

you used long channel model to calculate some parameters. but it is not the purpose of gmid methodology as I think. The long channel model equation is only for understanding the relationship between parameters, not for get a precise number. so we should use "CHART" to be more accurate :)

Thank you. Please can you help point to where I've used the long channel model? I thought I've only followed the steps highlighted in some notes from UC Berkeley for the gm/id methods. And I'd be glad if you can help me with any material with detailed steps for the gm/id methods.

your first equation : gm=2Id/Vov why do you think like that? where gm and Id and Vov come from?