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Design >> Analog Design >> Why the non-linearity
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Message started by Fausto on Jul 26^th, 2022, 10:04pm

Title: Why the non-linearity
Post by Fausto on Jul 26^th, 2022, 10:04pm

Hi everyone, I'm designing a Fully Differential Folded Cascode OPAMP; to verify the input linearity range and the circuit well behavior, I plot the Vo+ Vo- and Vo-differential vs. the differential input voltage.

In the graph, I can appreciate a linear response in the range of +-80uV; I thought that the non-linearity was due to a transistor not working in saturation, so I also graphed the region of operation of each transistor and saw that all were in saturation in the range of +-80mV. So I don't understand the non-linearity from 80uV to 80mV.

Some can help me, please. Thank You.

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Title: Re: Why the non-linearity
Post by Horror Vacui on Jul 28^th, 2022, 2:18pm

I personally wouldn't rely on the region parameter. It's there for the model, but there is no abrupt behaviour around the transition point. Rather check currents and voltages. Break down the problem by inspecting functional blocks. Annotate the DC node voltages and OP parameters at different input voltages where the non-linearity starts.