qqic
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Posts: 2
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Hi all, I design an opamp, when I double the width and length of input differential pair, the phase margin changed from 60 degree to 40 degree, the structure is folded caccode. the reason is that when input pair W*L increase, the cds increase quickly! some of the input differential pair's capacitors is as following according to DC operation point simulation of spectre: cds=72f cdg=-72.5f cgd=-10.7f cgs=-120f
I have 3 questions: 1> why the cdg is minus in DC simulation of spectre ? 2> why cgd is not the same to cdg, and cdg is much high? 3> why cds is much high? in my opinion cds should be very small!
thank you very much!
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