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NMC op amp transfer function calculation

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xinzhao

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Hi guys,
I've a question for calculating the transfer function of a 3 stage NMC op amp (see figure).
I assume each stage output resistance is infinite. My calculation is based on the output current of each gm stage. Three gm stages give me three equations. For example, first equation in the figure is gm1 output current equals the current flowing through C1. The Vout/Vin result shows correct pole locations, but only one zero. The NMC transfer function should have two zeros but I can not get the second one from my calculation. Anyone have any idea about what I'm missing? Thanks.
NMC.jpg
 

well the last zero is +gm2/C1. Just to let you know the zero's in a three stage amplifier are usually negligible. Also you have some sign issues in there. It should be +gm1 in the block diagram. Also all poles should be positive and the zero you have is correct and but should be negative.
Hope this helps
Jgk
 

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