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Overshot and ringing of fully differential opamp design

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tompham

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Hi all

Im designing fully differntial folding cascode opamp and my opamp has: gain = 70dB, phase margin = 80. When I test for overshoot and setling time I see a 20mV overshot but the worse is taking long time to settle. Anyone can give a suggestion how to fix this.? Thanks a lot

 

yuvan

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I guess it is difficult to explain the problem without the circuit...
 

LvW

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I doubt if the phase margin really is 80 deg. How did you perform the corresponding analysis?
 

tompham

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Hi SunnySkyguy and yuvan

This is detail of my schematic and simulation waveform. I hope you can help me with this. My technology is smic 65nm









I think overshot and ringing because my cmfb design not well can cause the problem. I read in the book they say the GBW of
cmfb should greater or equal to main opamp , my design satisfied this condition but still have issue. The phase margin small also the factor to make ringing of the op amp too.
 

LvW

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Tompham - what about capacitive loading during ac and time domain tests?
 

tompham

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Hi LvW
I use 1pF loading capacitor at each output. I may figure out (not sure) the problem come from the sizing of each transistor (I use L=1um maybe too big for 65nm, since I want high gain but it make big cap at each node in opamp. During transient simulation the charge and discharge time of those caps will afect the circuit behavior. I will reduce the length to 0.2um (keep same W/L ratio) to see what happen
 

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