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    2stage fully differential opamp output distortion

    Hi all

    Im desinging 2 stage fully differential opamp with continous cmfb. My design using smic18mmrf with VDD=3.3v and input cm = output cm = 1.65v. I test transient with apply sin wave 20uV peak to peak
    to see how output behave. From the waveform (see attached) , the output distortion at below output common mode. I have hard time to figure the problem even the opamp has good output swing. Anyone can help me with this. Thanks a lot








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    Re: 2stage fully differential opamp output distortion

    You see in picture 4 what the actual output swing is (either if it's considered good or not). Shouldn't the CM bias be centered to the output voltage range?

    The other question is why the output clips below 1.5 V. I fear , this can't be seen from your selected circuit snippets.



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    Re: 2stage fully differential opamp output distortion

    Hi FvM

    Thanks for your response. I use common mode reference = 1.65v = vdd/2
    This is clip below 1.5v from my simulation



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    Re: 2stage fully differential opamp output distortion

    It's look like your current sink controlled by CMFB has very poor performance. Add some degeneration to them or cascodes and check what will happen.



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    Re: 2stage fully differential opamp output distortion

    The clipping at 1.5V reminds me of typical op amp behavior.
    Example, LM741, whose output cannot go closer than a couple volts above its bottom supply rail.



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  6. #6
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    Re: 2stage fully differential opamp output distortion

    Quote Originally Posted by BradtheRad View Post
    The clipping at 1.5V reminds me of typical op amp behavior.
    Example, LM741, whose output cannot go closer than a couple volts above its bottom supply rail.
    Yeah, but not for cmos process with 3.3V supply . I gues that diff pair is loaded by simple current source maded on one nmos so the lower rail limitation is equal to VDSsat voltage around 0.2-0.3V.



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