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[Help]About the CMFB simulation.

xxgenerall

Hi,

A fully differential amplifier is like below.

Parents

  • Dear xxgenerall,

    > I'm not sure which one is correct, the 1st or the 2nd smulation.

     

    I am not sure if others are experiencing the same problem, but It is very difficult to see any detail in your various JPEG files. I cannot make out the axes of your stability plots and I also can not tell how you are simulating the stability. From what I can tell, it appears you are doing two stability analyses - one with the opamp outputs open (i.e. no connects) and one with the opamp outputs feeding back to the inputs through a feedback factor. I can barely make out the DC operating point values annotated on each JPEG file, but it appears in the file "2.sch_closed", the opamp input values are the same whereas in file "3.sch_open", the two DC input values are signficantly different. This suggests that the opamp input devices in the analysis referred to by "3.sch_open" does not have the opamp devices biased at or near their expected operating points. Hence, I would not expect any results from a stability analysis using this topology to necessarily correspond to the stability of the opamp in its closed loop configuration (such as shown in 2.sch_closed).

     

    Shawn 

Reply

  • Dear xxgenerall,

    > I'm not sure which one is correct, the 1st or the 2nd smulation.

     

    I am not sure if others are experiencing the same problem, but It is very difficult to see any detail in your various JPEG files. I cannot make out the axes of your stability plots and I also can not tell how you are simulating the stability. From what I can tell, it appears you are doing two stability analyses - one with the opamp outputs open (i.e. no connects) and one with the opamp outputs feeding back to the inputs through a feedback factor. I can barely make out the DC operating point values annotated on each JPEG file, but it appears in the file "2.sch_closed", the opamp input values are the same whereas in file "3.sch_open", the two DC input values are signficantly different. This suggests that the opamp input devices in the analysis referred to by "3.sch_open" does not have the opamp devices biased at or near their expected operating points. Hence, I would not expect any results from a stability analysis using this topology to necessarily correspond to the stability of the opamp in its closed loop configuration (such as shown in 2.sch_closed).

     

    Shawn 

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