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fully differential opamp stb simulation

AllenD

Hi team

The schematic of the fully differential amplifier is in the attached pictures. I want to simulate the differential gain/stb and common mode feedback loop stb

Fig1 fully differential amplifier circuit testbench

Fig 2 Inside the fully differential opamp is my core fully differential amplifier and CMFB amp

Fig 3 My core amplifier and I put the CMDMprobe here

My question is that I did transient and stb simulation with cmdm probe. in the plot below, the transient plot is on the left. Red is the input the green is the output, which suggests a stable amplifier with a positive gain. But the stb simulation with CMDM=-1 suggests the plot on the right--the gain is negative. 

Can you please let me know what I did was wrong? Or what other info do you need to help me diagnose?

Thanks

MD

  • ShawnLogan

    Dear AllenD,

    I must be overlooking something, but I don't see any instances of the differential probe diffstbprobe  in your test bench nor in your differential amplifier. Am I overlooking it?

    Shawn

  • AllenD
    in reply to ShawnLogan

    Hi Shawn

    Thanks for your reply! 

    I did not use that probe because I used CMDM probe (in fig 3) and put "－1" in the probe parameter. To the best of my knowledge, this will simulate the differential mode...?

    I tried to insert the diffstbprobe, as you suggested, at the exact same location of my previously CMDM probe. Then updated the probe instance terminal, mode type to differential. But the result is still wrong (as yellow and purple). But the common mode simulation is in agreement with CMDM probe with parameter “=1" 

     

  • ShawnLogan
    in reply to AllenD

    Dear AllenD,

    AllenD said:
    did not use that probe because I used CMDM probe (in fig 3) and put "－1" in the probe parameter. To the best of my knowledge, this will simulate the differential mode...?

    I apologize, but I must be blind - I'm not sure which of your figures is Figure 3, but I don't see an instance of the CMDM probe you mention - sorry!

    In any case, the use of diffstbprobe is recommended for both common-mode and differential mode stability analyses as documented at URL:

    https://support.cadence.com/apex/ArticleAttachmentPortal?id=a1Od0000000nSkWEAU&pageName=ArticleContent

    Further its location is quite important in your circuit. There is also an example on the Cadence support site at URL:

    https://support.cadence.com/apex/ArticleAttachmentPortal?id=a1Od0000000nSorEAE&pageName=ArticleContent

    in case you have not seen that already.

    I apologize, once again, if I am overlooking your probe. Maybe my display is not showing your entire graphic.

    Shawn

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