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PSTB loop-gain Phase-plot : error at lower frequency

jdp721

Hi.

I am simulating the common-mode loop gain of common-mode feedback loop of a fully differential amplifier, through PSS/PSTB analysis (using "diffstbprobe").

But, the phase-plot obtained shows a low-freq (dc) phase of 0° (instead of 180°) !

Please see the attached screenshot - the left half phase plot is as expected (obtained from another circuit), but the right half plot seems erroneous at low frequencies as I stated above!

I am using: PSS errpreset=conservative, and reltol=1e-5, vabstol=3e-8, iabstol=1e-13.

Can you please suggest a way to obtain the actual phase plot profile from the PSTB.

PSTB plots

Thanks!

Parents
  • jdp721
    in reply to Frank Wiedmann

    THANK YOU Frank for pointing to the relevant thread... Slight smile

    But, I am not able to locate any asymmetry in my differential circuit, as this is at schematic level, and with only single f/b loop (the CMFB loop that's having the diffstbprobe).

    Circuit is having a folded cascode amplifier followed by source-follower (in differential open-loop config), and the peculiar dc phase of 0° is coming when I am using n-MOS source-follower in place of p_MOS source-follower!

    Can it be because of testing in open loop config?

  • Frank Wiedmann
    in reply to jdp721

    Without a detailed examination of your circuit, it's difficult to say what exactly is causing this behavior in your case. However, as discussed in the thread, it probably does not indicate a stability problem (and the thread also explains how to confirm this).

Reply
  • Frank Wiedmann
    in reply to jdp721

    Without a detailed examination of your circuit, it's difficult to say what exactly is causing this behavior in your case. However, as discussed in the thread, it probably does not indicate a stability problem (and the thread also explains how to confirm this).

Children
  • jdp721
    in reply to Frank Wiedmann

    Okay...thanks Frank once again.

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