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Can the stb simulation in Spectre calculate the negative phase margin?

zuiying

When I was simulating a two-stage op amp, I tried to use stb to simulate its stability.

 At first I did not add a compensation capacitor (later I used ac simulation and the calculator to find that the phase margin at this time was a negative value).The stb simulation shows "PM metric not found".

And when I added the compensation capacitor, the stb simulation showed the phase margin.

Therefore, can the stb simulation only calculate the positive phase margin, and the negative phase margin will be considered "PM metric not found"? (If this is the case, then I may prefer to use ac simulation and calculators in the future.)

Thank you very much.

Parents

  • I just had a look at the documentation and discovered that there is a (new?) parameter margininfo_unstable with the default value "no". So you will probably get a result if you set margininfo_unstable=yes.

  • in reply to Frank Wiedmann

    I also did not find the "margininfo_unstable" parameter.

    I am using the spectre emulator in virtuoso (version information is as follows).

    I'm just more used to negative phase margins. But stb may think that a negative phase margin means no phase margin.

    Is there any way for my version of the software to make stb output negative phase margin? (This may cure my obsessive-compulsive disorder. Hahaha)

     Thanks a lot.

  • in reply to zuiying

    It's the spectre version that matters, not the IC version. Even in the latest IC version we've not added that parameter into the UI (because it's likely to be rarely used, I think).

    So, check the spectre version from the spectre log file (it will be near the top), and assuming you're using SPECTRE19.1 ISR9 or later, you can then type margininfo_unstable=yes into the additionalParams field on the stability analysis options form.

    If you're using an older Spectre version and can't update it for some reason, then you'd have to send the loopgain to the calculator and then use the phaseMargin function on that. That should work too.

    Andrew

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