Phase Margin issue

Brian Li

Genius 10780 points

Other Parts Discussed in Thread: OPA365, OPA189

Hi all,

I'm using TINA to simulate the stability of the opamp, and I find a question about phase margin.

The AC transfer characteristic is as below:

When the Gain=0dB, the phase margin is 34deg.

But the phase margin can be 1deg when the Gain is 19dB.

I wonder can this 34deg means the circuit is stable?

over 2 years ago

LeonardEllis over 2 years ago

TI__Guru 71526 points

Hi Brian,

TINA is a DesignSoft/TINA program, therefore questions are handled by using the DesignSoft support page: http://www.tina.com/technical-support/ .

The DesignSoft/TINA team is very helpful and this is the most direct way for TINA support. Please advise by Reply when the issue has been resolved.

Regards,

~Leonard

kai klaas69 over 2 years ago

Guru 140200 points

Hi Brian,

I would say that the circuit is not stable.

Because of manufacturing tolerances fo OPamp and imperfections of the model, the phase margin should not only be >45° exactly at the point of 0dB gain (122kHz), but also in the immediate surroundings. And because the phase margin goes all the way down to 0° at 100kHz, I would say that the circuit is not stable.

Can you post your TSC-file and tell more about your application?

Kai

Brian Li over 2 years ago in reply to kai klaas69

TI__Genius 10780 points

Hi Kai,

It is still the same circuit but with other opamps( opa365, opa189) : https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1168359/opa277-opa188-oscillation-issue/4405577#4405577

Thanks,

Brian

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Phase Margin issue