Hi Guys,

My high-pass filter begins to oscillate, but I cannot explain it in the Bode plot. May I have other simulation tool to find why it becomes non-stable.

The following picture is the output of the high-pass filter where it begins to oscillate.

Any comments are appreciated.
Yawei

Hello Ywguo,
You can't judge stability from bode, since you have non-monotonic Phase plot , use Nyquist to judge. Looks like you have some encirclements. Why don't you show schematic or try to plot Nyquist in cadence?

Regarding Oscillation Frequency i am not sure why it depends on UGB. May be Frank Wiedmann can explain better.

Thanks,
Raj.

hello,
plot real vs imaginary of the loop gain, this will give you half of the Nyquist plot means for +ve frequency's. Just imagine other symmetric half  if you want total Nyquist plot better export real and imag data into matlab and do some processing(but it is bit tricky).

But looks like you have serious problem with small signal approximation, because your oscillation magnitude is around 1V which can create large shift in the lop dynamics.

Thanks,
Raj.

dear Ywguo,
i think you don't need to calculate exact TF, very hard job for a transistor based problem. Nyquist criterion says

clockwise encirclements= closed loop RHP- openloop RHP. So please plot nyquist plot to check how many encirclements, based on your schematic you may guess open loop RHP, so now you can estimate stability.

Thanks,
Raj.

I zoom in the picture to show the plot near origin.

hello ywguo,
you have one clockwise encirclement and there is no RHP in the open loop means 1 RHP in the closed loop(please refer my previous post).

Thanks,
Raj.

Post the schematic. Seems like you have complex poles after the zero, there is a 180 phase shift around 40MHz.
From the looks of it this happens well below the 0dB gain, so system should be stable unless your not breaking
the loop correctly(you definitely have multiple loops or resonance happening).

Regards,
Aman

hello Ywaei,
this circuit is more or less a voltage regulator, why do you say it is high-pass, your transient results shows oscillations and Nyquist confirms it by having 1 closed RHP.

Thanks,
Raj.