what happens to the phase shift of a 2-stage opamp?

[zhangljz]

Hello,

I have a 2-stage opamp, and I use nulling resistor + miller capacitor for frequency compensation. The opamp is connected as an unit gain buffer. When I do 'stb analysis' to find the phase margin, for some voltage the phase has a 'V' shape shift at high frequency. Why is it?



Will this affect the stability?


shouldn't it be monotonically decreasing like this?



Thank you in advance

 

[erikl]

View attachment 116497 Will this affect the stability?

I suspect so: there is 180° phase shift at gain > 1

shouldn't it be monotonically decreasing like this? View attachment 116496

Of course.

 

[D.A.(Tony)Stewart]

The phase starts at 180 deg as it should then should be negative after going thru 0 with 3rd and 4th order effects , but appears to be inverted. check calcuation error.

It appears there is No compensation built in with dominant 1st order filter.

 

[zhangljz]

I changed 'stb' sweep type from 'automatic' to 'logarithmic', then the plot looks better. like this


Maybe because of calculation error

 

[nitishn5]

This sort of behaviour shows up in Loop Gain analysis of different kinds.

On can even see similar plots from tools like MatLAB as well.

I think it is a result of how the tool handles multiple poles at the same point or complex poles.

Perhaps it is not able to solve for the phase accurately when given multiple poles/zeroes at the same point or close by. So it shows "kinks" in the phase curve.

 

[D.A.(Tony)Stewart]

Thats better
always use log scale