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

Status
Not open for further replies.
Z

zhangljz

Member level 5
Joined
Oct 19, 2013
Messages
81
Helped
1
Reputation
2
Reaction score
1
Trophy points
8
Activity points
648
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
 

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.
 

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


Maybe because of calculation error
 

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.
 

Status
Not open for further replies.

Similar threads

Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…