Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

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

Status
Not open for further replies.

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?

charge pump0000000000012.jpeg

Will this affect the stability?


shouldn't it be monotonically decreasing like this?
charge pump0000000000013.jpeg


Thank you in advance
 

SunnySkyguy

Advanced Member level 5
Joined
Sep 26, 2007
Messages
6,743
Helped
1,675
Reputation
3,348
Reaction score
1,644
Trophy points
1,413
Location
Richmond Hill, ON, Canada
Activity points
50,732
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

Member level 5
Joined
Oct 19, 2013
Messages
81
Helped
1
Reputation
2
Reaction score
1
Trophy points
8
Activity points
648
I changed 'stb' sweep type from 'automatic' to 'logarithmic', then the plot looks better. like this
charge pump0000000000015.jpeg

Maybe because of calculation error
 

nitishn5

Full Member level 5
Joined
Mar 27, 2011
Messages
308
Helped
79
Reputation
158
Reaction score
79
Trophy points
1,308
Location
Bangalore, India, India
Activity points
3,366
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.

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top