The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Simulators >> RF Simulators >> SC BPF simualtions
https://designers-guide.org/forum/YaBB.pl?num=1270715333

Message started by smalin on Apr 8^th, 2010, 1:28am

Title: SC BPF simualtions
Post by smalin on Apr 8^th, 2010, 1:28am

Hi all,

I am new on the forum. Hi to everybody. Hope you can help me.

I am trying to make set of simulations for BPF on the picture. I did PSS+PAC for transfer function - all works fine.
Now I want to do PSS+PSTB to check the stability of each opamp but here I have problems. results are strange. I placed two probes (one for each opamp) in marked places. Performed simualtions for each probe individually. results are not as expected.

How shoul the testbench look like to make proper simulations. Any hints.

Of course for this simulation input is connected to analog ground.

BR

Title: Re: SC BPF simualtions
Post by Frank Wiedmann on Apr 9^th, 2010, 12:32am

Your probes are both in the local loop around the opamp (via the capacitor) and in the global loop, so the loop gain you are getting is the sum of the local and the global loop gains (and not only the local loop gain as you probably expected).

Title: Re: SC BPF simualtions
Post by smalin on Apr 9^th, 2010, 12:56am

Yes, you are right. To obtain the OL gain of opamps only I should open the global loop or keep it closed?

Where do you suggest to put probes and which loops should stay closed and which opend?

BR

Title: Re: SC BPF simualtions
Post by Frank Wiedmann on Apr 9^th, 2010, 4:52am

Although your loop gain results may look unusual, you can still use them to examine the stability of your circuit. Please see http://www.designers-guide.org/Forum/YaBB.pl?num=1244840188 for some more details.

Title: Re: SC BPF simualtions
Post by smalin on Apr 9^th, 2010, 8:48am

Hi,

Below I place Nyquist diagramms. Can you help me to interpreat results?
I have to go in details with stability criterion during weekend to understand this better.

Any way thanks for your help. It is always for me to learn something new.

BR

Title: Re: SC BPF simualtions
Post by Frank Wiedmann on Apr 10^th, 2010, 1:30am

Your plots are a little hard to interpret but to me it looks like the curve encircles -1 but not +1. So, if your plot shows the loop gain as calculated by the pstb analysis, for which the critical point is +1 due to the sign inversion (see replies #3 and #5 of http://www.designers-guide.org/Forum/YaBB.pl?num=1244840188), there would be no encirclement.