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smalin

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austria

SC BPF simualtions
Apr 08^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

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bpf.jpg

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Frank Wiedmann Community Fellow Offline Posts: 678 Munich, Germany	Re: SC BPF simualtions Reply #1 - 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).
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smalin New Member Offline Posts: 4 austria	Re: SC BPF simualtions Reply #2 - 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
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Frank Wiedmann Community Fellow Offline Posts: 678 Munich, Germany	Re: SC BPF simualtions Reply #3 - 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.
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smalin New Member Offline Posts: 4 austria	Re: SC BPF simualtions Reply #4 - 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
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Frank Wiedmann

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Posts: 678
Munich, Germany

Re: SC BPF simualtions
Reply #5 - 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.

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