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redentore

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Differential OTA stability...
Nov 27^th, 2006, 8:59am

Hi! I need to plot gain loop of a OTA with capacity feedback.
I tried to use spectre's STB analysis,, but the results doesn't look good.
How do I select the probe for the analysis in a differential circuit (I tried to select one transistor of the differential pair and tried to break the one feedback with a probe, but doesn't seem to work)?
Can someone write me a step by step tutorial?
Thank you very much!

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Ken Kundert Global Moderator Offline Posts: 2386 Silicon Valley	Re: Differential OTA stability... Reply #1 - Nov 27^th, 2006, 6:38pm Try "spectre -h stb". There are instructions on how to apply stb to differential circuits there. -Ken
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redentore New Member Offline Posts: 4	Re: Differential OTA stability... Reply #2 - Nov 28^th, 2006, 2:20am One last question: how do I converti this netlist into a circuit? subckt diffprobe x1in x2in x1out x2out ibranch inout x1out iprobe vinj inout x1in iprobe evinj x2in x2out x1in x1out vcvs gain=0 fiinj 0 x2out pcccs probes=[ibranch vinj] coeffs=[0 1 1] gain=0 ends diffprobe
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ACWWong Community Fellow Offline Posts: 539 Oxford, UK	Re: Differential OTA stability... Reply #3 - Nov 28^th, 2006, 3:19am you can draw it based on the netlist using analoglib iprobe, vcvs and pcccs.... Or if you have access to it you can use the drawn for you component: analoglib.cmdmprobe
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redentore New Member Offline Posts: 4	Re: Differential OTA stability... Reply #4 - Nov 28^th, 2006, 7:22am Ok, I found the component. I plotted the loop gain but there is a little problem: the zero crossing frequency from the loop gain is different from the frequency of the domint pole. Can this be possible? In theory the domint pole frequency and the zero crossing of the loop gain should be the same. Thank you very much!
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ACWWong Community Fellow Offline Posts: 539 Oxford, UK	Re: Differential OTA stability... Reply #5 - Nov 28^th, 2006, 8:44am The dominant pole freq is when the loop gain is reduced by 3dB. It is NOT the frequency at which loop gain is 1 (0dB). When the loop gain does cross 0dB you need to ensure good phase margin.
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redentore New Member Offline Posts: 4	Re: Differential OTA stability... Reply #6 - Nov 28^th, 2006, 9:06am Sorry, I think I haven't made myself clear. Since H=A/(1+T) =~A/T -> T=a/H -> (T)dB = (a)dB - (H)dB, being a the open loop gain, H the closed loop gain and T the loop gain. In this approximation the closed loop -3dB frequency should be the zero crossing frequency on the loop gain.
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Croaker

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Re: Differential OTA stability...
Reply #7 - Nov 29^th, 2006, 4:24pm

Yes, sounds like you are plotting H=1/B (closed-loop gain) and A (open-loop gain) on the same graph, and the difference is the loop-gain. OK.

Your assumption is correct assuming the A*B >> 1 and 1/B intersects at a -20 dB point. Maybe you have a second pole in your open-loop response. If so, there will be an increase in downward slope, though sometimes it's a little tricky to see.

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