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Design >> Analog Design >> Differential OTA stability...
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Message started by redentore on Nov 27^th, 2006, 8:59am

Title: Differential OTA stability...
Post by redentore on 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!

Title: Re: Differential OTA stability...
Post by Ken Kundert on Nov 27^th, 2006, 6:38pm

Try "spectre -h stb". There are instructions on how to apply stb to differential circuits there.

-Ken

Title: Re: Differential OTA stability...
Post by redentore on 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

Title: Re: Differential OTA stability...
Post by ACWWong on 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

Title: Re: Differential OTA stability...
Post by redentore on 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!

Title: Re: Differential OTA stability...
Post by ACWWong on 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.

Title: Re: Differential OTA stability...
Post by redentore on 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.

Title: Re: Differential OTA stability...
Post by Croaker on 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.