Sorry, I didnot finish the upload owing to the file is too largeborodenkov said:you forgot the attachment..
yeah it is rightcarpa said:Check bias first. Is the output DC magnitude the same as you calculated?
You are right .The OPAMP is two stage folded cascode OPAMP,the second one's output is connected to it's input for Miller conpensation,How could i set about to eliminate the phenomenonsutapanaki said:First, is there a second stage in this folded cascode opamp - the one that you use Miller compensation around it? If there is, it is known that a 2 stage folded cascode opamp can have a gain peaking around the unity gain frequency due to a pair of complex conjugate poles. You may want to check a paper by Milles Copland on that matter. It is an old one and I don't remember the title, but I think it is cited in P.Allen book.
If your opamp is not a 2 stage one you should be able to compensate it well just by the load capacitor. Run a small signal step responce to see if it oscillates. Run also a large signal step responce to see if there are additional instability problems.
Joestar said:When i design a Folded Cascode OP AMP(Follow the step by the P.E.Allen 's book),The curve of Magnitude-frequence appear a peak at the vicinity of the second pole.Firstly I thought it maybe cause by the the right half plane zero,but I couldnot solute the issue after added the compensation capacitor and resistance.So i think the problem may be cause by the bias circuit of the cascode active load . who can interpret the bias circuit and the phenomena of the curve.
The attachment is the curve of Mag-Fre
fehler said:Joestar said:When i design a Folded Cascode OP AMP(Follow the step by the P.E.Allen 's book),The curve of Magnitude-frequence appear a peak at the vicinity of the second pole.Firstly I thought it maybe cause by the the right half plane zero,but I couldnot solute the issue after added the compensation capacitor and resistance.So i think the problem may be cause by the bias circuit of the cascode active load . who can interpret the bias circuit and the phenomena of the curve.
The attachment is the curve of Mag-Fre
Hi, is your opamp single-ended? It seems that there are a
complex-pole-complex-zero-pair in your design. I dont think it will exist in
fully differential design. So, if you are single ended, please check with a
smaller load cascode transistor L, and to verify the simulation results.
If your design is fully differential, please add isolating caps in bias, and
try to find the source of this complex-pole-complex-zero-pair.
sutapanaki said:do a pole-zero analysis. depending on the software you use, it should have this option. Eldo has it, I think hspice has it, very probable that spectre has it too.
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