From: Antonio <tri...@gm...> - 2006-06-25 16:18:31
Attachments:
opamp.tar.gz
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HI to the list, I need a model of an opamp which has gain, voltage limit, two poles, and possibly the current biasing and voltage offset and common mode gain. In this way I could simulate the various techniques for the external compensation and see how they compare each other. I've created a three-port model that simulate such opamp (you can find it in the project file attached). It seems to work, in a way (although seems the simulations quite slow). The inconvenient is that I have to enter the sub circuit and modify there some sub-circuit component parameter if I want to modify the opamp parameters. I can live with it for now, but I wonder how difficult would be to add such model to the qucs library. I think a better (yet idealized) model for the opamp in qucs would be very useful. Returning to my problem. Testing the opam model (opamp-sub.sch) in an open loop configuration (open-gain.sch) I get the expected graph for the open loop gain, with the two poles. If I test the opamp in a simple inverting configuration (inverting-config.sch) I get strange results. At very high frequency the gain begins to became constant again, as there were two zeros (to compensate the -2 slope) but the phase diagram does not increase (as should if there were the zeros). Is there an error in my model or is this a numerical artefact of some kind? Moreover the resolution of the diagram seems quite low and I can't increase it increasing the point in the simulation (seems a sort of numerical noise). Zoom on the diagram in inverting-config.dpl to see what I mean. The same strange behaviour is present in various other configurations you can see in the project file. Any suggestion is appreciated... Cheers, ~ Antonio |
From: Stefan J. <st...@gr...> - 2006-06-26 07:10:17
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Am So, 25.06.2006, 18:18, schrieb Antonio: > HI to the list, Hi there! > I need a model of an opamp which has gain, voltage limit, two poles, > and possibly the current biasing and voltage offset and common mode > gain. In this way I could simulate the various techniques for the > external compensation and see how they compare each other. > > I've created a three-port model that simulate such opamp (you can find > it in the project file attached). It seems to work, in a way (although > seems the simulations quite slow). The inconvenient is that I have to > enter the sub circuit and modify there some sub-circuit component > parameter if I want to modify the opamp parameters. I can live with it > for now, but I wonder how difficult would be to add such model to the > qucs library. I think a better (yet idealized) model for the opamp in > qucs would be very useful. ... on the TODO list. :-( > Returning to my problem. Testing the opam model (opamp-sub.sch) in an > open loop configuration (open-gain.sch) I get the expected graph for > the open loop gain, with the two poles. > > If I test the opamp in a simple inverting configuration > (inverting-config.sch) I get strange results. At very high frequency > the gain begins to became constant again, as there were two zeros (to > compensate the -2 slope) but the phase diagram does not increase (as > should if there were the zeros). Is there an error in my model or is > this a numerical artefact of some kind? > > Moreover the resolution of the diagram seems quite low and I can't > increase it increasing the point in the simulation (seems a sort of > numerical noise). Zoom on the diagram in inverting-config.dpl to see > what I mean. > > The same strange behaviour is present in various other configurations > you can see in the project file. > > Any suggestion is appreciated... Hm. I'll play around with it... Cheers, Stefan. |
From: Antonio <tri...@gm...> - 2006-06-26 18:07:39
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Hi, On 6/26/06, Stefan Jahn <st...@gr...> wrote: > Am So, 25.06.2006, 18:18, schrieb Antonio: [cut] >> I wonder how difficult would be to add such model to the > > qucs library. I think a better (yet idealized) model for the opamp in > > qucs would be very useful. > > ... on the TODO list. :-( Ok, better then nothing :-) > > Returning to my problem. Testing the opam model (opamp-sub.sch) in an > > open loop configuration (open-gain.sch) I get the expected graph for > > the open loop gain, with the two poles. > > > > If I test the opamp in a simple inverting configuration > > (inverting-config.sch) I get strange results. At very high frequency > > the gain begins to became constant again, as there were two zeros (to > > compensate the -2 slope) but the phase diagram does not increase (as > > should if there were the zeros). Is there an error in my model or is > > this a numerical artefact of some kind? > > > > Moreover the resolution of the diagram seems quite low and I can't > > increase it increasing the point in the simulation (seems a sort of > > numerical noise). Zoom on the diagram in inverting-config.dpl to see > > what I mean. > > > > The same strange behaviour is present in various other configurations > > you can see in the project file. > > > > Any suggestion is appreciated... > > Hm. I'll play around with it... Many thanks :). > Cheers, Stefan. Cheers, ~ Antonio |
From: Stefan J. <st...@gr...> - 2006-07-03 10:53:52
Attachments:
Antonio_Op_AMP.tar.gz
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Am Mo, 26.06.2006, 20:07, schrieb Antonio: > Hi, Hello Anotonio, >> > Returning to my problem. Testing the opam model (opamp-sub.sch) in an >> > open loop configuration (open-gain.sch) I get the expected graph for >> > the open loop gain, with the two poles. >> > >> > If I test the opamp in a simple inverting configuration >> > (inverting-config.sch) I get strange results. At very high frequency >> > the gain begins to became constant again, as there were two zeros (to >> > compensate the -2 slope) but the phase diagram does not increase (as >> > should if there were the zeros). Is there an error in my model or is >> > this a numerical artefact of some kind? >> > >> > Moreover the resolution of the diagram seems quite low and I can't >> > increase it increasing the point in the simulation (seems a sort of >> > numerical noise). Zoom on the diagram in inverting-config.dpl to see >> > what I mean. >> > >> > The same strange behaviour is present in various other configurations >> > you can see in the project file. >> > >> > Any suggestion is appreciated... >> >> Hm. I'll play around with it... > > Many thanks :). I was asking Mike Brinson (in the CC) about your problem with the 2-pole opamp. Please read the attachment. Probably it gives you an idea what goes wrong... Cheers, Stefan. |
From: Antonio <tri...@gm...> - 2006-07-05 20:52:07
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Hi, On 7/3/06, Stefan Jahn <st...@gr...> wrote: > Am Mo, 26.06.2006, 20:07, schrieb Antonio: > > > Hi, > > Hello Anotonio, > > >> > Returning to my problem. Testing the opam model (opamp-sub.sch) in an > >> > open loop configuration (open-gain.sch) I get the expected graph for > >> > the open loop gain, with the two poles. > >> > > >> > If I test the opamp in a simple inverting configuration > >> > (inverting-config.sch) I get strange results. At very high frequency > >> > the gain begins to became constant again, as there were two zeros (to > >> > compensate the -2 slope) but the phase diagram does not increase (as > >> > should if there were the zeros). Is there an error in my model or is > >> > this a numerical artefact of some kind? > >> > > >> > Moreover the resolution of the diagram seems quite low and I can't > >> > increase it increasing the point in the simulation (seems a sort of > >> > numerical noise). Zoom on the diagram in inverting-config.dpl to see > >> > what I mean. > >> > > >> > The same strange behaviour is present in various other configurations > >> > you can see in the project file. > >> > > >> > Any suggestion is appreciated... > >> > >> Hm. I'll play around with it... > > > > Many thanks :). > > I was asking Mike Brinson (in the CC) about your problem with > the 2-pole opamp. Please read the attachment. Probably it gives you an > idea what goes wrong... Thank you. The explanation is very clear. Even if my model was idealized (and oversimplified) the stray capacitance of the resistors can explain such "constant gain" behaviour seen at very high frequencies. BTW, the macro-model for the opamp provided in the attachment it's really complete! I will use it for my simulations, many thanks. PS: Sorry for the delay, I'm really busy with my electronic exam and I've not checked the mail. > Cheers, Stefan. Cheers, ~ Antonio |