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Op-Amp Folded Cascode help....

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kart339

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Hello ppl,
I could use your opinion on a problem I am facing with my op-amp design. It's single stage folded cascode. Trouble is I'm not able to get the current of 25uA (am getting only 21uA) to flow in the two folded cascode branch.(spice file attached) I have changed the bias voltages and sizes but without an idea of why it's happening, it's hard to troubleshoot!

Second problem is that my DC transfer characteristic seems shifted to the left of the origin (image attached). And I am really concerned if it's something I should be worried about. All help highly appreciated! Thanks in advance!

Regds
Kart
 

The symptons you mentioned in the email seems to be related with the improper bias of the OTA, which also causes the systematic offset at your transfer curve. You can find a lot of literature (for example, Ken Martin's Analog Integrated Circuit Design) to cover the design of folded cascode OTA and proper choice of the input branch versus output branch. Sorry I don't have time to open up your zipped files.
 

Dear kart339,
You seem to be doing a voltage biasing.
Could you please try current biasing. Currents would be exact ratios if VDS are same for the transistor mirroring the current and the one in which it is getting mirrored.
Try mirroring the current with help of a simple current mirror. You could read on varius buasing schemes in John Martin chap 5 and 6

As for your DC offset. You are looking for O/P=0V for I/P=0V. This could only be manipulated by sizing of the PMOS and NMOS in common gate string. This is an issue related to systematic offset, please read John Martin he talks of systematic offset for a 2-stage opamp.
Hope it helps
 

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