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Newbie level 4

Hi everyone,
I'm trying to design a rail-to-rail amplifier with a folded-cascode stage (differential to single-ended stage). I have a 20µA bias. However, I have not succeeded in having a large bandwidth (it should be 40MHz) and I don't know how to do to obtain it. I can't increase current and sizes too much.
Do you know how can I do to have a correct bandwidth ?
And also, how are cascode transistors sizes usually chosen ?
I have designed smaller structures but for this one, I am a bit confused on the method.
Thanks
I'm trying to design a rail-to-rail amplifier with a folded-cascode stage (differential to single-ended stage). I have a 20µA bias. However, I have not succeeded in having a large bandwidth (it should be 40MHz) and I don't know how to do to obtain it. I can't increase current and sizes too much.
Do you know how can I do to have a correct bandwidth ?
And also, how are cascode transistors sizes usually chosen ?
I have designed smaller structures but for this one, I am a bit confused on the method.
Thanks