[MOVED] opamp linearity at higher freqs

[GSarris]

hello
i am trying to design an opamp for high linearity at high frequencies (100MHz) and i have used a folded cascode followed by a simple cascode at a unity gain feedback. At lower freqs the linearity is ok, but at higher frequencies it is horrible. Can you please tell me what can be possibly wrong? The schemartic, loopgain and the linearity performance are attached. Thank you in advance.

 

[erikl]

Seems some of your FET DC OPs are not ok: check M1, M5 & M6: |Vds| < 0

 

[GSarris]

Sorry for that.. They are ok. I don't know why they seem like that in this picture. Op point is ok actually. I think there is some other problem butno clue so far..

 

[erikl]

... butno clue so far..

Nor have I. Just guesswork:

1. Are you sure your input is symmetric?
2. Is the non-linearity already visible at the connections to the cascode followers (M5-M10 & M6-M11)?

 

[monsoon]

Hi
What is the vdsat (overdrive) of M13,M14,M15 and M16? If you give a smaller amplitude signal, I believe distortion will be less. It is difficult to achieve good linearity near the unity gain frequency, especially if the swings are "large", because your loop gain has reduced. At 100MHz, loop gain is only 10db. Either you have to reduce your output swing or increase unity gain frequency.