Hi, my name is JH.
I am designing OTA. In order to high gain, I use cascode current
mirror  load.
But when output common mode level is changed, rout also changes dramatically.
The attatched image is main problem of OTA.
How can I keep rout to be nealy constant with vout level changed?

Thanks for read.

Thanks for reply.
But I checked region of cascode device. Both transistors are in saturation region.

Hello-
You should have posted the operating point rather a std cascade stage in paint. You wrote impedance is changing with common mode, means devices are moving out of sat region. Best way is check your Vdsat and make sure your common mode I snot below 2*vdsat.
What is your ro at the point where you have biased? Multiply this with gm*ro of the cascade, should match reasonably close to your sim.