Is there any open source opamp cell in sky130 available? like lm741? in undergrad we designed circuits using discrete opamp, curious to try out to design asic version of those old school circuits. something like creating a sky130 version of this: https://community.element14.com/members-area/personalblogs/b/frank-milburn-s-blog/posts/op-amp-made-from-discrete-components

for an on-chip realization, do you need that whole output stage shown in the linked page? I've often seen on-chip op amps that act a bit more like OTAs without a buffered output stage and they're sufficient for driving on-chip loads, especially capacitive ones like FET gates of further stages/feedback. For a fully diff amplifier I did in the past, the common mode feedback op amp was the simple/minimalistic 5-transistor OTA and that was more than enough for the CMRR and stability I needed

Thanks for the info. For 5 transistor OTA tail current ref how do you go about generating?

I'd share a current reference if one was already available for other circuits, but the tail current isn't too critical on its own so I could theoretically see someone getting away with simpler than a temperature-compensated bandgap reference + v->i conversion