two stage op-amp - nmos or pmos input question

[arunsomanathan]

two stage op-amp

I was just wondering in a two stage opamp suppose the first stage is nmos input why do we always use pmos input in the second stage or vice-versa. Can't we use nmos or pmos input in both the stages.

 

[jerryzhao]

two stage op-amp

"nmos input why do we always use pmos input in the second stage or vice-versa" the second input mirror the current of the first stage's output in DC point.
"Can't we use nmos or pmos input in both the stages." I never do it. maybe it work, but DC point setup is not easy. The input and output range may be limited. you can try and simulate it.