OP AMP why input pmos transistor

[olzanin]

pmos transistor

Hello,

why most of the opamps I have seen have pmos differential input stage and not nmos. Is it regarding
noise or other effects too?

Thanks.

olzanin

 

[mikersia]

op amp,transistor

due to noise reasons and ground referenced signal design style mostly

 

[tlihu]

pmos opamp

One reason is that PMOSFETs have lower flicker noise.

 

[rfsystem]

pmos inpute opamp

Next reason is that the body voltage of the MOS differential pair, which is the NWELL for PMOS, could follow the input common mode voltage. Therefore the CMRR is much higher because the body effect and the mismatch of the body effect of the two differential pair MOS is avoided.

If the process have an isolated PWELL you can also use NMOS.