2 stage OTA rather basic question

[diarmuid]

Basic question which struck me:

I have a 2-stage OTA.

Stage 1 is a conventional 5 transistor OTA with NMOS input differential pair.

Stage 2 is a PMOS common source amp.

Question: In such an 2-stage OTA, why with an NMOS diff pair do we use a PMOS common source amp?
Likewise, why with a PMOS diff pair do we use a NMOS common source amp?

Books always have this sort of complimentary architecture. What would happen if say I used an NMOS
diff pair with NMOS common source amp? Or PMOS diff pair wiht PMOS common source amp?

Nothing strikes me as obvious except for ease of biasing maybe.

Thanks,

Diarmuid

 

[LvW]

I think, the main reason is the following:

In case of succeeding stages of the same type the dc operating point must be increased from stage to stage (because no ac coupling capactors are allowed)
In contrary, if you combine an NMOS with a following PMOS (or vice versa) there is a necessity for a slight increase only (if any). Thus, you can design for lower supply voltages.

 

[nitishn5]

The Output of an NMOS input 5-pack amp is supply referred. So for good PSRR, it would be best to give it to a PMOS gate as any supply coupling will come differentially to the CS amp and will cancel out to the first order.

But ease of biasing is the main motivation in my opinion.