Structure of Differential pair with MOS

[hausofgaga]

i'm doing the assignment about designing a comparator and i have found the design. anyone can explain what is different when i change XM1 and XM2 from P-Mos to N-Mos;the advantage of choosing Pmos instead of Nmos and what is the function of XM3 and XM4. Thank in advance.

 

[AMS012]

Changing XM1 and XM2 will alter the bias conditions and also reduce the gain from the differential input to the single ended output of the basic diffamp which is not desirable for any comparator. XM3 and XM4 act as the active current source load for the basic diffamp. And also, the current mirroring configuration of XM3 and XM4 will increase the single ended gain to the value which would have been obtained in the case of differential or double ended case.