Question about widths of PMOS transistors

[p.sivakuamr]

Hi
this siva,Now i am designing comparator as a receiver.Generally we are giving two inputs ,one is vo+ and other is vo- for that we are setting both Pmos transistors(input level) Width of cmparator is same for zero hysteresis.In my designing one input is pulse wave form(1to3.3v)and other input is fixed to maintain the Hysteresis (165mv).You can see the My comparator circuit in jacob Baker(pag no 689).

Is need to set the pmos transistors widths are same ?

if it is yes can u tell me the reason ?

In my design we are using different level of inputs,so can i set different widths for pmos transistors to get the rail to rail o/p with 165mv hysteresis ?

Actually i designed with same widths of pmos transistors for that i got one o/p was rail to rail swing but other one (which is less compared to other input) i got lower swing(not rail to rail) with less hysteresis. thatswhy i am asking above questions

regards
siva

 

[suhas_shiv]

Re: doubts on comparator

Usually for matching purpose, the widths of the 2 input transistors are kept the same in a diff amp.

 

[snoop835]

Re: doubts on comparator

Same width for the input transistor is for layout matching purpose. to reduce offset you can set the width bigger but there are limitation. You can put some sort of a buffer to get back rail-to-rail output swing. For example use inverter.

 

[Syukri]

Re: doubts on comparator

Same width is for matching...

YEs u can use different width but it's recomemded that u are using method to remain the matching of those transistor like a dummy trasistor