about layout match of asymmetric transistors

[needforspeed]

Hi,
I have an operational amp implemented by using asymmetric transistors. I guess it's not a good idea to share drain and source in a pair of transistor concerning match. Do you guys have any idea about layout matching of asymmetric transistors?

 

[JoannesPaulus]

choose a unit size for the width and and length and make the two transistors made of a different number of fingers, i.e. if M1 is 3u/0.4u and M2 is 7u/0.6u choose the unit transistor as 1u/0.2u. M1 will be made of 3 branches of 2 parallel transistors and M2 will be made of 7 branches of 3 parallel transistors. Than you can do a common-centroid layout.