differential comparator design
Are you sure you wanna use simulink?
if you just use comparator as a block in your design, you can choose a sign block in Simulink Library/Math Functions/Sign
it seems you need to design a comparator using circuits elements. Simscape provides capacitors, op-amp, etc. which you can utilize in your system. Besides, in SimElectronics there're also MOSFETs and SPICE-compatible blocks.
SimElectronics doesn't exist in Matlab earlier than 7.7 (R2008)
Again, I guess few people use Simulink to deal with pure circuits design.