what is the main difference in the design of an op-amp with an single ended output and a differential output..... is it possible to design opamp that can have its output swing to both supply rails...
For differential output opamps you need cmfb circuit to set the output common mode. For single ended opamps its set by the current mirror load.
About designing opamps to get rail to rail output .... its simply not possible. The best output dynamic range you can get from an output stage is by using an inverter. But even this will require two Vdsat to operate in saturation.ie max op swing = VDD - 2Vdsat.