I am trying to design the fully differential opamp. I put all the values according to mathematical calculation and put those values in simulator. After some simulation, some transistors are going to linear again against the calculations. I found a very high value of common mode voltage at output node. I tried all the means to get all MOSFETs into saturation. But i was not able. Can somebody suggest how to make these into saturation and setting output commonmode voltage to VDD/2

I am attaching the schematics
Click image for larger version. 

Name:	1.png 
Views:	11 
Size:	18.7 KB 
ID:	136786

Click image for larger version. 

Name:	2.png 
Views:	9 
Size:	20.9 KB 
ID:	136785