amarnath,
I am still lost here. Okay, let me put it in this way:
Say one side outputs the minimum voltage, if the other side outputs the maximum voltage?
In this case, if one side outputs Vin,CM-Vthn, if the other side must output Vdd-(|Vgs3|-|Vthp|)?
bamboo,
I think Vout(min)≈Vin,CM-Vthn is because input signal is rather small compared to Vin,CM. Say if Vin,CM=1.5V. Vin1-Vin2 maybe only 50mV. So Vout(min) roughly equal to Vin,CM-Vthn.
I don't know if my understand for this is correct. If I am not. please let me know what it is really going on here.
I am really disappointed by my poor foundation. Maybe I mixed up some basic concepts.