Here I am taking an example of common source configuration(refer figure). Generally head current source is used as load with nMOS and tail cuurent source with pMOS. Why can't be this other way around? I wanted to know the reason behind this.
In single device amplifier topolpgy the term "common XX" means terminal XX is at AC ground. Various biasing techniques may end up with terminal XX with a DC voltage, but they will be AC ground or "common". As a current source has infinite impedance it makes no sense to place it between the common terminal and ground (in the AC analysis circuit) In the above circuits VDD would be assumed to be a voltage source referenced to ground (the triangle symbol) and hence AC ground.
Another way to think of it is that for common source configuration, the load ( a current source in this case) is in the drain circuit.