differential pair

A differential pair uses input NMOS devices with W/L=50/0.5 and a tail current of 1mA. What is the equilibrium overdrive voltage of each transistor?
这里面的tail current和overdrive voltage是什么意思?

tail current就是尾电流，input pair 的电流之和。
overdrive voltage是过驱动电压，vgs-vth