It's a little complicate to explain it.The circuit is assumed to be a part of my bandgap circuit,I want it to generate a current which can be expressed as I=a*T square+b*T+c (T means Temperature) to do a second order curvature compensation.The principle is if the Vx is fixed,and the voltage of Vbias equals -kT+b,then according to the equation Id=1/2*u*cox*w/l*(vsg-vth)square,the I is generated.Now the only problem is how to fix the Vx to a constant value while the vdd is changing.Finally if this current is injected to the output branch,the compensation may be achieved.