How can I find the origin of equation about this filter circuit?
I cannot explain , but these transfer functions are derived from standard VI nodal matrix calculations.

I hope this may help you.  http://nopr.niscair.res.in/bitstream/123456789/11677/1/IJEMS%2018(2)%2097-101.pdf

Below  in 1996 the DDCC "Differential Difference Current mode Conveyor" is then combined to next filter. It has 4 inputs x,y1,y2,y3 and  differential outputs z1,z2 .  This symbol can be simplified to show differential +- inputs y2,y3 and x,y1.  These devices depend on perfectly matched components.



e.g.