This is a 4 NMOS version of:
http://everycircuit.com/circuit/5760014432075776
Each NMOS (N-channel MOSFET) sink ~ 1/4 of the load. At low current each NMOS requires a ballast resistor. For details read:
http://www.eevblog.com/forum/reviews/maynuo-m9711-programmable-dc-electronic-load-photos/msg220156/#msg220156
