 Of course, only real op amps are current-limited, not nullors.The remaining variation of the current with the voltage "V"CC is due to the Early effect, which causes the β of the transistor to change with its collector-to-base voltage "V"CB according to the relation β = β0(1 + "V"CB/"V"A), where "V"A is the so-called Early voltage.Analysis based upon a nullor leads to the output resistance of this current sink as "R"out = "r"O(β + 1) + "R"C, where "r"O is the small-signal transistor output resistance given by "r"O = ("V"A + "V"CB)/"i"out.