 That is, based upon the idealization of a nullor, the output current is controlled by the user-applied input voltage "v"IN and the designer's choice for the reference resistor "R"R.

The purpose of the transistor in the circuit is to reduce the portion of the current in "R"R supplied by the op-amp.Without the transistor, the current through "R"C would be "i"OUT = ("V"CC − "v"IN)/"R"C, which interferes with the design goal of independence of "i"OUT from "V"CC.Another practical advantage of the transistor is that the op amp must deliver only the small transistor base current, which is unlikely to tax the op amp's current delivery capability.