Nullor

A nullor is a theoretical two-port network consisting of a nullator at its input and a norator at its output.

[1] Nullors represent an ideal amplifier, having infinite current, voltage, transconductance and transimpedance gain.

[4] The sink is intended to draw the same current iOUT regardless of the applied voltage VCC at the output.

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

The purpose of the transistor in the circuit is to reduce the portion of the current in RR supplied by the op-amp.

Nullor electronic symbol (balanced version)
Nullor electronic symbol (unbalanced version)
Figure 1: Current sink based on an operational amplifier. Because the op-amp is modeled as a nullor, its input variables are zero regardless of the values of its output variables.