A nullor is a theoretical two-port network composed of a nullator at its input and a norator at its output. Nullors represent an ideal amplifier, having infinite current, voltage, transconductance and transimpedance gain. Its transmission parameters are all zero, that is, its input-output behavior is summarized with the matrix:

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In negative feedback circuits, the circuit surrounding the nullor determines the nullor output in such a way as to force the nullor input to zero.

Inserting a nullor in a circuit schematic imposes mathematical constraints on how that circuit must behave, forcing the circuit itself to adopt whatever arrangements are needed to meet the conditions. For example, an ideal op amp can be modeled using a nullor, and the textbook analysis of a feedback circuit using an ideal op amp uses the mathematical conditions imposed by the nullor to analyze the circuit surrounding the op amp.

Read more about Nullor:  Example: Voltage-controlled Current Sink

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