In the theory of electrical networks, a nullator is a pathological two-pole (Fritsche / Seidel) through which no current flows and where no voltage drops. The operating point in the U / I diagram is therefore (0/0) (short circuit and open circuit at the same time). The nullator provides two restrictive network equations, and , with which the network is over-determined. In order to compensate for the additional network equation, there must also be a norator for each nullator , which represents the dual two-pole to the nullator. The pair of Nullator and Norator is called Nullor . A nullor is mostly used to model an ideal operational amplifier in the linear range or an ideal bipolar transistor in forward operation. The nullator is linear, non-polar and lossless. It is a special case of the fixator .
This should be explained here practically using a negative feedback operational amplifier: The input current of the inputs is minimal (theoretically 0, in reality at a few nA). The first condition (I = 0) is thus fulfilled. Due to the negative feedback from the output of the OpAmp to its inverting input, the second condition (U = 0) is also fulfilled. The negative feedback practically forces the norator at the output to establish the state U + = U - .