1. A system for characterizing a circuit, the system comprising: a computer configured to: supply a first stimulus, having a first value, to an input of a model of a circuit, wherein the model includes the input and an output; supply a second stimulus, having a second value, to the output of the model of the circuit; sweep the first value through a first set of swept values; sweep the second value through a second set of swept values; and characterize an output current of the model of the circuit as a function of the first and second swept values.