1. A system for analyzing 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; supply a second stimulus, having a second value, to an output of the model; sweep the first value through a first set of swept values; sweep the second value through a second set of swept values; and obtain an output current of the model as a function of the first and second swept values, wherein the model includes multiple controlled sources, each of which having a current value derived from the first value.