Electric circuits are present in several applications.
The development of integrated circuit requires numerical simulation. Modern modelling techniques like the modified nodal analysis lead to differential-algebraic equations. The analytical and numerical solutions of these systems depend strongly on the structure and the index. The paper deals with lumped circuits containing voltage sources, current sources as well as general nonlinear but time-invariant capacitances, inductances, and resistances. Furthermore, the index is shown to be limited to 2 for the considered model class.