This paper presents an innovative approach to extend the S-parameter definition to multiport networks having conventional single-ended and differential ports, as is the case for operational amplifiers, transformers and baluns. To give maximum generality to this technique, for example, allowing for complex S-parameter reference impedances, the mathematical derivation will be carried out with the most general definition of the S-parameters. The presented approach gives the same results already published for circuits with differential ports only when the required simplifications are applied