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A unified investigation is presented for low-sensitivity and limit-cycle-free filter structures. It is shown that important properties like boundedness and pseudopassivity are closely related to the symmetry of the system matrix description. Negative elements can be incorporated in certain prototypes leading to real advantages in switched-capacitor (SC) realisations. Stability and noise problems are also discussed. According to the realisation, component variations can result in matrix symmetry being maintained, or more generally, in the introduction of asymmetry. Sensitivity considerations are outlined for both situations. Implementation by passive RLC, SC and digital circuits are considered.