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International audience; Most of the negative group delay (NGD) circuit recent investigation were focused on the classes of lowpass and bandpass categories. Few research works are currently available in the literature on the typical high-pass (HP) NGD function. This paper introduces an original HP-NGD circuit theory of four-port passive topology constituted by a single lumped capacitor. The S-matrix equivalent model of the innovative topology is established from the admittance matrix. Then, the basic frequency analytical responses are expressed. Then, the GD model is derived in function of the capacitor. It was found analytically that the four-port mono-capacitor passive circuit is susceptible to behave originally as an unfamiliar HP-NGD function. An innovative HP-NGD analysis is explored in addition to the expression of synthesis design equation in function of the expected NGD cutoff frequency which is linked to NGD optimal value. The validity of the established HP-NGD circuit theory is verified with a proof of concept (POC). As expected, the HP-NGD behavior showing a good correlation between analytical model and simulation is obtained.

International audience; This paper investigates an innovative negative group delay (NGD) theory of ''li'' geometrical shape topology. The li-topology is an outstandingly simple and fully distributed circuit comprised of a coupled line (CL). The li S-parameter model taking into account the CL coupling coefficient, delay and attenuation is established. The NGD analysis showing the possibility to generate NGD condition with respect to the li topology parameters is developed. The NGD characteristics as NGD value, center frequency, bandwidth, transmission and reflection coefficient are expressed. The li-NGD theory is validated with two proofs-of-concept implemented in microstrip technology. Calculated models, simulations and measurements are in good correlation. As expected, bandpass NGD presenting center frequency at approximately 2.56 GHz and 0.92 GHz with NGD level of approximately −0.9 ns and −3.7 ns were realized with the small and large li prototypes. Outstanding time-domain analyses explaining the bandpass NGD meaning, with innovatively low attenuation output, were also presented. The time-domain results highlight li-output pulse signal envelopes in time advance without violating the causality. INDEX TERMS Distributed circuit, li-topology, microwave theory, negative group delay (NGD), S-matrix modelling.