This paper presents a systematic design methodology for amplifier-based active sensors, with stability as a central consideration. The proposed sensor consists of a passive resonant structure and an active element connected in series to form a single-port amplifier operating in reflection. A lumped-element equivalent circuit was first employed to determine an initial value for the impedance of the active element, which was used as starting point for the design using realistic models of a Complementary Split-Ring Resonator (CSRR), for the passive resonant structure, and a Clapp oscillator, for the active element. The Clapp oscillator was optimized until a threefold improvement in the quality factor ( Q -factor) compared to the passive case without compromising the stability of the system. The sensing performance of the proposed active sensor was validated both in simulation and measurements on lossy samples, showing improved performance without compromising its sensitivity. System stability was confirmed by examining the Normalized Determinant Function (NDF), in simulation, and the spectral response, in both simulation and measurement. Despite promising results were obtained, further optimization of the active element is required to increase the stability margins and the Q -factor. Moreover, the integration of both the passive resonant structure and the active element subsystems into a single compact circuit, as well as the analysis of nonlinear effects, should be addressed in future works.
