These research works aim to develop analytical tools for the analysis and design of differential systems. While the use of these circuits in RF reception/transmission chains is increasingly growing, there is no accurate method to study their stability. Therefore, the first part of this work is to develop a CAD tool which can be rigorously use to investigate the extrinsic stability of linear differential systems. The second part refers to the design of a three-ports differential component for space telecommunication application. Indeed, for system requirements, it is often necessary to convert a differential access into a single-ended access. This operation allows the differential device to communicate with other components in the system. For this purpose, a broadband differential active balun is designed using BiCMOS technology. Differential measurements are performed and the results are compared to the initial performances of the spatial application technical specifications