Investigation of stability of negative impedances for use in active metamaterials and antennas

During the last years a lot of attempts to use negative non-Foster impedances to improve the bandwidth of electrically small antennas (ESAs) or the bandwidth of metamaterial structures have been realized. Unfortunately, negative non-Foster impedances are not absolutely stable circuits which could be a serious problem in practical applications. In this paper, we study the stability of negative impedances in several circuits that model realistic situations.