Active microwave filters

Design principles and state-of-the-art of microwave filters employing FET, avalanche transit time diodes, Gunn Diodes, varactors, or other microwave semiconductors as active elements with negative dynamic impedance were reviewed and various design approaches were examined. Three main directions in the active filter development, which include the technology, the design and modeling were outlined. It was pointed out that the technological aspects of the active filter development are now focused on finding ways of using active structures or active media for amplification or signal generation. A heterostructure of GaAs substrate with epitaxial active semiconductor n-layer and ohmic contact of AgGaIn alloy on one side of the substrate and dielectric-SiO-metal-Cr-Cu-Ni on the other side can serve as an example of such a structure. It can be used for Q-switched resonators in the two cm range. Prospects of developing monolith microwave filters with GaAs substrates are very attractive.