Coupling Matrix-Based Design of Waveguide Filter Amplifiers

This paper extends the conventional coupling matrix theory for passive filter to the design of "filte amplifi rs," which have both f ltering and amplifi ation functionality. For this approach, extra elements are added to the standard coupling matrix to represent the transistor. Based on the specifi ation of the filt r and small-signal parameters of the transistor, the active N + 3 coupling matrix for the "f lteramplif er" can be synthesized. Adopting the active coupling matrix, the last resonator of the filt r (adjacent to the transistor) and the coupling between them are modif ed mathematically to provide a Chebyshev response with amplifi ation. Although the transistor has a complex impedance, it can be matched to the f lter input by the choice of the coupling structure and the resonator frequency. This is particularly useful as the f lter resonators can be of a different construction (e.g., waveguide) to the amplifie (e.g., microstrip). Here, an X-band f lter-amplifi r is implemented as an example, but the technique is general.