Power Combiners, Impedance Transformers and Directional Couplers
Many RF applica-tions require power combiners or dividers, impedance transformers and direc-tional couplers. In the case of combiners, it is critical, particularly at higher frequencies, that the correct types are used to achieve the desired power performance when combing individual active devices to achieve higher power.The methods for configuration of the com-biners or dividers differ, depending on the operating frequency, frequency bandwidth, output power, and size requirements. Coaxial cable combiners with ferrite cores are often used to combine the output powers of power amplifiers intended for wideband applica-tions. The device output impedance is usually low at high power levels; so, to match this impedance with a standard 50-ohm load, coax-ial-line transformers with specified impedance transformation are used. For nar-row-band applications, the N-way Wilkinson combiners are widely used due to their simple practical realization. For microwaves, the size of combiners should be very small and, there-fore, the hybrid microstrip combiners (includ-ing different types of the microwaves hybrids and directional couplers) are commonly used to combine output powers of power amplifiers or oscillators. In this paper, a variety of differ-ent combiners, impedance transformers and directional couplers for application in RF and microwave transmitters is given with descrip-tions of their schematics and operational prin-ciples.
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