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RF and Microwave Transmitter Design by Andrei Grebennikov

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4

Power Transformers, Combiners, and Couplers

It is critical, particularly at higher frequencies, that the special types of combiners and dividers are used to avoid insufficient power performance of the individual active devices. The methods of configuration of the combiners or dividers differ depending on the operating frequency, frequency bandwidth, output power, and size requirements. Coaxial cable combiners with ferrite cores are used to combine the output powers of power amplifiers intended for wideband applications. The device output impedance is usually sufficiently small for high power levels; so, to match this impedance with a standard 50-Ω load, coaxial-line transformers with specified impedance transformation are used. For narrow-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, therefore, the hybrid microstrip combiners (including different types of the microwaves hybrids and directional couplers) are commonly used to combine output powers of power amplifiers or oscillators. In this chapter, the basic properties of three-port and four-port networks are presented, as well as a variety of different combiners, transformers, and directional couplers for application in RF and microwave transmitters.

4.1 BASIC PROPERTIES

Basic three-port or four-port networks can be used to divide the output power of a single power source or combine the output powers ...

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