Circuit Modeling of Coaxial-Cable Baluns in Microstrip-Mounted, High-Power, Very-High-Frequency Amplifiers [Application Notes]

Baluns have been studied for several decades. They have been used in antennas as unbalanced-to-balanced signal convertors and have sometimes taken on the role of impedance transformers. Apart from their use in antennas, baluns can be found across a variety of applications, ranging from mixers and samplers to power dividers and amplifiers. Good reviews of the most common balun types can be found in [1]-[3]. Because technology limits the maximum voltage that can be applied to a transistor, the simplest way for a component vendor to increase power capabilities is to increase the current. This can be done by connecting many transistor units in parallel, which in turn causes a decrease of the transistor input as its power increases. Currently, laterally diffused transistor modules for very-high-frequency bands, with 1-kW output power, have a roughly 1-Ω impedance [4]-[6] . A further increase in power capabilities will cause an additional drop in input impedance, which eventually must be matched. Transforming standard 50-Ω impedances down to 1-Ω becomes quite difficult and forces designers to use modules with groups of two transistors and drive them deferentially, effectively placing them in series. Baluns play a critical role in this process by preserving symmetry between the transistors, maximizing obtainable power, and relaxing the criteria for the transformation network by halving its transformation ratio, yielding novel types of baluns [7]-[9].