Improved amplifier efficiency under back-off conditions can also be achieved by changing the load impedance in a proper way. To reach that with the very high power and bandwidth necessary for modern RF power amplifiers, tunable elements or varactors with high power capability and good tunability are needed.
In the RF Power Lab a novel type of varactors for RF power applications is investigated. The varactors are fabricated using a thick film of barium-strontium-titanate (BST). By applying high voltage over the ferrite thick film, the polarization of the material changes. This polarization, in turn, causes a shift in dielectric coefficient. Using BST as a dielectric it is possible to fabricate different types of varactors, planar interdigital or vertical metal-insulator-metal (MIM) varactors. They show a large breakdown voltage and can handle high powers with very good linearity. The ceramic layers are fabricated using screen-print or inkjet printing at KIT in Karlsruhe, but the varactors themselves are fabricated at the Technische Universität Darmstadt (TUD).
Since BST is a quite novel material for RF power applications much work remains in investigating and improving tunability and thermal stability of the varactors. One of the current projects is conducted as an ESA ITI project aiming to develop and demonstrate a 20 W discrete tunable pre-matching (TpM) GaN-HEMT transistor where a BST varactor is integrated in the package of the GaN-HEMT. This is a concept that was developed in cooperation between TUD and FBH in 2010. Such devices enable tunable, frequency-agile power amplifiers for future telecommunication systems.
Another project, financed by the German Research Foundation (DFG), aims at improving BST RF power varactors as discrete components to be used in applications above 100 W. As improved discrete components they can find use in, e.g., frequency-agile telecommunication systems and plasma oscillators. The aim of the project is to improve the power handling by optimizing the material and design of the varactors to make them a viable alternative for very high power applications.