NOVEL COMBINER CIRCUITS FOR A DOHERTY RF POWER AMPLIFIER
Original Publication Date: 2002-Jul-22
Included in the Prior Art Database: 2002-Jul-22
Using a Doherty circuit is one good way to improve RF Power amplifier (PA) efficiency, but it has found only limited application in practice because there are two critical transmission line lengths which must be set correctly in order to minimize distortion products such as IMD and spectral regrowth. While there are methods which can be used to approximately predict the required transmission line lengths, these methods are usually not accurate enough to give first pass success, so the design must be iterated. Iteration of this type of design is complicated by the fact that changing the length these transmission lines forces all the components before or after them to move on the layout. This paper describes several ways to eliminate these critical transmission lines by using various types of quadrature hybrid combiners to replace the combiner structures in prior art Doherty PAs. This, in turn, allows the Doherty PA to be "tuned" for minimum distortion by varying the length of a transmission line stub or changing the value of a reactive component, which can be done without significantly altering the existing circuit board layout.