M.Sc Thesis

M.Sc StudentScherzer Tamir
SubjectAnalysis of an Enhanced Doherty Configuration for Use in
Modern Highly Efficient Power Amplifiers
DepartmentDepartment of Electrical and Computer Engineering
Supervisor PROFESSOR EMERITUS Israel Bar-David


One of the main emphases in modern RF power amplifier (PA) design research is on improving efficiency, while at the same time, maintaining good linearity. The Doherty PA system, invented by William H. Doherty, attempts to improve the overall efficiency by cleverly combining two similar class-B amplifiers, main and peaking, while maintaining the linearity of a system where the same amplifiers are combined in a dual-hybrid configuration.

            In attempting to realize the Doherty system using modern RF transistors, designers fail to achieve the promised potential. This is caused by the fact that RF transistors do not behave as ideal current sources, as assumed by the classical Doherty system analysis. We show that the maximum output power matching impedance to maximum small-signal gain matching impedance ratio, , is a good indicator for the expected improvement of a Doherty system over a dual-hybrid system, and that as it decreases, the overall efficiency and linearity improve, eventually reaching ideal values for .

            The ratio, , is determined by the transistors used in the system. This ratio can be calculated using data presented in device datasheets, and this is demonstrated on Motorola’s MRF873 power BJT and MRF1550 power LDMOS transistors. Since in the modified, -dependent analysis efficiency and linearity are degraded, methods for improvement are presented and analyzed, showing promising results for future Doherty system realization.