Linearization of Three-Stage Doherty Amplifier

In this paper, the operational behavior of three-way Doherty amplifier loaded with harmonic control circuits is analyzed, including the efficiency and linearity. Amplifier is designed in configuration with two quarter-wave impedance transformers in the output combining circuit with LDMOSFETs in carrier and peaking amplifiers with the same periphery and in periphery relations 1:2.5:2.5. The signals for linearization (the fundamental signals’ second harmonics-IM2 and fourth-order nonlinear signals-IM4 at frequencies that are close to the second harmonics) are extracted at the output of peaking cells biased at various points. The Doherty amplifier is designed with the frequency diplexer at the outputs of the Doherty cells that separates the fundamental signals and signals for linearization. The diplexer includes harmonic control circuit (HCC) which, in combination with the matching circuits at signals for linearization, provides an optimal impedance for their adequate power level and either an open or short circuit for the third harmonics. The linearization technique results in the suppression of the thirdand fifth order intermodulation products of Doherty amplifier. Key-words: Doherty amplifier; harmonic control circuit (HCC); fourth-order nonlinear signals; linearization; power-added-efficiency; second harmonics