Harmonic-controlled three-way Doherty amplifier with improved linearity

In this paper the efficiency and linearity behavior of three-way Doherty amplifier is analyzed. The amplifier is designed in the configuration with two quarter-wave impedance transformers in the output combining circuit with LDMOSFETs in carrier and peaking amplifiers. 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 matching circuits, provides an optimal impedance for an adequate power level of signals for linearization, and either open circuit for the third harmonics (HCC class-3F) or short circuit for the third harmonics (HCC class-3IF). The linearization technique results in the suppression of the third- and fifth-order intermodulation products of Doherty amplifier.

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