Inverted Doherty Output Network for Broadband RF Power Amplification

The patent inverts the conventional Doherty amplifier topology by placing the impedance inverter between the peaking amplifier and the combining node, rather than between the main amplifier and the combining node. This inversion allows the peaking amplifier to provide a virtual ground at the combining node during low-power operation, enabling broader RF bandwidth while maintaining the power amplification function. The impedance inverter configuration with specific phase delay (e.g., 270 degrees) and impedance transformation ratios resolves the bandwidth limitation without requiring fundamental redesign of the entire circuit topology.

The impedance inverter in the patent serves multiple functions simultaneously: it provides impedance transformation, introduces the required phase delay, and creates the virtual ground condition at the combining node during low-power operation. By consolidating these functions into a single component configuration rather than using separate impedance-matching networks, the patent achieves broad signal bandwidth (e.g., 700 MHz or more) while minimizing the increase in device complexity. The impedance inverter's characteristic impedance and electrical length are specifically designed to achieve multiple objectives with one component.

The patent systematically varies the asymmetry factor (α) as a design parameter to optimize power transfer efficiency. By adjusting α (the ratio of peaking amplifier power to main amplifier power) and corresponding impedance transformation ratios, the patent achieves maximum power transfer while maintaining broadband performance. The impedance inverter's characteristic impedance is specifically designed as Z0 = sqrt(Zoptm * RL) / α, where Zoptm is the optimal impedance for the main amplifier and RL is the load resistance. This parameter optimization approach allows efficient power transfer across a wide bandwidth without requiring complex multi-stage impedance matching networks.