Linearity Improved Doherty Power Amplifier Using Coupled-Lines and a Capacitive Load

A Doherty power amplifier, using a pair of coupled lines loaded by a capacitor, is proposed for linearity improvement. The conventional λ/4 transmission line is substituted by capacitor-loaded coupled lines to perform impedance inversion, and to compensate phase delay. At the same time, the proposed structure is also utilized to suppress the second and higher order harmonic of the output of the main amplifier. Furthermore, the phase distortion of the peak amplifier at a high power level can be corrected by tuning capacitor-loaded coupled lines. Compared to a conventional Doherty PA, the third-order intermodulation distortion of the proposed Doherty PA is reduced by 23 dB.