A Low Distortion 25 W Class-F Power Amplifier Using Internally Harmonic Tuned FET Architecture for 3.5 GHz OFDM Applications

An ultra low distortion class-F power amplifier for base stations of broadband access systems is presented. This amplifier adopts internally harmonic tuned FET architecture (IHT-FET) to improve the linearity under class AB operating conditions. The feature of this architecture is an on-chip input 2nd harmonic tuning circuit placed in front of each FET unit cell to achieve accurate control of input 2nd harmonic impedance. With the proposed IHT-FET architecture, a single-chip multi-cell FET for verification exhibits a low distortion of a -51 dBc ACPR and a 19% PAE with a 11.8-dB associated gain at a 10-dB back-off output power level under a 3.5-GHz 3GPP W-CDMA signal test. This ACPR corresponds to a 10-dB reduction in ACPR of a conventional FET. In addition, a 25 W power amplifier with two IHT-FET chips successfully achieves a 1.5% EVM (error vector magnitude) at an output power of 34.6 dBm under a 3.5-GHz WiMAX (IEEE 802.16a) compliant OFDM signal test, where the output power is a 10-dB back-off level