A GaN Switchless-Class-G-Doherty-Continuum Power Amplifier With 140% Fractional Bandwidth

Abstract

This article presents the design and analysis of an ultra-wideband back-off efficient power amplifier (PA) based on switchless-Class-G-Doherty-continuum theory. Switchless Class- G (SLCG) PA has a great potential in the RF bandwidth (BW), but shows limited signal BW and di fficulty in parasitics absorp- tion. The situation for Doherty PA (DPA) is exactly the opposite. To combine the merits of SLCG PA and DPA, we propose an SLCG-Doherty-continuum PA (SDCPA), which is constructed by simply inserting a frequency-adaptive transmission line (FATL) into SLCG PA. When the phase shift of FATL deviates from 0 ◦ to ±90◦, SDCPA transfers from SLCG mode to Doherty mode gradually. The theoretical performance of SDCPA is analyzed in depth. Furthermore, a coupled resonator (CR) with inherent parasitics absorption and supply path is proposed to realize broadband FATL on chip. An SDCPA prototype is implemented in a 0.25-µm GaN-HEMT process for validation. The fabricated SDCPA achieves a saturated output power of 36.6–38.6 dBm and a 6-/7.5-dB back-off drain efficiency (DE) of 38%–56%/37%–54% from 1.35 to 7.6 GHz with a compact chip size of 2 .1×1.6 mm2. The fractional BW (FBW) is up to 140%. Applying a 100-MHz LTE signal with a 7.5-dB peak-to-average power ratio (PAPR), an average DE of 35%–49.2% is measured over the entire BW, and adjacent channel power ratio (ACPR) before and after digital predistortion (DPD) are better than −25.3 and −46 dBc, respectively. Applying a 100-MHz 64-QAM signal with a 7.9