20.7 An RF-PA supply modulator achieving 83% efficiency and −136dBm/Hz noise for LTE-40MHz and GSM 35dBm applications

Recently, supply modulation (SM), instead of a constant supply voltage, has raised interest in enhancing the overall SM-PA efficiency. In the average-power-tracking (APT) method, a buck converter simply generates stair-case voltages for a PA according to its required output-power level. In a more advanced envelope-tracking (ET) method, typically a hybrid structure of a low-loss but slow buck converter and a high-speed linear amplifier is used to track the fast-changing signal envelope with its efficiency performance becoming important for high peak-to-average power-ratio (PAPR) signals, like LTE [1–4]. The challenge is how to design an SM-IC that can achieve both a large-envelope-tracking bandwidth and good system-power efficiency, while achieving low output noise to minimize receive-band sensitivity degradation.