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As mobile communication systems evolve to handle higher data rates, their modulation schemes only become more complicated, generating signals with large bandwidth and high peak-to-average-power ratio (PAPR). To amplify such signals with high efficiency, the power amplifier (PA) should have high efficiency not only at the peak power level but also at low power, especially over the maximum power generation region. To realize these PA characteristics, the drain bias voltage of the transistor can be modulated on the basis of the input envelope power to minimize the dc supply power. Thus, the drain bias voltage should follow the envelope of the modulated signal, and this is called envelope tracking (ET). Usually, the envelope is shaped to realize the optimum performance from the ET PA. The PA is biased close to class B, and the dc current is automatically adjusted to the power level. The resulting PA has high efficiency for all power levels, comparable to the maximum efficiency of the PA at high power. In practice, the efficiency is degraded somewhat at low voltage because of the knee effect, lower transconductance, and mismatch effect for different drain bias voltages.