An AC-coupled hybrid envelope modulator for HSUPA transmitters with 80% modulator efficiency

Abstract

High-data-rate wireless technologies such as HSUPA and LTE are power-hungry because of the fundamental correlation between data rate and transmit power. Furthermore, the high peak-to-average power ratio (PAPR) of the modulated signals causes a degradation in PA efficiency, since the supply voltage of the PA must be high enough to provide the peak output voltage without loss of linearity. Envelope tracking modulators have been proposed to improve the efficiency and linearity of transmitters. Modulators using multiple input voltages have been shown to improve efficiency, but generating multiple supplies imposes substantial overhead. Several recent works have demonstrated a hybrid modulator, combining the output current of a buck output stage with that of a linear amplifier. The hybrid modulator has the notable advantage that the low-frequency power (which constitutes the majority of power, even in high-PAPR scenarios) can be provided through the efficient buck stage. The inherently less efficient linear amplifier stage needs only to supply the high-frequency power. The efficiency of hybrid modulators degrades at moderate power levels, when the ac amplitude is much less than the supply voltage of the linear amplifier. At low power levels, the power savings afforded by the modulator do not offset its own quiescent current, resulting in a lower efficiency than can be achieved using a fixed-drain, average-power-tracking (FD-APT) supply.