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A hybrid supply modulator consisting of a parallel operation of a high-drive, low output impedance, wideband class-AB linear amplifier and a high-efficiency, wideband, low-ripple switching amplifier is presented for the application of polar transmitters. At system level, a two-phase switching is employed to lower the inductor current ripple so that both the output ripple and power loss are reduced. On-chip feed-forward bandpass filter is used to extend the tracking bandwidth of the switching amplifier, without hurting the stability of parallel control loop or the need to increase switching frequency. At circuit level, the output impedance of the linear amplifier is lowered by optimizing the design of super source-follower output stage. Inductor current sharing and two-phase ramp generator are implemented for realizing the two-phase switching scheme. Fabricated in a 0.35-μm CMOS process, the prototype chip measures 9 % static efficiency improvement over the conventional single-phase switching design in the back-off power level. Dynamic efficiency is enhanced by 8-12% by enabling the bandpass filter. Successful tracking of a 4 MHz 0.4-2.8 V full-wave rectified sine wave and a WCDMA envelope signal is demonstrated.
Date of Publication: Dec. 2010