Design of a Wideband High Efficiency Dynamic Supply for RF Power Amplifiers in Polar Transmitters

Abstract

This thesis presents a new amplitude modulator for polar transmitters. A common method for polar amplitude modulator is to use the low drop out regulator (LDO) which inherently suffers from low efficiency. The most efficient energy conversion from a battery is achieved by means of a switching power converter, though this approach results in a limited bandwidth due to the limitation imposed on the switching frequency. To achieve both high efficiency and wide bandwidth, Master-Slave Switching Amplitude Modulator (MS-SAM) and Parallel Switching Amplitude Modulator (P-SAM) are presented. MS-SAM consists of a combination of two step-down modules with different switching frequencies and different switch sizes. Master module was designed for wide bandwidth and slave module for high efficiency. A new current-sensing circuit based on a sample and hold circuit operable under a high switching frequency (> 40 MHz) is also presented. The chip was fabricated in a standard 0.35 \mum CMOS process with an area of $6.45 mm^{2}$. The MS-SAM could drive the amplifiers of up to 4 W RF power in the experiment. We obtained an efficiency of 89% and the converter’s bandwidth was wide enough to meet the EDGE spectral requirements. P-SAM consists of a parallel combination of a step down converter and a pseudo step down converter. This modulator achieves the maximum efficiency of 87.5% and 5 MHz bandwidth with -71.5 dB ripple voltage satisfying EDGE spectral mask. The number of off-chip component is the same as the conventional step-down converter. It can drive up to 4 W and the chip area is $4.4 mm^{2}$. Two suggested methods are good candidate topologies for polar applications. These overcome the difficulties of designing high efficiency and wide bandwidth amplitude modulator.