무선통신용 포락선 추적 전력 증폭기와 디지털 RF 송신기에 관한 연구

Abstract

This thesis describes the analysis and implementation of an envelope tracking power amplifier and a digital RF transmitter for wireless communication. For low power consumption and longer battery usage time, an envelope tracking technique and RF DAC based digital quadrature transmitter are employed for the power amplifier and transmitter, respectively. The envelope tracking power amplifier is a suitable structure for highly efficient amplification of high PAPR signals such as WiMAX, LTE, and LTE Advanced. The RF DAC based digital quadrature transmitter directly converts digital I and Q signals to modulated RF signals. This digital structure has many advantages of high controllability, size and cost reduction, and low power consumption.For the optimized operation of the envelope tracking power amplifier, an envelope shaping function following the sweet spot and a crest factor reduction technique are proposed. Besides, a simple and accurate PA model is used for the design of a CMOS hybrid switching supply modulator, which consists of a high efficiency switching amplifier and a high speed linear amplifier. For the WCDMA/LTE 10 MHz signals, the implemented envelope tracking PA shows the efficiencies of 48.8/42.2 % at the peak output power of 31.5/28.9 dBm, respectively.For the accurate tracking of a wideband signal up to 30 MHz channel bandwidth, a supply modulator should follow the envelope signal accurately and fast. By inserting a capacitor in a feedback path of a linear amplifier, a negative zero and a negative pole are generated. Because the frequency of the zero is lower than the pole, the bandwidth of the linear amplifier increases about 2 times without any additional power consumption and stability problem. For 10/20/30 MHz LTE signals, the envelope tracking PA and the supply modulator have efficiencies of 40.0/39.4/38.5 % and 75.5/74.2/72.3 %, respectively.For the improved efficiency of the switching amplifier, the switching amplifier employs fast and slow slew rate buck converters and realizes an adaptive slew rate control of the current. The adaptive slew rate control enables a faster output current tracking and a higher efficiency. The implemented dual-switch hybrid switching supply modulator has improved the efficiency by about 2 % for 10 MHz LTE signal.A gain compensated predistortion is proposed for linearity improvement of the envelope tracking power amplifier. Generally, gain of a power amplifier decreases as its supply voltage reduces, degrading linearity. For the linearity improvement, a gain compensated predistortion is adopted to the RF input as well as an envelope shaping of the supplyvoltage. By the predistortion, AM-AM response of the envelope tracking power amplifier is improved significantly.A highly efficient and wideband digital quadrature transmitter employing RF DACs, with an array of class-D PA and a distributed capacitor power combiner, directly generates RF signal from baseband digital I and Q signals. By applying a disable opposite cell technique, the digital transmitter delivers high efficiency and good linearity. The measured drain efficiencies and total efficiencies of the implemented transmitter are 14.4/11.0/10.6 % and 10.8/6.7/6.3 % at peak output power of 7.87/0.26/-0.11 dBm for CW/LTE 5-MHz/LTE 10-MHz signals, respectively.