Personnel

FICs - Prof. Wing-Hung Ki

Description

In wireless communications, RF power amplifiers are usually operated in the Class-A or Class-AB mode in order to preserve modulation accuracy. For a Class-A RF power amplifier, the DC current and voltage are kept constant, and its efficiency is proportional to the output power. For a Class-B RF power amplifier, the supply voltage is kept constant while the DC current is proportional to the output RF current. Thus, for a Class-A RF amplifier, the efficiency decreases in proportion to P_out/P_out(max), while for a Class-B RF amplifier, the efficiency decreases in proportion to (P_out/P_out(max))^1/2. The efficiency of a Class-AB RF amplifier lies in between these values, and can be improved greatly if the supply voltage changes dynamically in accordance with the output power. Some existing designs showed that the loss of power amplifiers can be reduced by two times or more by dynamic supply voltage control. However, a high-efficiency wideband supply modulator is needed to achieve both high efficiency and high linearity for the RF power amplifier. Both linear regulators and switching converters could be used for supply modulation. The efficiency of a linear regulator is very poor when the output voltage is low, while a switching converter is very difficult to achieve a loop bandwidth three times of the signal bandwidth. Many of the existing dynamic supply biasing designs are restricted to narrowband applications such as EDGE, CDMA, and WCDMA whose signal bandwidth is of the order of 100 kHz or a few MHz. For emerging 4G high data rate applications such as LTE or WiMAX, the baseband signal bandwidth is as high as 20 MHz and the PAR (peak-to-average ratio) is about 10 to 12dB for the advanced OFDM modulation scheme. We propose to study and design high-efficiency wideband supply modulators with dynamic supply voltage control. The bandwidth of the supply modulator should be as high as 20MHz with efficiency near or over 90%. To achieve both high efficiency and wide bandwidth, a linear-assisted switching converter would be a good candidate. In this architecture, the switching converter is used to supply most of the power confined within the low to medium frequency range, while the linear regulator (may be a low dropout regulator) supplies power for the high frequency range. Existing designs either employ hysteretic-mode control or voltage-mode control. In this research, we propose to explore new control methods, for example, linear-assisted switching converters with current-mode control.

Source: RGC | ITF