Efficiency improvement techniques of CMOS power amplifiers for wireless communications

Abstract

A power amplifier (PA) is a key element in wireless communication systems. Since the PA consumes most of the battery power in mobile handsets, its efficiency directly affects overall talk time. Therefore, efficiency improvement has been a major issue in mobile PAs. Generally, amplifiers have high efficiency at peak output power, but the efficiency is decreased drastically as the output power decreases. Furthermore, PAs of mobile handsets in some applications operate mostly in the low power region. In those applications, efficiency boosting at the low power region will be very important. Recently, CMOS (Complementary Metal-Oxide Semiconductor) power amplifiers (PAs) have been studied assiduously, though few works have focused on CMOS PAs for polar transmitter applications. Polar transmitters are expected to be the next-generation transmitters. The polar transformer (I-Q to R-$\theta$) decomposes the input of a power amplifier into two types of input signals. The first of these is an RF phase signal, which is applied to the input of a power amplifier, and the second is an envelope signal, applied as a supply voltage. The supply voltage has to be applied through a DC-DC converter or a low drop output regulator circuit in order to supply sufficient power. Polar transmitters can use switching-mode power amplifiers, as the input signals do not contain envelope information. These include Class-D, Class-E, and Class-F amplifiers, which are nonlinear, but very efficient. A CMOS power transistor is known to be viable for switching power amplifiers rather than linear amplifiers; thus, it common to study CMOS switching power amplifiers for polar transmitters. There are two important specifications of power amplifiers for polar trans-mitters. The first is the output dynamic range. It is crucial to obtain enough dynamic range with a given supply voltage range. The second specification is related to efficiency at a low output power. In general, a power efficiency close to...