Efficiently Amplified

Modern wireless communication systems have evolved to support increasing numbers of subscribers and provide higher data rate services within the limited frequency resources. As part of this evolution, many transmitted signals in the new standards, such as WCDMA, long term evolution (LTE), and worldwide interoperability for microwave access (WiMAX), now utilize a high peak-to-average power ratio (PAPR) caused by complex modulation schemes, generating the rapid change in the magnitude of signal as shown in Figure 1(a). In particular, as depicted in Figure 1(b), the PAPRs of the signals exceed 9 dB at 0.01% level of complementary cumulative distribution function. Use of high PAPR signals result in the power amplifier (PA) operating at a large enough back-off to satisfy the stringent linearity requirement. However, in this region, efficiency of the PA is very low. To bolster low efficiency in the back-off region, various efficiency boosting techniques have been considered over time. Recently, due to high-efficiency capabilities [1]–[13], supply voltage modulated PAs, such as envelope elimination and restoration (EER), hybrid-EER (H-EER), and envelope tracking (ET) technique, have received a lot of attention. Excellent experimental results have been reported using the various device technologies and modulated signals, as summarized in Table 1.