Linearity and efficiency enhancement study for envelope tracking and Doherty transmitter

Abstract

In modern wireless communication systems such as 3GPP LTE (Long Term Evolution) and mobile WiMAX, since a modulation scheme utilizes orthogonal frequency division multiplexing (OFDM), which has a high peak-to-average ratio (PAPR) resulting in a degradation of efficiency, the high efficiency of the power amplifier (PA) is an important issue, including an improvement in thermal management, size reduction, and the enabling of a lower-cost base station implementation, such as in RRH (Remote Radio Head) systems. It is a challenge, therefore, to achieve highly efficient PA architectures. Recently, advanced transmitters for high efficiency, such as Doherty amplifiers and ET (Envelope tracking) amplifiers incorporated with a DPD (Digital Pre-Distortion) function, have been vigorously investigated. For the high-efficient transmitter, highly efficient PAs are greatly needed. The harmonic tuned amplifier such as Class-F/F-1/J are explored and analyzed. With those fabricated amplifiers, the ET and Doherty transmitter are introduced in conjunction with the linearity and efficiency improvement techniques. For ET transmitter, a high efficiency envelope-tracking (ET) transmitter incorporating a novel efficiency-boosting function is proposed and implemented. An inverse Class-F power amplifier (PA) is utilized and optimized using the proposed output loading condition, which enhances its efficiency at the high probability region. This matching network can be conveniently implemented by controlling the non-linear capacitance of the power transistor. For an accurate analysis, the output waveforms are modeled in terms of the non-linear capacitance, and the efficiency and output power are subsequently analyzed and successfully optimized. For a high efficiency envelope amplifier (EA), we propose a new envelope amplifier utilizing a two-bit switching stage in place of a single-bit switching stage. This proposed architecture effectively reduces the ripple current, improving the effic...