System Linearity-Based Characterization of High-Frequency Multilevel DC–DC Converters for S-Band EER Transmitters

In modern telecommunications systems, the requirements for the amount and speed of transmitted data are increasing rapidly. For better spectral efficiency, complex-type modulations [quadrature amplitude modulation (QAM) and orthogonal frequency-division multiplexing (OFDM)] are used, resulting in high peak-to-average power ratio (PAPR) signals. As a consequence, when conventional linear amplifiers (classes A and AB) are used in transmitters, both prominent linearity and poor efficiency are assured. The efficiency of the transmitters can be significantly improved using some of the popular techniques: Kahn envelope elimination and restoration (EER) principle, envelope tracking (ET) technique, Doherty amplifier, and Chireix's amplifier. In this article, two different approaches in supplying a 2.4-GHz class-E power amplifier (PA) (S-band) based on the EER technique are analyzed and compared. The consistent part of the EER transmitters is a switched-capacitor-based voltage divider combined with a switching structure-analog multiplexer, in the envelope amplifiers (EAs) and a class-E PA. The analog multiplexer is assisted with a series linear regulator in the first system and a fourth-order LC filter in the second system, providing supply modulation for the PA. The systems are compared in terms of efficiency and system linearity using the standard metrics [error vector magnitude (EVM) and adjacent channel power ratio (ACPR)]. Special consideration is devoted to the characterization of how the EAs affect the quality of the RF output signal. It is shown that the tested EER transmitters can reproduce 16-QAM and 64-QAM signals up to 2.5 MHz, manifesting an average efficiency of about 32% with the first transmitter, and 16-QAM signals up to 660 kHz RF bandwidth and average efficiency of 44% with the second transmitter. A software tool that accurately estimates the EA linearity is utilized and successfully tested on both EER transmitters.