2.4 A 2.4V 23.9dBm 35.7%-PAE -32.1dBc-ACLR LTE-20MHz envelope-shaping-and-tracking system with a multiloop-controlled AC-coupling supply modulator and a mode-switching PA

Abstract

Long-term-evolution (LTE) communication enables high data-rates but degrades the efficiency of the power amplifiers (PAs) due to high peak-to-average power ratios of transmitted signals. Envelope tracking (ET) and envelope-elimination-and-restoration (EER) techniques have been proposed to improve the PA efficiency by adapting the PA supply voltage to the envelope. Linear PAs in ET systems are mostly implemented in non-CMOS technologies for high efficiency [1]. However, the growing demand for low-cost integrated systems has motivated the use of CMOS PAs [2]. The main drawback of this is that linear CMOS PAs have poor efficiency. The high efficiency of switching PAs makes them a promising candidate in CMOS [3]. In EER systems, where switching PAs are used, the supply modulator must satisfy stringent noise and bandwidth specifications in order to recover the amplitude information of the LTE signal. Thus, it is a challenge for supply modulators to maintain high efficiency. Recently, a number of methods have been adopted to improve supply-modulator efficiency. In [1], a dual-switching topology is proposed, but it requires an additional inductor and results in an unpredictable noise spectrum. In [4], an AC-coupling topology is adopted to reduce the supply voltage of the linear amplifier. However, due to the slow response of the switching amplifier, the efficiency is still low.