A 60MHz Bandwidth High Efficiency X-Band Envelope Tracking Power Amplifier

This paper presents a high efficiency X-band envelope-tracking power amplifier (PA) that achieves a 60MHz modulation bandwidth. The system consists of a wideband envelope modulator and a single-chip gallium- nitride (GaN) MMIC X-band PA. The envelope modulator achieves >70% efficiency while delivering >7W into an 8Ω resistive load for a 60MHz LTE-A modulated signal with a 6.6dB PAPR. The envelope tracking system attains a PAE of 35.3% (including the power dissipation of the envelope modulator) with 1.1W of average output power at 9.23GHz using a 20MHz LTE signal. The X-band PA itself was measured to achieve a drain efficiency of 81.6% with a gain of 7.6dB under these conditions.

I. Introduction

As next generation systems attempt to increase the data throughput of wireless links, the signal peak-to-average power ratio (PAPR) and channel bandwidth increase. This stresses the power amplifier, which must attain good linearity and high efficiency. Conventional PAs reach maximum efficiency when they are pushed into saturation, yet to meet linearity specifications while amplifying signals with high PAPRs, the PA must be operated with the average power backed-off from compression. This results in low average efficiencies. Many techniques for improving PA efficiency under these conditions have been investigated. One of the most popular PA designs that improves average efficiency under high PAPR signals is the Doherty architecture [1]. Unfortunately the Doherty design is inherently low bandwidth due to it reliance on a quarter-wave impedance transformer.