Dual-Mode CMOS Doherty LTE Power Amplifier With Symmetric Hybrid Transformer

This paper presents a dual-mode Doherty long-term evolution (LTE) power amplifier (PA) for efficiency enhancement at power back-off. The amplifier utilizes transformer-based Doherty operation to achieve high back-off efficiency with good linearity. In addition, the PA has two power modes to further reduce the power consumption when low output power is required. The PA combines four push-pull amplifiers by using an symmetrical hybrid transformer that ensures low combining loss and high amplifier efficiency. The two-stage LTE amplifier is fabricated in standard 40 nm CMOS technology and it achieves 28 dBm output power with 1.5 V supply. The peak power-added efficiency (PAE) of the amplifier is 34% and the PAE at 6 dB and 12 dB back-off levels are still as high as 25.5% and 19.7% respectively. The amplifier is tested with 20 MHz LTE signal and it satisfies the stringent ACLR and EVM requirements at 23.4 dBm output power with a PAE of 23.3%. In addition, the amplifier achieves 18.4% PAE when 6 dB lower power is transmitted and 11.1% PAE at 12 dB back-off under LTE modulation.