A CMOS Wideband Current-Mode Digital Polar Power Amplifier With Built-In AM-PM Distortion Self-Compensation

This paper presents a fully integrated wideband current-mode digital polar power amplifier (DPA) in CMOS with built-in AM-PM distortion self-compensation. Feedforward capacitors are implemented in each differential cascode digital power cell. These feedforward capacitors operate together with a proposed DPA biasing scheme to minimize the DPA output device capacitance C-d variations over a wide output power range and a wide carrier frequency bandwidth, resulting in DPA AM-PM distortion reduction. A three-coil transformer-based DPA output passive network is implemented within a single transformer footprint (330 mu m x 330 mu m) and provides parallel power combining and load impedance transformation with a low loss, an octave bandwidth, and a large impedance transformation ratio. Moreover, this proposed power amplifier (PA) output passive network shows a desensitized phase response to C-d variations and further suppresses the DPA AM-PM distortion. Both proposed AM-PM distortion self-compensation techniques are effective for a large carrier frequency range and a wide modulation bandwidth, and are independent of the DPA AM control codes. This results in a superior inherent DPA phase linearity and reduces or even eliminates the need for phase pre-distortion, which dramatically simplifies the DPA pre-distortion computations. As a proof-of-concept, a 2-4.3 GHz wideband DPA is implemented in a standard 28-nm bulk CMOS process. Operating with a low supply voltage of 1.4 V for enhanced reliability, the DPA demonstrates +/- 0.5 dB PA output power bandwidth from 2 to 4.3 GHz with +24.9 dBm peak output power at 3.1 GHz. The measured peak PA drain efficiency is 42.7% at 2.5 GHz and is more than 27% from 2 to 4.3 GHz. The measured PA AM-PM distortion is within 6.8 degrees at 2.8 GHz over the PA output power dynamic range of 25 dB, achieving the lowest AM-PM distortion among recently reported current-mode DPAs in the same frequency range. Without any phase pre-distortion, modulation measurements with a 20-MHz 802.11n standard compliant signal demonstrate 2.95% rms error vector magnitude, -33.5 dBc adjacent channel leakage ratio, 15.6% PA drain efficiency, and +14.6 dBm PA average output power at 2.8 GHz.