Load-Modulation-Based IMD3 Cancellation for Millimeter-Wave Class-B CMOS Power Amplifiers Achieving EVM < 1.2%

Abstract

This letter presents a novel load-modulation-based 3rd-order intermodulation distortion (IMD3) cancellation technique for class-B CMOS power amplifiers (PAs). In a class-B PA, the IMD3 generated by the 3rd-order transconductance ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$g_{m3}$ </tex-math></inline-formula> ) and the gain compression have opposite signs, and thus, they can cancel each other at specific bias and loading conditions. The proposed Doherty topology allows adjusting the gain compression by modulating the effective loading, facilitating IMD3 cancellation over the entire load modulation region. The proposed approach is verified using a 28 GHz 40 nm CMOS series-Doherty PA (DPA) topology. The experimental result demonstrates 10/17 dB IMD3 improvement compared to class-B/DPA operation. Without using any digital pre-distortion, the measured EVM of the proposed technique for a 50 MHz 64-QAM OFDM signal with 8.9 dBm average output power is −38.7 dB (1.2%), which is 5.7/11 dB better than a standard class-B/DPA operation.