A Ka-Band Pseudo-Doherty Load-Modulated Balanced Amplifier With Gain Flatness Enhancement Technique

A millimeter-wave (mm-wave) fully integrated pseudo-Doherty load-modulated balanced amplifier (PD-LMBA) with a gain flatness enhancement technique is demonstrated in this article. Unlike the conventional LMBA topology based on using two balanced amplifiers (BAs) in the class-AB/B condition and a control amplifier (CA) in the class-C condition, the CA is used as the main amplifier in the class-B condition and the BAs as the auxiliary amplifiers in the class-C condition in the proposed configuration. Therefore, the proposed architecture only needs to satisfy the load condition at saturation rather than in both saturation and power back-off (PBO) conditions. This design can more easily and flexibly achieve a large dynamic power range. Furthermore, a gain flatness enhancement technique based on a variable cross-coupled pair (VCCP) is proposed to improve AM–AM characteristics without decreasing efficiency. The proposed fully integrated PD-LMBA is fabricated in a commercial 65-nm CMOS process and occupies 1.1 mm2. Adopting continuous-wave testing, the amplifier is found to provide a peak gain of 25.5 dB with a 3-dB gain bandwidth from 24.3 to 28.2 GHz. At 27 GHz, a measured saturated output power ( $P_{\mathrm{ sat}}$ ) of 22.2 dBm with a peak power added efficiency (PAE) of 30.4% and output 1-dB compression point power ( ${\text {OP}}_{1\,\text {dB}}$ ) of 20.1 dBm with 23.6% PAE are achieved. The measured PAE at 8-dB PBO from $P_{\mathrm{ sat}}$ still reaches 15.0%. Without applying any predistortion technology, the prototype demonstrates an ultralow AM–AM distortion characteristic that is less than 0.6 dB in the linear region at 28 GHz. Modulated signal measurements are also performed at the center frequency of 27 GHz. The amplifier provides 13.2 dBm average output power ( $P_{\mathrm{ avg}}$ ), 13.8% average PAE, −26.3 dBc adjacent channel leakage ratio (ACLR) and −25.9 dB EVM for a 100 MSym/s single-carrier 64-quadrature amplitude modulation (QAM) signal with a 7.1-dB peak-to-average-power ratio. For a 2.4-Gb/s (400 MSym/s) signal, also at a carrier frequency of 27 GHz, this proposed PD-LMBA still demonstrates 12.3-dBm $P_{\mathrm{ avg}}$ and 11.9% average PAE with −25.5 dB rms EVM and −25.7 dBc ACLR.

I. Introduction

With the increasing requirements for high-speed wireless communication, a high-quality and efficient transmission data rate is needed. The millimeter-wave (mm-wave) frequency has been greatly developed and widely used in various fields due to the sufficient available spectrum resources. To satisfy the requirements of low latency and high capacity for wireless communication, higher order quadrature amplitude modulation (QAM) signals with a higher peak-to-average power ratio (PAPR) are adopted to improve spectrum utilization. As a consequence, the requirements of efficiency and linearity for a power amplifier (PA) are becoming more stringent. The PA achieves high efficiency at the saturation output power level with serious nonlinear distortion [1], which degrades linearity performance. Therefore, the PA usually works at an output power back-off (PBO) level to sacrifice efficiency to meet the linearity requirement. It is still very challenging to design a PA with good linearity and high efficiency simultaneously.