A 26–28 GHz Quadrature-Phase Load-Modulated Balanced Amplifier With Inherent Gain Compression Elimination

Load-modulated balanced amplifiers (LMBAs) offer compelling efficiency advantages over broad frequency ranges, but their inherent gain compression in the power back-off (PBO) region poses a significant linearity challenge for modern 5G millimeter-wave applications where digital predistortion (DPD) is difficult to deploy. This article presents a quadrature-phase LMBA (QLMBA) that addresses this limitation through a quadrature-phase control signal injection technique. By establishing a 90 degrees phase offset between the balanced amplifier (BA) and control amplifier (CA), the proposed architecture isolates BA and CA power contributions, suppressing the nonlinear gain compression inherent to conventional LMBA designs. A theoretical analysis demonstrates that, with a properly selected CA to BA drive ratio, gain compression can be fully eliminated. Additionally, a reactive compensation network is integrated into the BA output matching network (OMN) to mitigate the imaginary admittance variation introduced by quadrature load modulation (QLM), improving back-off efficiency. A proof-of-concept 26-28 GHz QLMBA fabricated in a 0.12-& micro;m gallium nitride (GaN) HEMT process achieves saturated output power of 32.3 dBm, drain efficiency of 29.3% at peak and 20.71% at 6-dB PBO at 27 GHz, with less than 1 dB gain compression across the full power range. When driven by 100/400 MHz 5G New Radio (NR) frequency range 2 (FR2) 64 QAM signals, the QLMBA achieves an adjacent channel leakage ratio (ACLR) of -34.2/-30.6 dBc and an error vector magnitude (EVM) of 4.5%/5.5% without DPD, demonstrating its suitability for next-generation millimeter-wave transmitters.