32.7 A 25.2dBm PSAT, 35-to-43GHz VSWR-Resilient Chain-Weaver Eight-Way Balanced PA with an Embedded Impedance/Power Sensor

Power amplifiers (PAs) are one of the essential parts of the mm-wave 5G phased-array transmitters (TXs), as they usually define the TX linearity, reliability, and efficiency. In the hybrid/analog beamforming architectures, the signal reaches the PA input with a low-power level due to losses of preceding stages. Therefore, the PA must provide a high gain and peak output power to achieve the required EIRP. Besides, the PA must satisfy the ever-tightening linearity specifications of the complex modulation schemes employed in 5G systems [1]. In view of this, a key challenge is achieving the required performance and maintaining it over the antenna VSWR variation. Since the mm-wave 5G TXs must support modulation bandwidths (BWs) up to 1.4GHz, the antenna impedance can vary significantly over such a large BW. This dependency becomes more complex when considering a time-varying VSWR caused by the beam-angle-dependent mutual coupling and environmental changes. As illustrated in Fig. 32.7.1 (top-left), the time and frequency-dependent VSWR deteriorates PA gain-flatness, OP
1dB
, linearity, and reliability. The gain-flatness correction consumes the link budget, while the PA needs to be over-dimensioned to satisfy the required EIRP, linearity, and reliability over the signal BW to handle the worst-case VSWR scenario. Reconfigurable matching networks (RMNs) are introduced to address the VSWR issue [2, 3]. However, since the PA can be tuned only for a specific antenna impedance (i.e., at a chosen frequency), RMNs do not offer frequency-dependent VSWR compensation. Balanced PAs (BPAs), on the other hand, provide an inherent VSWR resilience by relying on the cancellation of the reflected wave by the quadrature-hybrid coupler (QHC) [4]. The QHC also provides broadband output matching, thus alleviating frequency-dependent gain variation. In Fig. 32.7.1 (top right), the gain variation of a BPA is compared with a single-branch PA (Γ
OUT
=-4dB) for VSWR<4:1. While the gain of the single-branch PA varies by 7dB, the BPA shows only <1.94dB gain variation caused by mismatch loss of the antenna (1-|Γ|
²
). Nevertheless, the linearity of a BPA is defined by the combined performance of the two PAs whose non-optimum loadings degrade the overall linearity and reliability. Moreover, as shown in Fig. 32.7.1 (middle left), the gain deviation of the BPA over a VSWR circle heavily depends on the QHC amplitude and phase error. Besides, a VSWR resilient impedance/power sensor is an essential part of such a PA for regulating the gain, beam pattern, and EIRP [5].