Broadband Power Amplifier Design with High Power, High Efficiency and Large Back-off Range

Abstract

As modern communication system technology develops, the demand for devices with smaller size, higher efficiency, and larger bandwidth has increased dramatically. To achieve this purpose, a novel architecture of load modulated balanced amplifier (LMBA) with a unique load-modulation characteristic different from any existing LMBAs and Doherty power amplifiers (DPAs) was presented, which is named as Pseudo-Doherty LMBA (PD-LMBA). Based on a special combination of control amplifier (carrier) and balanced amplifier (peaking) together with proper phase and amplitude controls, an optimal load-modulation behavior can be achieved for PD-LMBA leading to maximized efficiency over extended power back-off range. More importantly, the efficiency optimization can be achieved with only a static setting of phase offset at a given frequency, which greatly simplifies the complexity for phase control. Furthermore, the co-operations of the carrier and peaking amplifiers in PD-LMBA are fully de-coupled, thus lifting the fundamental bandwidth barrier imposed on Doherty-based active load modulation. However, since PD-LMBA has CA over-driving concerns, a new load-modulated power amplifier (PA) architecture, Asymmetric Load-Modulated Balanced Amplifier (ALMBA), is proposed based on PD-LMBA. And a subsequent improved type-continuous mode Hybrid Asymmetric Load Modulation Balanced Amplifier (H-ALMBA) has been developed. The two sub-amplifiers (BA1 and BA2) of the balanced topology in an LMBA are set as peaking amplifiers with different thresholds when cooperating with the control amplifier (CA) as the carrier, forming a hybrid load modulation behavior between Doherty and ALMBA. Compared to standard LMBA, the proposed H-ALMBA has a three-way load modulation with CA, BA1 and BA2 through proper amplitude control and phase alignment. Thus, this new mode offers extended power back-off range and enhanced back-off efficiency without suffering from difficulty and complexity in wideband design as imposed on three-way Doherty PAs. Based on comprehensive theoretical derivation and analysis, the proposed H-ALMBA is designed and implemented using commercial GaN transistors and wideband quadrature couplers. Moreover, the continuous-mode matching is applied to the carrier amplifier achieving a maximized wideband efficiency at power back-off. This is the first time that continuous mode and ALMBA have been used in combination, and very satisfactory results have been achieved, exhibiting the highest 10-dB output power back-off (OBO) drain efficiency (DE) ever reported for wideband load-modulation PAs. The developed prototype experimentally demonstrates wide bandwidth from 0.55-2.2 GHz. The measurement exhibits an efficiency of 63-82% at peak output power, 51-62% for 5-dB OBO, and 50-66% for 10-dB OBO within the design bandwidth. When stimulated by a 20-MHz long term evolution (LTE) signal with 10.5-dB peak to average power ratio (PAPR), a 50-55% average efficiency is measured over the entire bandwidth at an average output power around 33 dBm