Extension of Output Backoff Range in Three-Stage Load Modulated Balanced Amplifier Using Asymmetric Coupling and Non-Z₀ Load

In this article, the extension of the output backoff (OBO) range of the load-modulated balanced amplifier (LMBA) is reported by manipulating the asymmetric output coupling and the non- $Z_{0}$ termination. Closed-form formulas are derived based on the established model. It is demonstrated that by appropriately selecting the relevant parameters, high efficiency can be maintained over a large dynamic power range, such as 11/13/15/17 dB OBO. Furthermore, the strong saturation of the main power amplifier (PA) [like the pseudo-Doherty LMBA (PD-LMBA)] is circumvented in the proposed LMBA due to the specific scheme of modulation, effectively tackling the stability concerns associated with PD-LMBA and eliminating the linearity concern caused by the over-driven issue of the main PA in PD-LMBA design. The procedure and the graphical illustration are provided in comprehensive detail. For validation, the LMBA with the 15 dB OBO, operating at 1.80–2.05 GHz, is designed and implemented utilizing the gallium nitride (GaN) transistor. Measurements indicate that the prototype offers drain efficiency (DE) of 48.2%–63.2%/50.8%–59.5%/37.9%–49.2% at saturation/7.5 dB OBO/15 dB OBO range with saturation output power of 42–44.4 dBm. Under the excitation of long-term evolution (LTE) signal with bandwidth of 10 MHz and peak-to-average-power-ratio (PAPR) of 10.2 dB, the prototype offers average efficiency of 49.8%–55.5% at the average output power level of 31.8–34.2 dBm, with the adjacent channel leakage ratio (ACLR) of around −25 dBc.