This papers proposes a novel design methodology to enhance bandwidth (BW) and efficiency in single-input Sequential Load Modulated Balanced Amplifier (S-LMBA). It investigates the impact of lossy off-state impedance of the auxiliary devices on efficiency degradation at back-off (BO) power and discusses the optimum choice of the auxiliary stage’s output matching network to minimize this impact. Additionally, a practical approach to extract the optimum main-to-auxiliary phase shift profile that maximizes bandwidth and efficiency is proposed. The proposed methodology is experimentally validated through the design and measurement of a 43dBm S-LMBA prototype. Between 3-4.2 GHz, the measured prototype achieves 42.1-43.5 dBm of P sat , a drain efficiency (DE) of 56.5%-66% and 45%-53% at peak and 9-dB BO, respectively.