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Abstract This study presents a switchable Doherty power amplifier (S‐DPA) based on dual compensating reactance and adjusted drain voltage with 124% fractional bandwidth (FBW) using single‐input architecture. The proposed S‐DPA can be operated in wideband mode (WB‐MOD) and dual‐band mode (DB‐MOD) with a larger power back‐off region through one‐click switching directly. The needed back‐off impedance in both operation modes is achieved by using the dual compensating reactance for bandwidth extension. Meanwhile, a load modulation enhancement technique through adjusting the drain voltage of both the carrier and peaking amplifiers is proposed for proper load modulation in DB‐MOD. To verify the proposed techniques, an S‐DPA was designed and fabricated. Measurement results show that the proposed S‐DPA has an efficiency of 51.3%–70.3% at saturated power of higher than 43.4 dBm over the frequency band of 0.8–3.4 GHz with 124% FBW. The 6‐dB back‐off efficiency in WB‐MOD (1.3–2.9 GHz) is 45.3%–56.1% and the 8‐dB back‐off efficiency in DB‐MOD (0.8–1.3 GHz & 2.9–3.4 GHz) is 41.6%–57.9%. 80‐MHz modulated signals with 6.2‐dB and 8.2‐dB peak to average power ratios are used to evaluate the modulated signal performance in two modes. The DPA can achieve an average efficiency of up to 56% with high linearity after digital pre‐distortion linearisation.