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A new signal processing method that partially encodes the envelope signal using a delta sigma modulator (EDSM) into a bi-level signal according to the magnitude of the envelope, has been introduced to utilize a high efficiency class-C amplifier for linear amplification. The RF signal is synthesized through the proposed envelope waveform, which is encoded into a pulse signal in the lower power region and bypassed in power regions higher than the base threshold. In this respect, the magnitude of the encoded envelope is always high enough to drive an efficient power amplifier (PA) biased as a deep class-C amplifier. For verification, a 2.6 GHz harmonic-tuned class-C PA using a commercial 10 W GaN device has been fabricated and is driven by the proposed signal. The experimental results show that the PA delivers a drain efficiency (DE) of 45.3% at an average power of 33.7 dBm, which is approximately 7% higher than that of a PA with the original complex RF signal.