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A three-state signal coding scheme is proposed for driving Class-S amplifiers with higher efficiency than conventional techniques. The approach is formulated in the framework of polar modulation, where the magnitude is controlled by a timing code that specifies positive and negative rectangular pulses as well as a third zero state, and the phase is varied by modulating the clock delay. In particular, high efficiency is attained when the active pulse widths are a half-cycle of the carrier frequency, and the zero-state is formed by skipping pulses. In this case, full-amplitude modulation has the same efficiency as a Class-D amplifier, and is only slightly degraded as the amplitude is decreased. In addition, the technique operates at only 2X sampling and is very simple to implement. Spectral analysis is performed for baseband signals consisting of single tones, 2-tone suppressed-carrier, and bandlimited random noise. The latter case emulates real communication signals and is shown to exhibit good spectral characteristics and low error-vector magnitude in the time domain. An important application of this technique is in all-digital, power-efficient power amplifiers for cellular base stations and handsets.