Combined Three-State/PWM Signal Coding for Wideband High Efficiency Class-S Amplifiers

A three-state signal coding scheme was recently proposed for driving Class-S amplifiers with higher efficiency than conventional techniques [1]. The magnitude is controlled by a timing code that specifies positive and negative rectangular pulses as well as a third zero state. 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. Good spectral characteristics and low error-vector magnitude are achieved for fixed carrier frequency, where output filtering can be effectively employed. However, for applications requiring operation with signals spectrally located anywhere within a much wider service bandwidth, intermodulation effects limit the adjacent channel power ration (ACLR). In this paper, we explain this fundamental limitation and propose combining 3-state coding with pulsewidth modulation (PWM) as a remedy. The combined three-state/PWM scheme enables a tradeoff to reduce ACLR while not seriously compromising the efficiency.