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The following paper presents a novel 900MHz pulse-width-modulated current-mode class-S power amplifier. The conversion from an analog to a digital signal, which can be handled by the subsequent switch-mode-stage, is implemented using a fast pulse-width-modulator. The complete class-S amplifier is characterized using clipped WCDMA signals with 5MHz and 80MHz bandwidth, respectively. For the 5MHz signal, the measured spectrum at the amplifier's output shows good EVM of < 12% (approx. 1.5% degradation assumed for class-S PA) and also meets the 3GPP ACLR requirements within 9dB input power range. The capability to support large signal bandwidths is demonstrated, using an 80MHz WCDMA signal, thus meeting the expectations on class-S amplifier for the first time with respect to signal bandwidth.Index Terms -Class-S amplifier, switch-mode operation, linearity, large signal bandwidth, PWM.

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