Analysis of Far-Out Spurious Noise and its Reduction in Envelope-Tracking Power Amplifier

Abstract

This paper describes analysis of the far-out spurious noise and a method to reduce this noise in an envelope-tracking power amplifier (ET PA). The ET PA delivers a higher efficiency and a better linearity than the stand-alone PA, simultaneously. However, the ET PA generates spectral regrowth at a far-out spurious emission domain. The noise sources are analyzed, and the noise effects on a RF PA are estimated using mathematical models of the RF PA and supply modulator. To reduce the noise, a wideband supply modulator is designed using 40-nm CMOS process for a high speed operation, and an additional capacitor is connected to the supply line of the PA. These noise reduction techniques significantly reduce the far-out spurious noise of the ET PA to a level similar to that of the stand-alone PA. For a 10-MHz bandwidth and 6.5-dB peak-to-average power ratio long term evolution signal, the ET PA delivers a power-added efficiency of 41.8%, a gain of 23.8 dB, and an evolved universal terrestrial radio access adjacent channel leakage ratio of $-34.3~{\rm dBc}$ at an average output power of 27 dBm and an operating frequency of 1.85 GHz. The noise power spectrum densities of the ET PA are $-130.3/-133.7~{\rm dBm/Hz}$ at 1.93/1.99 GHz, respectively, which is limited by our PA itself.