A X-band GaN power amplifier with Bitstream modulations and active noise filtering

Power amplifiers (PAs) using Bitstream modulations such as Envelope Delta-Sigma Modulations (EDSM) have been proposed to overcome the trade-off between the linearity and efficiency in RF transmitters for signals with complex modulations. The essential idea of Bitstream amplifiers with EDSM is to translate the analogue envelope to digitized envelope so the PA can settle in several discrete driving points for high efficiency operation. The quantization noise generated due to the Bitstream modulation, however, must be rejected in a lossless fashion to guarantee the high power efficiency. The suppression of the noise power has been traditionally achieved with a passive filter, which incurs additional power loss and bulky volumes. In this paper, an active noise filtering technique is applied to suppress the quantization noise power in Bitstream transmitters by leveraging on the load modulation between the multiple identical units of PAs driven under different phase delays. A GaN PA operating at X-band has been fabricated based on Triquint's 0.25um discrete GaN HEMT devices. Two units of PAs are combined for the purpose of the active noise filtering. Driven by a single-channel WCDMA signal with 5MHz bandwidth and peak-to-average ratio (PAPR) of 5.3dB, it delivers a 35.6 dBm output power with 42.8 % drain efficiency (DE) and 33.5 % power added efficiency (PAE). The active noise filtering characteristics leading to a significant noise power reduction has also been observed in the output spectrum of the PA.