Highly efficient envelope tracking transmitter by utilizing sinking current

This paper presents an envelope shaping function of the hybrid switching amplifier (HSA) to achieve high efficiency. In the envelope tracking (ET) transmitter, the switch current of HSA can not follow the wideband load current due to limitation of the switching frequency. Thus, the switch operates as a quasi-constant current source and the sourcing and the sinking currents of the linear stage generates the accurate load current. The sinking current is a critical efficiency reduction factor since it is a wasted power. To reduce the sinking current, the gate bias of the PA should be increased, so that the sinking current is delivered to the PA and is utilized for amplification. The efficiency of the HSA can be increased over 80% from around 70%. The implemented ET transmitter using Cree CGH40045 GaN HEMT achieves drain efficiency of 59.1% at 40.3 dBm output power for long term evolution 10-MHz signal with 6.5 dB PAPR at 889MHz. Moreover, the ET transmitter could be linearized to -45 dBc with using the digital predistortion technique.