An HBT Saturated Power Amplifier With Minimized Knee Effect for Envelope Tracking Operation

In this letter, a highly efficient saturated power amplifier (PA) based on a heterojunction bipolar transistor (HBT) is employed to maximize the efficiency of an envelope tracking (ET) PA. In this PA, output voltage waveform of the PA is shaped as a half-sinusoidal voltage waveform peaked by a second harmonic. The knee voltage effect is minimized in this voltage waveform due to the large fundamental voltage for a given supply voltage, which is very important for an ET PA with a low average supply voltage. Based on the saturated PA, the ET PA is tested at 1.85 GHz using a long term evolution signal. The ET PA delivers a power-added efficiency of 46.5%, an $ {\rm E}- {\rm UTRA}_{\rm ACLR}$ of $-33.4~{\rm dBc}$ , and an error vector magnitude of 3.0% at an average output power of 27 dBm.