An envelope tracking system has an envelope tracker that is configured to generate a power amplifier supply voltage that changes is relation to an envelope of a radio frequency signal, and a power amplifier comprises at least a first amplification stage having an input terminal receiving a radio frequency (RF) signal to be amplified. The power amplifier has a first coupling unit, and a second coupling unit inductively coupled with the first coupling unit, the second coupling unit provides radio frequency-coupled feedback to the input terminal of the first amplification stage through a radio frequency-coupled feedback path.

A power amplifier comprises a first amplification stage having an input terminal receiving a radio frequency (RF) signal to be amplified and having a first coupling unit, a second amplification stage outputting an amplified radio frequency signal and having a second coupling unit and a third coupling unit providing RF feedback to the input terminal of the first amplification stage through an RF feedback path, the second coupling unit being coupled to the first coupling unit, and the third coupling unit being coupled to the first coupling unit.

A power amplifier system for amplifying a radio frequency signal can have a driver transistor coupled to a radio frequency signal input. The system can also have a transformer balun with a main primary coil connected between the driver transistor and a voltage supply node of the power amplifier system, a secondary coil magnetically coupled to the main primary coil, and an additional primary coil configured to generate a feedback signal related to a signal induced in the main primary coil. A neutralization diode can be configured to use the feedback signal to reduce a gain variation resulting from variations in a voltage supplied from the voltage supply node of the power amplifier system. The neutralization diode can be connected between the additional primary coil and the driver transistor. Through envelope tracking, voltage supplied through the voltage supply node can change in relation to an envelope of the radio frequency signal.

This application relates to methods and apparatus for compensating for distortion in an output drive signal of a switched-mode amplifier which arises from non-linearity of an inductor in an output path for the drive signal. A distortion predictor is configured to predict the distortion in the output drive signal. The distortion predictor has a current predictor configured to determine a predicted current in the inductor based on, at least, an indication of an input voltage for the inductor. A model then models the distortion based on the predicted current and a corrector determines a correction term based on the predicted distortion that can be additively applied to an input signal to at least partly compensate for the distortion in the drive signal.