A novel apparatus and method of improving the power efficiency of a
digital transmitter for non-constant-
amplitude modulation schemes. The power efficiency improvement mechanism of the invention leverages the high efficiency of a switched-mode power supply (SMPS) that supplies the high
DC current to the
transmitter's power
amplifier, while compensating for its limitations using
predistortion. The
predistortion may be achieved using any suitable technique such as
digital signal processing, hardware techniques, etc. A
switched mode power supply (i.e. switching
regulator) is used to provide a slow form (i.e. reduced bandwidth) of
envelope tracking (based on a narrower bandwidth distorted version of the envelope waveform) such that the switching
regulator can use a lower switching rate corresponding to the lower bandwidth, thereby obtaining high efficiency in the switching
regulator. The resulting AM-AM and AM-PM distortions in the power
amplifier are compensated through
predistortion of the digital amplitude modulating
signal which dictates the envelope at the PA input. Similarly, the
phase modulation is also compensated prior to the PA, such that once it undergoes the
distortion in the PA, the end result is sufficiently close to the desired phase.