A power amplifier driver using self-oscillating pulse-width modulators

A polar transmitter for use with non-constant envelope signals is presented. Techniques for efficient behavior of both the amplitude (AM) and phase (PM) path are introduced. The envelope linearization technique is based on asynchronous pulse-width modulation of a phase-modulated RF signal. The efficiency of the switching amplifier is less dependent of the output power, compared to class B operation. Using a self-oscillating, asynchronous type of pulse-width modulator, the spurs are concentrated in narrow bands well separated from the signal band. These switching spurs are filtered out by the filters already present in the transmitter. The feasibility of this RF-pulse-width-modulation is proven by measurements with UMTS-signals on a prototype in 0.18 mum CMOS, yielding a peak output power of 8.26 dBm and a drain efficiency of 35%. In the phase-modulation path of the polar transmitter, injection locking of an oscillator is used as a single-stage high- gain amplifier. Avoiding multiple stages and related power loss improves the overall power efficiency.