Highly Efficient 3-Bit Digital Power Amplifier for OFDM Waveform Amplification

In this work, we present a digital power amplifier (DPA) and a signal-optimized control technique suitable for the amplification of orthogonal frequency division multiplexing (OFDM). OFDM is a high peak-to-average power ratio (PAPR) signal that is naturally arising from high spectrum efficiency modulations. A 3-bit DPA is implemented with three power-scaled transistors which are turned on and off based on the signal amplitude, while phase modulation is restored using the radio frequency (RF) input signal. The unavoidable nonlinearities at the DPA output due to PA switching are minimized by accounting for the OFDM signal probability density function (pdf). This pdf is a priori knowledge to design an optimal quantizer that minimizes distortion by distributing the DPA power levels where the signal amplitude is more similar to the original one. Back-off efficiency within 2³=8 possible states is then optimized by implementing a load-modulating power combiner. Theory and an example design of the combiner network are provided and demonstrated for this DPA. The reported DPA prototype operates at 1.5 GHz with a 3-bit control and achieves a maximum power-added efficiency (PAE) of 64.3% and maintains a drain efficiency greater than 47% over the output power range from 36.6 to 45.2 dBm (8.6-dB range).