The theoretical efficiency in digital envelope power amplifiers for WLAN OFDM polar transmitters

Recently a number of polar transmitters for the multi-mode transmission of Wireless-Local-Area-Network (WLAN) and Bluetooth signals have been proposed. These polar transmitters use envelope modulators based on digital-to-analog converters (DACs) as power amplifiers. Such an approach is very useful when the maximum theoretical efficiency of the power-amplifier digital-to-analog converter (PA-DAC) is comparable or better than state-of-the-art architectures. This architecture has the advantage that the phase-modulated input signal can drive the circuit in class-A or class-B, without causing any distortion in the overall signal path. Only at the combination point of the phase-modulated signal and the envelope modulated signal, the distortion in the combined signal becomes important. Here, a calculation of the efficiency for such a PA-DAC is performed for both class-A and class-B using WLAN Orthogonal-Frequency-Division-Multiplex (OFDM) signals. When driving the phase-modulated input of the envelope PA-DAC in class-A, an efficiency of 28% can be reached for a peak-to-average-power-ratio (PAR) of 6 dB, while 20% can be reached for a PAR of 9 dB. Similarly, when driving the phase-modulated input of the PA-DAC in class-B, an efficiency of 44% can be reached for a PAR of 6 dB, while 31% can be reached for a PAR of 9 dB.