Study on load transformation networks for differential common drain class-B RF power amplifier

Common drain class-B PAs have the potential of providing high linearity and efficiency. This work analyses the performance of differential common drain class-B power amplifiers (PAs) in terms of linearity, efficiency and time-domain waveforms depending on the load transformation network (LTN) used. Four different LTNs, which provide the required impedance conditions for class-B operation are analysed and the circuit performance of common drain PAs is verified and compared in simulations. The transformer based LTN shows maximum linearity due to the high degree of symmetry. Third-order intermodulation distortion, IMD₃ at 3 dB output power back-off from 1 dB compression point, P_1dB is 36 dBc while second harmonic rejection is 103 dBc at an input power of 20 dBm. Peak power added efficiency, PAE of 40.6 % was achieved with this configuration. An equivalent off-chip load network achieves peak PAE of 30 % with IMD₃ rejection of 31 dBc at 3 dB back-off from P_1dB and second harmonic rejection of 24.7 dBc for an input power of 20 dBm. A compromise between area and integration complexity can be obtained with the other two designs analysed. The results confirm that differential common drain class-B PAs have the potential of providing good linearity also for modern digitally modulated signals.