Linearity Analysis of a 40 Watt Class-G-Modulated Microwave Power Amplifier
Fig. 1: Nonlinear amplitude amplification (AM/AM) plot for class-G modulation (top). AM/AM plot after correction using digital predistortion (bottom).
Fig. 2: Efficiency against average output power with and without class-G modulation.
Fig. 3: Normalized mean square error against average output power with and without class-G modulation.
Microwave power amplifiers (PA) are key components in today's wireless infrastructure. They amplify the transmitted signal to the necessary output power of a mobile communications base station and consume most of the dissipated power. Complex modulation schemes like orthogonal frequency division multiplexing (OFDM) are used to achieve high spectral efficiency, i.e., the ratio between data rate and signal bandwidth. The resulting signals show a high peak-to-average power ratio (PAPR). Therefore, the PA has to operate most of the time in the so-called back-off region, i.e., at output powers below its maximum output power. For a linear PA, however, the efficiency drops with decreasing output power. Hence the efficiency degrades when the PA is operated in back-off. A possible solution to solve this problem is to use supply modulation by means of a class-G modulator.
In class-G modulation, the supply voltage of the PA is controlled in discrete steps related to the instantaneous power of the transmit signal. The amplitude of the modulated RF signal's envelope is used as reference. For small amplitudes, the supply voltage is reduced. As a consequence, the losses in the PA decrease as well which raises efficiency. However, the switching of the supply voltage influences the linearity of the amplifier because its gain depends on the supply voltage. This is illustrated by the upper graph in Fig. 1, which shows the amplitude distortion as a function of the input power. At the FBH, a newly developed envelope tracking system is investigated in terms of its linearity and the possibility to improve the linearity using digital predistortion (DPD).
The basis is a PA with 40 watt peak output power and a dual-level class-G modulator. Both parts were developed at the FBH based on FBH GaN transistors. Investigations reveal that the linearity of the system is significantly degraded by the class-G modulation. Using DPD linearity can be improved so that the results become comparable to those of the system with constant supply voltage. At the same time, efficiency is improved by 10 - 15%. This is shown in Fig. 2, varying the switching threshold. In Fig. 3, the normalized mean square error (NMSE) is plotted demonstrating the influence on linearity. With these results it was shown that class-G modulation is a very promising approach capable to contribute to a large efficiency improvement in PAs for mobile communications systems in the future.
Publication:
N. Wolff, O. Bengtsson, M. Schmidt, M. Berroth, W. Heinrich, “Linearity Analysis of a 40 W Class-G Modulated Microwave Power Amplifier,” Proc. 45th European Microwave Conf. (EuMC 2015), Paris, France, Sep. 7-10, pp. 1216-1219 (2015).