FBH research: 02.11.2016

Can Class-G supply modulated systems replace power amplifiers in single-input, single-output systems?

Time domain waveforms of envelope and supply voltage
Fig 1: Time domain waveforms of envelope and supply voltage showing the reduced losses for a discrete (class-G) modulated system
Simplified schematic of the gate modulated class-G PA system
Fig 2: Simplified schematic of the gate modulated class-G PA system. Common GND omitted for supplies, RF input and RF output signals
Switch modulator control signals
Fig 3: Switch modulator control signal extracted from the original (bottom), pre-distorted signal for a classical class G system (top), and for the predistorted signal for a gate modulated class-G system
AM/AM and AM/PM distortion for gate modulated class-G system
Fig 4: AM/AM and AM/PM distortion for the gate modulated class-G system

Microwave power amplifiers (PA) are core components in modern communication systems, used to amplify the transmitted signal to the necessary output power of a base station for mobile communication. 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 at output powers below its maximum in the so called back-off region and only sometimes at full output power. For a linear PA the efficiency increases with rising output power. Therefore the efficiency is degraded when the PA is operated in back-off. A possible solution to solve this problem is to apply supply modulation using a class-G modulator.

The supply voltage of the PA in class-G modulation is controlled in discrete steps related to the instantaneous power of the transmit signal. Due to the reduction of the supply voltage the maximum output power of the PA is decreased. For similar output power at a lower supply voltage, the losses in the PA are reduced and the efficiency increases, see Fig. 1. However, the discrete supply modulated system is complex, providing two inputs, the modulated RF input and the switching signal for the supply modulator and one output, the modulated RF output. Such setups are commonly known as dual input single output (DISO) systems. Most RF power amplifiers work at a constant supply voltage, which makes them inefficient. They only have one input and one output and are known as single-input single output (SISO) systems. If a class-G system can be operated as a SISO system with maintained linearity using similar linearization techniques it would be possible to improve the efficiency of SISO systems tremendously. It would also enable to use them as drop-in replacements which could improve the efficiency for telecommunication systems as much as 10 - 20%-points.

In order to operate a class-G structure as a SISO system the control signal for the modulator will have to be extracted from the modulated signal itself. A wideband peak power detector working as envelope detector would do the job. However, for the linearization to work, the detected switching signal may not change between the iterations during linearization. The integrity of the originally identified switch control signal has to be maintained. Recent work at FBH presented at EuMW 2016 has shown that by introducing a discrete gate modulator working synchronously with the discrete drain modulation illustrated in Fig 2, the gate voltages can be selected in such a manner that the integrity of the switching signal is maintained as shown in Fig. 3. This means that with an added discrete gate modulation with proper gate voltages the identified switching signal can be made identical to the original signal, which does not apply to the predistorted classical class-G signal. Hence, convergence in the DPD process and success in the linearization can be assured with added gate modulation.

Furthermore, the discontinuity in AM/AM distortion that is always observed in class-G system can be almost totally eliminated with a proper selection of gate voltages, as can be seen in Fig. 4. This makes it a good candidate for linear systems whenever the baseband information is not available for digital predistortion as e.g. communication systems for space applications. The additional phase distortion introduced by the gate modulation shown in the AM/PM curve is expected to be compensated using varactors.

This work is one step in the direction to show the tremendous potential of class-G systems and make them a true candidate for future telecommunication applications.

Reference

N. Wolff, W. Heinrich, and O. Bengtsson, "Discrete Gate Bias Modulation of a Class-G Modulated RF Power Amplifier",  presented at European Microwave Conf., London, 2016.