Amplifier efficiency is an important requirement for achieving cost-effective operation of any wireless network. High power-amplifier (PA) efficiency will no doubt be needed for Fifth-Generation (5G) wireless networks, and it was the focus of an investigation into the use of supply modulation or envelope tracking (ET) technology to enhance the efficiency of several different types of PAs. The research was performed by Zoya Popovic, a graduate professor with the University of Colorado at Boulder.
In a PA with ET, the power-supply voltage to the PA is dynamically varied according to the signal envelope to keep the amplifier in compression where efficiency is high. One challenge in implementing ET in a PA is maintaining broad bandwidth. In addition, supply modulation is inherently nonlinear, and additional linearization circuitry is often needed to compensate for some of the effects of the dynamic power supply. For an envelope modulator with high efficiency and large bandwidth, this implies a large slew rate. For a switch-mode power supply with pulse width modulation (PWM), high switching frequencies are needed for large bandwidths. Because of the complex impedance between the supply and the PA, a special filter is oftentimes needed to meet the envelope bandwidth requirements.
Traditional PAs with ET have been designed based on gallium arsenide (GaAs) device technology at lower (2 GHz and less) microwave frequencies. For his research, Professor Popovic chose to explore higher-frequency amplifiers based on gallium nitride (GaN) active devices—notably, those based on the Qorvo (TriQuint) 150-nm GaN on silicon-carbide (GaN-on-SiC) process. Amplifiers were first evaluated by measuring static power-added-efficiency (PAE) performance based on different supply voltages, such as +3.2 to +20 V dc for an X-band PA. These test results were used to determine a trajectory between the supply voltage and the input signal envelope so that a desired tradeoff could be achieved among efficiency, gain linearity, and output power.
Supply modulation was used to increase the efficiency of various PA architectures, including Doherty amplifiers, and ET PA designs were found to provide the best solutions for applications in which the output power was required to change over a large range, such as daytime versus night-time conditions. When sufficient ET bandwidth can be achieved, amplifiers designed with integral supply modulation can provide significant improvements in efficiency even when compared with Doherty amplifiers.
See “Amping Up the PA for 5G,” IEEE Microwave Magazine, Vol. 18, No. 3, May 2017, p. 137.