GaN-HEMTs in Envelope Tracking systems for telecommunications
Fig. 1: Two-tone signal and illustration of the losses (grey) for a fixed supply voltage system (green) compared to a modulated supply voltage system (red).
Fig. 2: Circuit topology of envelope tracking system based on a HSA as voltage modulator, switch part of the HSA see red rectangle.
In the past years, much effort has been devoted to increase the efficiency in base-station systems for telecommunications. The high peak-to-average-power-ratio (PAPR) signals used in modern wideband systems like WCDMA or long-term-evolution (LTE) force the power amplifiers (PAs) to work in a very low efficient back-off region to meet the system linearity requirements. Therefore, much attention is now devoted to increase the back-off efficiency. One way to achieve back-off efficiency is to modulate the supply voltage with the instantaneous power of the signal, i.e. the envelope. The concept is known as envelope tracking (ET) and the principle is shown in Fig. 1.
When the instantaneous signal power in a modulated signal is low, the voltage difference between supply voltage (Vdd) and signal is large, leading to large losses. If the supply voltage for the amplifier is varied with the instantaneous power of the signal instead, the losses can be minimized and a very efficient transmission of the signal is achieved. One of the most promising transmitter topologies to achieve modulation with the required bandwidth and power is the so-called hybrid switching amplifier (HSA) as shown in Fig. 2.
Fig. 2. Circuit topology of an envelope tracking system based on a HSA as voltage modulator. The switch part of the HSA can be seen in the red rectangle. It consists of a wideband linear amplifier connected to a highly efficient switching stage shown in the red rectangle. When the RF power amplifier (PA) requires more current, the voltage drop across the sense-resistor and the switch (SW) goes active and provides the required current.
Traditionally the switching stage is implemented in Si-MOSFET technology. Therefore, a project was initiated to investigate the FBH GaN-HEMT technology as replacement for Si-MOSFET in hybrid switching amplifiers for wide bandwidth signals. A modular design where the alternative switch technologies could be tested under equal conditions was developed. The project was successful and although un-optimized for the modulator design, the GaN-technology showed already similar performance as Si-MOSFETs. The work was presented at the German Microwave Conference (GeMiC) in Ilmenau in March 2013 and received the best paper award.
The work is now being continued and an improved GaN-HEMT switching stage is being developed. A more powerful HSA that can handle 30 W at a 28 V supply voltage is also being designed. Together with a GaN-HEMT based PA optimized for supply modulation, which was also presented at GeMiC, it will be tested as a full envelope tracking (ET) supply modulated system in the coming months.
In parallel to the design work on the supply modulator, there are ongoing activities to improve the performance of the FBH GAN devices for supply modulated applications. Measurement and post-processing methods have been developed that emulate supply-modulated conditions directly during on-wafer characterization. These were presented last year in the Frequenz magazine.
Publications
R. Perea-Tamayo, O. Bengtsson, P. Landin (University of Gävle, Sweden), W. Heinrich, "A modular hybrid switching amplifier for wide-bandwidth supply-modulated RF power amplifiers", 7th German Microwave Conference (GeMiC), 2012, 12-14 March 2012, IEEE Conference Publication (2012).
A. Raemer, O. Bengtsson, W. Heinrich, "Software optimization of a supply modulated GaN-amplifier for baseband access ET systems", 7th German Microwave Conference (GeMiC), 2012, 12-14 March 2012, IEEE Conference Publication (2012).
O. Bengtsson, S. Chevtchenko, R. Doerner, P. Kurpas, W. Heinrich, "Load-Pull Investigation of a High-Voltage RF-Power GaN-HEMT Technology in Supply Modulated Applications", Frequenz. Volume 65, Issue 7-8, Pages 217–224, ISSN (Online) 2191-6349, ISSN (Print) 0016-1136, DOI: 10.1515/freq.2011.030, August 2011.
FBH research: 07.06.2012