FBH research: 21.07.2017

Efficient power amplifier solutions for the future communication infrastructure (IMS2017)

Class-G system and narrowband power spectral density
Fig. 1: Class-G system (schematic) with a three-level supply modulator (top) and narrowband power spectral density before and after DPD linearization at 38.5 dBm output power with class-G modulation (bottom).
Reverse buck converter system and floating ground amplifier
Fig. 2: Reverse buck converter system (top) and floating ground amplifier (inset) with LF-RF ground coupling capacitors (bottom).
System including BST varactors and modulator & PA
Fi.g 3: System including BST varactors and modulator (schematic, top) and PA - modulator switch-cards (left inset) and packaged thick-film MIM BST varactor (bottom, left inset)

FBH’s RF Power Lab was well represented at this year’s international microwave symposium (IMS2017) in Honolulu. In the continued work on extending the bandwidth of discrete level (class-G) supply modulated systems, a 75 MHz modulation bandwidth system with 38.5 dBm average output power for a 10.6 dB peak-to-average power ratio (PAPR) signal was presented. It demonstrates that the system PAE can be improved by 13%-points for a 1.8 - 1.9 GHz amplifier targeting LTE FDD applications by implementing a three-level discrete supply modulation as shown in Fig. 1, (top) [1]. Despite the large bandwidth and the discontinuities in the discrete supply modulation, in-band (EVM) and out-of-band (ACLR) linearity can be fully restored using iterative learning control-based DPD (see Fig. 1, bottom). These results are unique and state-of-the-art in power and modulation bandwidth. The work is an extension of a project financed by the DFG.

Another interesting topic that was presented concerns an envelope tracking system targeting space applications. It is based on a reverse buck-converter supply envelope amplifier. The system topology is shown in Fig. 2, top [2]. By letting the buck-converter GaN-HEMT switch towards ground, parasitic capacitances in the switch are reduced, rendering an isolated gate driver in the switch superfluous. This has the potential to enable faster switching in the envelope amplifiers for larger modulation bandwidth and, due to the reduced delay, for larger bandwidth in closed-loop operation. However, with such a topology, a floating ground RF power amplifier is necessary. The floating ground amplifier was implemented using coupling capacitances very close to the transistor die as shown in Fig. 2, bottom. The modular reverse-buck system shows 10.7%-points PAE improvement operating at 1620 MHz with 41.6 dBm average output power. The results were achieved for an 8 MHz OFDM signal with 8.6 dB PAPR. As a first demonstrator of the concept these are unprecedented results for a floating ground system. The project is directly financed by ESA as an NPI project.

The third topic presented was a power amplifier with discrete dynamic load modulation based on barium strontium titanate (BST) varactors. The schematic of the amplifier (top) and the final amplifier (bottom) are shown in Fig. 3 [3]. In the paper, dynamic load-modulation of the power amplifier for wideband signals using BST varactors is shown for the first time. The dynamic power consumption is presented as well as the possibility to linearize such a system using iterative learning control DPD. The PA reaches a peak output power of 44 dBm for modulated signals, and an average PAE of 27.3% was measured. The work is conducted in cooperation with TU-Darmstadt in a project financed by DFG.

In the final day of IMS these results were also presented at a workshop organized by Andreas Wentzel and Olof Bengtsson from FBH. The topic of the workshop was efficiency enhancement and linearization techniques for future wireless telecommunication systems. The possibilities to extend the presented methods to future K-band solutions based on integrated MMICs were presented and discussed. The workshop included other excellent talks from industry and academia and was very well attended and much appreciated by all attendees.

References - IMS 2017

[1] Highly Efficient Class-G Supply-Modulated Amplifier With 75 MHz Modulation Bandwidth for 1.8 - 1.9 GHz LTE FDD Applications

[2] A 14 W Wideband Supply Modulated System With Reverse Buck Converter and Floating-Ground RF Power Amplifier 

[3] Thick-Film MIM BST Varactors for GaN Power Amplifiers With Discrete Dynamic Load Modulation