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Ultra broadband envelope tracking systems for 5G utilizing class-S supply voltage modulator

Subject Area Electronic Semiconductors, Components and Circuits, Integrated Systems, Sensor Technology, Theoretical Electrical Engineering
Term from 2017 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 390907840
 
Final Report Year 2022

Final Report Abstract

The project addressed three topics: 1) A GaN power amplifier (HF-LV) for 26...28 GHz adapted for ET operation, 2) a digital modulator that modulates the supply voltage of the HF- LV according to the envelope of the signal, and 3) the integration of these two components into an envelope tracking (ET) system and its characterization. With regard to the MMIC power amplifier (HF-LV) one can state: • A coplanar HF-LV using 0.15µm GaN MMIC technology was realized at FBH, which delivers 2 W output power at 26 GHz. This is in line with the state-of-the-art for coplanar versions for frequencies above 25 GHz, but remains below the performance of microstrip versions. • For backoff operation, PAE is very low at 4% for 10 dB versus 11% for 0 dB. • This has a decisive influence on the potential for improvement by ET and Doherty concepts. In the project, an improvement of 2 percentage points from 1.7 % to 3.8 % was experimentally demonstrated with a static ET. • For Ka-band amplifiers with higher efficiencies, these values will increase accordingly, but a first estimate leads to improvements of only less than 10 percentage points even if 0 dB efficiencies reach 40 %. The digital modulator as the most critical component was investigated in particular detail: • As DC-DC converters do in general, the digital amplifiers based on the preliminary work show a degradation of efficiency at low output voltages (and thus low supply voltages of the PA). • Therefore, two other topologies based on the buck converter and boost converter were developed using FBH's 0.25 µm GaN technology. The boost converter could only be realized as hybrid circuit, because it requires powerful diodes, which were not available in a MMIC environment so far. Efficiencies in the 35...90 % range were achieved with the hybrid boost converter. • The demonstrated efficiencies are remarkable, especially in view of the parasitic effects caused by the design, but are not yet sufficient to realize an ET system in the K-band with convincing overall efficiency. With the components developed in the project, an ET system was set up and tested and its function demonstrated. Despite the use of a worldwide unique measurement set-up and a great deal of efforts, reliable broadband measurements with modulated signals proved to be a challenge and could not be carried out to the desired extent, so that the results had to be supplemented by extrapolations. Overall, three conclusions can be drawn from the results obtained: • The absolute improvement in power efficiency that can be reached by ET decreases with increasing frequency due to the deteriorating PAE values. This applies to the Doherty concept to a similar extent. The lower savings in DC power limit severely the power budget, which is available for additional components such as the modulator when using ET. • Envelope tracking with continuous modulation is of only limited use for larger bandwidths due to the low modulator efficiencies, even if digital approaches based on PWM are applied. Discrete modulation (class G), on the other hand, which only switches back and forth between different voltage values, promises better results and should be pursued. • Linearity becomes more important with increasing operating frequency, since with overall decreasing gains their voltage-related variation grows, which increases the linearization effort.

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