Digital Transmitters

The demand for power electronics having greater compactness, better manufacturability, and higher performance has motivated a strong increase in switching frequencies to lower GHz-range. High frequency DC/DC converters can significantly reduce size of the passive components used for conversion output networks and energy storage. This leads to reduced form factor and weight, higher power density and faster transient response. Moreover, there is potential for monolithic integration of very compact supplies with greatly reduced parasitic inductances. Based on FBH 0.25 µm GaN prcoess we realize monolithic converter ICs with highly efficient gate drivers. They represent the core unit for different applications like buck converters for 48 V PoL energy conversion or supply modulators.

In addition, as of 2022 AlN-based VHF power converters will be designed in the DiPA Lab in the framework of the DFG Priority Programme 2312 "Energy Efficient Power Electronics ''GaNius'".  

Patented digital modulator concept with highest linearity and integrated lossless DPD provides bit sequences perfectly tailored for digital switching PAs of any technology. Also, it can realize time delays with ps accuracy between several transmitter branches for phased arrays in digital beamforming networks.  
To expand the application fields the modulator concept is planned to be implemented on CMOS in a joint effort with TU Berlin and represents together with an integrated PA a novel transmitter concept for more flexible, energy-saving handsets.

The GaN-based digital transmitter activities focus on sub 6 GHz MIMO systems with digital beamforming. One path covers the integration with highly robust LNAs to digitally enhanced transceivers, mainly based on strained engineered GaN-HEMTs. Another path focuses on field tests in a real 5G environment. Access to such a testbed is granted through FBH membership in 5G Berlin e.V. association.

Besides the activities on the actual GaN-based digital amplifier part, there is another important block in the fully digital transmitter chain that we are working on:

The otherwise common SiGe or BiCMOS digital pulse sources cannot directly drive GaN transistors digitally due to insufficient voltage amplitudes. In addition to 5 V_pp amplitude, DC-coupled digital and multistage GaN power amplifiers require a fixed DC potential at the input. This must be dynamically adaptable to different broadband input modulation schemes. A novel GaN-based driver amplifier module with integrated DC potential shifting meets all these requirements. The central element of the compact module is a GaN chip developed in the Digital PA Lab. The preamplifier provides a controllable shift between -1.5 V and -11 V. A voltage gain of 11 is furthermore achieved at a GaN HEMT typical input impedance. The 3 dB bandwidth is DC -3.2 GHz and the typical power consumption is 2.3 W.

In addition, a CMOS-based solution with extended breakdown voltage is also being worked on in parallel. This is being pursued in collaboration with the TU Berlin and the Mixed-Signal Circuit Design - Group of Prof. Gerfers.

We think Green ICT not only in the here and now, but already well ahead for 6G applications and beyond. Green ICT will also be a major focus in intelligent phased array sub-THz transmitters up to 300 GHz. Most efficient novel transmitter concepts with switching amplifiers are applied to the InP-based 6G circuits for wireless communications through intelligent designs at the limits of the technology. These highly efficient transmitters drive the green aspect of the massive scale arrays required for 6G. Interconnection with suitable high-speed SiGe technology based on 3D heterointegrated chiplets for more functionality is also planned.