Digital Transmitters

Digitally driven, switched transistors are more and more THE core elements to push many areas of electronics to a new, higher level of development and thus make them future-proof. Fast and efficient switching enables the connection of power to high-frequency electronics (power meets RF). It provides more efficient power management capabilities and maximum flexibility as well as compactness for the infrastructure of future mobile communications from sub-6 to 300 GHz (5G/6G). The target is the complete digitization of the components. This paradigm shift will significantly preserve resources and reduce CO2 emissions – a decisive driver for the worldwide project of green communications technology (Green ICT). All these goals are being pursued by the Digital PA Lab, which is working on digital transmitters for various areas.


Our topics focus on different applications from 100 MHz ... 300 GHz. In all cases, highly efficient high speed switching is required as a difference maker to push performances. 

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 Vpp 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.