Study and Design of Integrated Distributed Amplifiers for wideband optical transmitters in AI and cloud computing applications

Recently, the volume of data in several application, such as Artificial Intelligence and cloud computing, has undergone an extremely rapid increase. This have led to the need of improving the performances of the data centers that host these applications. In particular, one of the fundamental requirements in this field is to achieve extremely fast links between the racks that contains servers, cables, and other hardware. This means designing communication systems with higher and higher speeds and, so, larger bandwidths. These requirements became so strict that they cannot be met using copper-based connections, so other technologies started to be used. Nowadays, optical systems are widely adopted in the realization of medium and long range links because of their better performances in terms of bandwidth, power consumption and cost. These are composed of a transmitter (TX) that converts the electrical data into an optical signal, an optical fiber channel and a receiver (RX), which contains a photodiode which converts light into an electrical signal. This thesis is focused on the transmission side of the optical communication system, where it is fundamental to realize a CMOS driver able to produce large-amplitude voltages with an extremely large bandwidths. The state of art in broadband amplification is the use of T-coil as bandwidth extension technique. This solution has some limitations that make it not suitable for the new 224 Gb/s-PAM4 standard for high-speed data communication in data centers. An alternative to the T-coil is the use of Distributed Amplifiers which are the main topic of this work. This thesis is structured as follows: • Chapter 1 provides an introduction to high-speed communication systems, focusing on the problems and on the currently used solutions, in particular, the T-coil; • Chapter 2 contains a theoretical explanation of the Distributed Amplifier’s operating principle and of its non-idealities; • Chapter 3 is an overview on the passive elements, transmission lines and inductors, that are fundamental components of Distributed Amplifiers, and on how these elements can be integrated in CMOS technologies; • Chapter 4 focuses the design flow followed during this work, describing the used models, the design choices and the simulations that led to the final circuit implementation in 5nm finFET technology; • Chapter 5 compares this Distributed Amplifier with the more classical T-coil solution.