1 Introduction
This chapter provides a short motivation and overview for the IC design process done during the lecture “Analogue and Mixed-Signal Circuit Design” that Prof. Dr.-Ing. M. Meiners gives in the graduate course Electronic Engineering M.Sc. at City University of Applied Sciences Bremen.
The chapter will start with the motivation behind the biquad IC filter design, outline the scope of work of the project, and ends with giving concrete specifications for the implemented filter.
1.1 Motivation
The design and implementation of analog filters is a cornerstone in signal processing, with applications ranging from audio processing to communication systems. Among these, second order filters, like the biquad filter, are versatile building blocks due to its ability to realize four types of second order filters - low pass, high pass, band pass, and band stop. This project focuses on the integrated circuit (IC) design of a biquad filter, to get insight into the theoretical and practical engineering considerations behind IC design.
For a deeper understanding of IC design, this project does not rely on off-the-shelf operational amplifiers for the filter design, but aims to implement the entire filter architecture at the transistor level. This approach not only deepens the understanding of analog filter behavior but also introduces the challenges and intricacies of IC design, such as layout constraints, power efficiency, and stability.
This project demonstrates the design process of IC design from theorectical modelling, over simulation and design constraints to prototyping and to learn hands-on experience with tools used during the design process.
1.2 Scope of the project
The scope of this project is supposed to follow a real-world design flow, starting at a theorectical analysis of the specified filter and - in the best case - end in a tape-out of a prototype. If that stage is reached the prototype can be compared to the theorectical and simulation results obtained during the design process and checked for functionality.
As a tape-out of a prototype is fairly unrealistic in the time given, the goal is to simulate the specified filter with templates for operational amplifiers and base the IC layout on these templates.
All in all, this project includes a systems analysis if the specified filter, simulation results with ideal components and real components, taken from provided templates, and a physical layout prototype.
1.3 Specifications
The main objective is to design a universal biquad filter, based on the filter design proposed in the ASLK PRO Board Manual from Texas Instruments (Rao and Ravikumar 2012). The biquad filter shall have the following specifications:
\[ f_0 = 1\,kHz \]
\[ Q = 10 \]
The circuit design is done in Xschem and the simulation in ngspice. For the design on transistor level the 130nm CMOS technology SG13G2 is used. All these tools and PDk are integrated into a docker image IIC-OSIC-TOOLS (Pretl and Zachl 2025) provided by Prof. Dr. Harald Pretl from Johannes Keplar University.
This documentation provides a development report, which documents the design process with the taken steps and decisions made.
1.4 Open-Source
All the results of this report and development approach to design a Biquad will be publicly available on GitHub. Everyone is invited and should feel free to use, change, and share this work. This whole course and project wouldn’t be possible without the great Open-Source tools provided by the amazing community of layout designers, enthusiasts, and developers. Here is a list with just a few of these programs: IC-OSIC-TOOLS, IHP Open PDK, Linux, Docker, Xschem, ngspice, KLayout, Quarto, Vim, Pandoc, LaTeX, TeXLive, Python, Git, CoCalc, LibreOffice.
1.4.1 Anthem
In realms where code is free to fly,
We build and share, our hearts reach high.
No walls, no locks, our wisdom streams,
In open light, we chase our dreams.
So sing the joy, the thrill, the spark,
In open source, we find our arc.
Together strong, we rise, explore—
In lines of code, forevermore.