Designing Analog Electronics

Part 3: Filters for AC Circuits

- By Andrew Levido

So far, the focus of this series has been on DC circuits and measuring DC quantities with some known degree of precision. In Part 1 (Circuit Cellar #424, November 2025), I discussed why error and uncertainty must be managed carefully in circuit design [1]. In Part 2 (Circuit Cellar #425, December 2025), I explored some techniques to extract a small signal of interest riding on a much larger offset [2].

Now it’s time to switch gears and look at AC circuits in more detail. Specifically, this installment will focus of the design of filters that are used to remove unwanted components of AC signals. It’s a big topic, so I am not aiming to be able to provide a comprehensive review; rather, I want to introduce some of the key concepts and leave you with some ideas and inspiration to explore further.

As usual, we will work through a couple of practical examples. But before we do that, we need to get a little theory under our belts.

WHAT IS A FILTER?

A filter is essentially a circuit with a gain that varies with frequency in a controlled manner. The signals with frequencies that we are interested in pass through, whereas the signals we want to reject are attenuated. These frequency ranges are called the “passband” and “stopband,” respectively.

Filters are described by four main characteristics—Shape, Order, Type, and Topology, which I will discuss in sequence.

FILTER SHAPE

The filter’s “shape” describes the relative arrangement of the “passband(s)” and “stopband(s).” These, respectively, are the ranges of frequencies that the filters either allow to pass through with little or no attenuation, or are blocked or heavily attenuated. Figure 1 shows the four main shapes filters can take.