Basic Introduction to Active Filters and its Applications

[John Santiago]

Here is a short introduction to active filters and its applications. Active filters have comparable frequency selective performance when compared to passive filters. Active filters require power since operational amplifiers are involved to provide gain but do not require inductors since they can be large and lossy, especially for low frequency applications.


Briefly, let's begin the talk about the electric filter invented during World War I. These filters and vacuum tubes triggered the growth of telephone and radio communications during the 1920s and 1930s.

With the dawning of integrated circuits in the 1960, the OP AMP allowed combining filtering and amplification functions now called active filters. Here, in these series of videos, we'll show you how to design a wide range of analog filters.

These filters can be applied in instrumentation systems audio systems, communication systems, and even digital systems

So what is an active filter? To put it simply, it's a signal processor that amplifies, attenuates or reshapes the frequency content of input signals.

There's a whole variety of applications for these filters.

In communications systems, use filters to suppress noise, to isolate a single communication from many channels, to prevent spillover of adjacent bands, and to recover the original message signal from modulated signals.

In instrumentation systems, engineers use filters to select a desired frequency components or eliminate undesired ones. In addition, we can use these filters to limit the bandwidth of analog signals before converting them to digital signals. You also need these filters to convert the digital signals back to analog representations.

In audio systems, engineers use filters in crossover networks to send different frequencies to different speakers. In the music industry, record and playback applications require fine control of frequency components.

In biomedical systems, filters are used to interface physiological sensors with data logging and diagnostic equipment.

We recall that passive filters contain only resistors, capacitors and inductors. Although these circuits can be highly selective when losses are low, the response is highly resonant. But they cannot provide passband gains greater than one. In addition, they suffer from loading effects which makes the chain rule in cascade design inapplicable.

Here, we will emphasize an active filter as a circuit that contains only resistors, capacitors,
and OP AMPS.

Some of the advantages of active filters includes similar frequency selectivity performance when compared to RLC circuits plus having passband gains greater than 1. Because these filters have OP AMP outputs, the chain rule applies in cascade design. Also, they do not require inductors which can be large, lossy and expensive, especially in low frequency applications.

In active filter design, creating circuits realize a given transfer function T(s).

The stages in the cascade are either first-order or second-order active filters. The real poles in T(s) are produced during first order building block developed earlier.

The complex poles are produced by second-order building blocks. These second order filters will use the damping ratio and the undamped natural frequency parameters. Consequently, we design an active filter by controlling the poles introduced by each stage in a cascade connection.

In the future, I will be providing sample videos in designing these active filters. However, you will need to have a knowledge of passive filter design as well as understanding the concepts of bode plots.