Active Filters: Types and Applications in Electronics
Active Filters
As you know, the characteristic that determines an electrical signal is frequency. In many practical cases, a circuit may carry more than one electrical signal, i.e., electrical signals with different frequencies can pass through. However, there may be circumstances where only one of these signals is of interest. This “selection” of an electrical signal by frequency is what filters do.
Initially, filters were composed solely of passive elements, i.e., resistors, capacitors, and inductors. However, the emergence of the operational amplifier has brought significant improvements to filter manufacturing, as it has made it possible to dispense with inductances. The improvements achieved by replacing inductances with operational amplifiers are significant in terms of response, energy use (less dissipation), size, and weight. Inductances cannot be integrated into a circuit and are, therefore, discrete elements with a considerable size.
The disadvantages of these filters (RC active filters) compared to filters made of passive elements (RLC filters) are the constraints in voltage and current levels and parasitic effects induced by active elements, such as voltage displacement, output current, bias current at the entrance, and so on. However, in most applications, the advantages of RC active filters over passive RLC filters are more numerous, hence they are gaining increasing importance in the field of engineering.
Active filters are circuits composed of resistors, capacitors, and operational amplifiers. Their purpose is to allow the passage of frequencies for which they were designed, thus attenuating the rest of the frequencies that are not of interest. This is done by attenuating or even canceling those whose frequency is not in the allowable frequency range.
Types of Filters
There are basically four types of filters:
- Low-pass filters: Allow the passage of low frequencies.
- High-pass filters: Allow only high frequencies to pass through.
- Band-pass filters: Allow only a certain range of frequencies to pass.
- Band-reject filters (or notch filters): Designed to attenuate or eliminate specific frequencies.
A low-pass filter only allows the passage of signals with frequencies less than f1.
A high-pass filter only allows the passage of signals with frequencies greater than f1.
In a band-pass filter, only signals with frequencies between f1 and f2 pass.
In a band-reject filter, signals with frequencies between f1 and f2 are the only ones that do not pass.
The range of band-pass filters obviously depends on the elements used in their construction and can, therefore, be selected as desired.
The use of filters has increased considerably in the last two decades, to the point that entire volumes are dedicated to them.
Schematic of a Band-Pass Filter
As an example, a basic active band-pass filter is shown below. The values of the capacitors and resistors, along with the characteristics of the operational amplifier used, will determine the range of frequencies that can pass through the filter.