Active filters are electronic circuits used to filter signals by selectively passing certain frequencies while attenuating others. Unlike passive filters that use passive components like resistors, capacitors, and inductors, active filters incorporate active components such as operational amplifiers to achieve greater control and performance. Active filters offer advantages like adjustable cutoff frequencies, better selectivity, and lower sensitivity to component tolerances compared to passive filters. Basic active filter circuits with a limited number of components can be relatively affordable. However, more advanced active filters with features like high-order filtering, programmability, and precision components can cost tens to hundreds of dollars, especially for specialized or high-frequency applications. When purchasing active filters, determine the desired filter type (low-pass, high-pass, band-pass, or band-stop) and the required cutoff frequency and filter order. Additionally, consider the input and output impedance compatibility with your system, as well as the voltage and current ratings to ensure proper integration.