Certain passive filters incorporate expensive inductors designed not to exceed unity gain. These filters may exhibit reduced attenuation efficacy at frequencies below the audio range.

Active filters consist of operational amplifiers, resistors, and capacitors.

Employed in audio processing, telecommunications, and power supply units, these filters are categorized as first-order low-pass, first-order high-pass, band-pass, and band-reject filters.

Consider the transfer functions and frequency responses of low-pass and high-pass active filters.

Unlike passive filters, the finite low-frequency gain in low-pass active filters and the high-frequency gain in high-pass active filters depend on the feedback-to-input resistor ratio.

Low-pass filters attenuate frequencies above the corner frequency, while high-pass filters suppress those below the corner frequency.

The combination of low-pass and high-pass filters results in a band-pass filter, while a band-reject filter combines both types with a summing amplifier.

Band-pass filters transmit frequencies between the corner frequencies and reject those outside; band-reject filters block frequencies within the range and allow external frequencies.