Band-Pass Filters

Combining a low-pass element and a high-pass element by connecting them in series results in a band-pass filter circuit. As its name suggests, this type of circuit allows passage of a particular band of frequencies while attenuating frequencies outside this band. The band-pass circuit shown in the example below comprises a series-connected LR low-pass element (green background) and CR high-pass element (blue background).

A band-pass circuit's frequency response is produced through superimposition of the low-pass and high-pass frequency responses. The diagram below illustrates this in terms of the amplitude response.

If the two circuit sections are decoupled by means of an operational-amplifier circuit, the band-pass frequency response corresponds exactly to the superimposition of both partial frequency responses (ideal case). However, in the case of passive circuit sections such as those illustrated above, the second filter section (i.e. high-pass) acts as a load on the preceding voltage divider (i.e. the low-pass filter), so that the overall frequency response deviates somewhat from the ideal model.