What’s the difference between a band pass filter and band stop filter?

Band pass filter and band stop filter (or notch filter) are devices designed to pass or block a specified range of frequencies. But what exactly is the difference between them, and how do they actually work?

In this article about band pass filters and band stop filters, we’ll give a brief overview about both of them and explain the theory behind them. 

Band pass filter

A band pass filter is a device specially designed to pass frequencies within a certain range, while rejecting (attenuates) frequencies outside of that range. It allows through components in a specified band of frequencies called its “passband”, while at the same time blocking components with frequencies above or below this band.

The band pass filter stands in contrast to low and high pass filters. Where the high pass filter allows components with frequencies above a certain threshold frequency, where the low pass filter does the opposite – allowing through components below a specific frequency. 

Both the band pass filter and band stop filter are characterized by their Q-factor, where the Q-factor is the reciprocal of the fractional bandwidth. A high-Q filter will have a narrow passband, while a low-Q filter will have a wide passband. Also referred to as “narrow-band” and “wide-band” filters.

A good example of a band pass filter would be the tuner on a radio, only allowing the desired radio station to pass, but rejecting all others.

What’s an active band pass filter?

An active band pass filter is a band pass filter that provides amplification to the input signal. It is a cascading connection of high pass and low pass filters. In contrast, a passive band pass filter is a combination of passive high pass and passive low pass filters. It only uses passive components like resistors, capacitors, and inductors. 

Band stop filter

While there is a difference between a band pass filter and a band stop filter, they are very similar. Instead of passing through frequencies within a certain range, a band stop filter stops/attenuates those in a specific range toi very low levels, and let’s most frequencies pass unaltered. 

A band stop filter would be like using a graphical equalizer on a sound system and pushing all controls to the top except one. As such, one range of frequencies would be cut way down, but all the other would be allowed to pass through.

And that’s pretty much it. In conclusion:

A band pass filter attenuates frequencies below and above a specified bandwidth and allows frequencies within a specified bandwidth to pass through. While a band stop filter attenuates frequencies between a specified bandwidth and allows all other frequencies to pass through.