PASSIVE AND ACTIVE FILTERS

THE MAGAZINE FOR COMPUTER APPLICATIONS
Circuit Cellar Online offers articles illustrating creative solutions
and unique applications through complete projects, practical
tutorials, and useful design techniques.

PASSIVE AND ACTIVE FILTERS

Technically Speaking Passing and Rejecting Signals Based on Their Frequency

by James Antonakos

Start ı A Filter by Any Other Name ı Whatıs the Frequency, Kenneth? ı A Closer Look ı Itıs Just a Phase ı Whatıs Load Got to Do with It? ı Activate That Filter! ı Filters to the Rescue ı Sources and PDF

A FILTER BY ANY OTHER NAME

There are four basic filter types (see Table 1). I will concentrate mostly on low-pass and high-pass filters. With a little creative thinking, itıs not difficult to see how band-pass and band-reject (or notch) filters are just combinations of low-pass and high-pass filters.

Filter	Operation
Low-pass	Passes low frequencies
High-pass	Passes high frequencies
Band-pass	Passes a range of frequencies
Band-reject	Rejects a range of frequencies
Table 1ıThe four basic types of filters can be seen here. Each filter reacts differently over its frequency response.

PASSIVE VERSUS ACTIVE

A passive filter is a filter that does not have any active components like transistors or op-amps. For example, a simple low-pass filter that is built out of a resistor and capacitor is a passive filter. The output signal can never have more power than the input signal, because resistors, capacitors, and coils have no gain themselves.

A filter that incorporates one or more transistors or operational amplifiers is an active filter. Here, the output signal can be larger than the input signal when it is passed by the filter.

PREVIOUS • NEXT

Circuit Cellar provides up-to-date information for engineers. Visit www.circuitcellar.com for more information and additional articles.
For subscription information, call (860) 875-2199, subscribe@circuitcellar.com or subscribe online. ıCircuit Cellar, the Magazine for Computer Applications. Posted with permission.