Filters are configured by connecting capacitors and inductors in networks, and their schematics suggest letters or other familiar symbols. The four most common configurations are the L, T, pi, and ladder. The positions of the elements are determined by the desired function of the filter (e.g., low pass or high pass). The L filter schematic is shaped like an inverted letter L, and the T filter is shaped like the letter T. The pi filter schematic looks like the Greek letter π, as shown in Fig. 1, and the ladder filter looks like a ladder.
All capacitors can pass AC, and high frequencies pass with less opposition than low frequencies. (Capacitive reactance is inversely proportional to frequency.) But because a capacitor has conductive plates separated by an insulating dielectric, DC is completely blocked. By contrast, inductors, basically coils of wire, easily pass DC and very low frequency AC, but their ability to oppose AC is directly proportional to frequency because inductive reactance is proportional to frequency. Thus, passive filters exploit the frequency-response characteristics of capacitors and inductors.
Figure 1 Pi filter for a power supply.
