Passive Filter

Filtering circuits are important to many electronic designs because they remove unwanted frequencies. Passive filters are a popular form; they are so-called because they do not need a power supply and depend only on passive inductive (L), capacitive (C) and resistive (R) components rather than active devices such as transistors.

Inductors pass low-frequency waveforms but offer increasing impedance to higher frequencies; capacitors do the opposite. Accordingly, low-pass filters can be built with a resistor in series with an inductor or in parallel with a capacitor as the reactive element. Similarly, a high-pass filter comprises a capacitor in series with or inductor in parallel with a resistor, so low frequency signals are attenuated while higher frequencies are passed.

These simple passive filters are called single element types as they have just one reactive element (L or C). More complex filters, with more reactive elements, are also available. An L filter comprises two reactive elements, one in series and one in parallel. Three reactive elements can be integrated into ‘T’ or ‘п’ topologies to implement low-pass, high-pass or band-pass filters. These filters can have very high impedance at low frequencies, and very low impedance at high frequencies, or vice versa. They find widespread application within transmission lines.

L, T and п filters can be further built into multiple element types, usually constructed as a ladder network. These types are typically used to improve some filter parameter such as stop-band rejection or slope of transition from pass-band to stop-band.