As the name suggest low pass filter circuits are designed to pass or conduct a preferred range of frequency lower or below a desired cut-off threshold, and attenuate or gradually block the frequencies above this value.
Normally opamps are employed for making such filter circuits, since opamps are best suited for these applications due to their extremely versatile characteristics.
The following graph provides the typical low pass filter frequency response with regards to the gain, we can clearly see how the response attenuates (gradually drops) as the frequency increases past the particular cut-off threshold.
The following images depict the standard opamp based low pass filter circuits. The first one needs to be powered by a dual supply, and the second one works using a single supply voltage.
The components R1, R2, and C1, C2 configured with the non-inverting (+) and the inverting (-) input pinouts of the opamp basically decide the cut-off range of the filter, and these need to be calculated optimally while designing the circuit.
For calculating these parameters and designing a low pass filter circuit quickly within minutes one can utilize the following formulas and the explained steps:
First we need to find C1 which we can do by selecting any value arbitrarily as per our convenience.
Next, we can calculate C2 with the formula:
C2 = C1 x 2
R1 and R2 can be identical, and can be calculated using the following formula:
R1 or R2 = 1 / 2 √2 x π x C1 x Frequency.
here the "frequency" is the range where the cut-off transition is expected to happen, or the desired cut-off range.
The values of Cin and the Cout shown in the single supply low pass filter are not critical and these can be anything 100 to 1000 times that of C1, meaning if you selected C1 as 0.1uF, then these could be anywhere between 10uF and 100uF etc. The voltage spec may be selected to be twice that of the supply voltage used.
The resistors are all 1/4 watt rated, 5% or 1%.
That's it!.... using the above simple technique you can design a reasonably good low pass filter quickly and use it for a specific application which could include a high bass music circuit, an active speaker cross network or a home theater system etc.