# RC Low Pass filter, capacitor in series or in parallel?

I've seen 2 variations of the RC low pass filter, sometimes with both the resistor and capacitor in series, and more commonly with the capacitor going straight to ground. What is the difference between these variations?

• The difference is the the 1st one is not a low-pass filer, while the 2nd one is... – brhans Aug 17 '20 at 0:52
• Ramsden, look up the dashpot/spring analogy for resistors and capacitors. And keep in mind (something totally missing in your schematics but should be there) that an "input" and "output" as you show it is actually always also involving a reference point (usually ground.) So, if the input rises, it rises with respect to ground (not shown.) If the output rises, it rises with respect to ground (also not shown.) So long as you are talking about voltages, anyway, there's always two points to consider. Do not lose sight of that fact when thinking about dashpots and springs. – jonk Aug 17 '20 at 3:12

Think of the capacitor as having very low resistance at high frequencies and very high resistance at low frequencies.

So in your top diagram the high frequencies zip through the cap to the output while low frequencies have a harder time. So it’s a high pass. It doesn’t make any difference if you put the cap in front of the resistor.

In the bottom diagram the high frequencies are shunted to ground, leaving only the low frequencies to make it to the output. So it’s a low pass.

• Thank you! Is there any benefit to using a high pass filter like in the first diagram, rather than the more commonly used version with a capacitor, then a resistor going to ground, like the second diagram with the components reversed? – J.Ramsden Aug 17 '20 at 1:37
• In your top circuit, very high frequencies see a total resistance of R between input and output (because C's resistance is low). If you put just a C between input and output, and R to ground, then the high frequencies see very low resistance between input and output. Why you'd choose one or the other will likely depend on the characteristics of the connecting input and output circuits. – td127 Aug 17 '20 at 2:09

First image is a high-pass filter. Second image is a low-pass filter with a -3dB frequency of $$\{1/2piRC}\$$. Without a resistive or inductive load the frequency (-3dB) of the high-pass filter is unknown.

• So what about the other 2 combinations (components swapped), with a capacitor then resistor in series, and a capacitor in series with a resistor in parallel? – J.Ramsden Aug 17 '20 at 1:08
• HFP vs LPF ?... – Tony Stewart EE75 Aug 17 '20 at 1:24
• @J.Ramsden In terms of components in series, it makes no difference what order they are in. A capacitor in series with a resistor to ground is a classic high-pass filter. The same equation applies, but it becomes the pass-band instead of the roll-off point. – user105652 Aug 17 '20 at 2:13
• I should add that after series RCL parts, whether it it high-pass or low pass depends on the next component to ground. If a capacitor, then it is a LPF (Low pass filter). If a resistor or especially an inductor it is a high-pass filter (HPF). With values close to the values in series then you get band-pass effects, especially with a LC to ground. The math for so many parts gets complex. – user105652 Aug 17 '20 at 2:20