# RC bandpass filter, what's first: low-pass filter or high-pass filter?

I want to do a RC bandpass filter can I use low-pass or high-pass at the beginning or does it have a specific sequence? First low-pass then high-pass, or first high-pass then low-pass?

• Maybe, you could consider using clear English phrasing, indicating such things as "should I put the low frequency suppression in the signal path first" in stead of "low first". – Asmyldof Oct 5 '14 at 21:22
• what sort of bandwidth are you considering and whereabouts in the spectrum. – Andy aka Oct 5 '14 at 22:13
• A pass-band from 220hz to 440hz, its for a project where I want to sample some music in that frequency range – Pulse9 Oct 6 '14 at 0:55
• What load impedance will be connected to the filter? I'm asking this because a bandwidth as stated is quite tight for RC filters to cope adequately. The arrangement is immaterial btw but the implementation may be difficult given what you expect the out of band attenuation to be. – Andy aka Oct 6 '14 at 7:37
• I want to connect this microphone link and filter just 220hz-440hz – Pulse9 Oct 6 '14 at 11:47

If you need a bandwidth of B=440-220=220 Hz the center frequency will be app. at Fo=311 Hz. As a consequence, the required quality factor of your bandpass will be Q=311/220=1.4

Please note that it is NOT possible to realize a bandpass with such a selectivity based on the mentioned approach (lowpass-highpass series, or vice versa).

Therefore, you either need a RLC bandpass configuration or an active RC bandpass topology.

If you put the low blocking stage first, your first capacitor will be very close to zero impedance to the higher frequency signal being cut off by your high blocking capacitor.

If you put your high blocking stage first, your high blocking will be close enough to high impedance to at low frequencies to have little effect.

At the point where they start interfering with each other, you are trying to create too narrow a band with a simple RC circuit and will need to look at LC stages or active filters.

First stage should be a low pass filter with the cuttoff frequency of 440Hz.So this low pass filter allows frequencies in the range 0 to 440Hz.

Second stage is high pass filter with the cuttoff frequency of 220Hz. So this high pass filter allows frequencies in the range 220Hz to 440Hz.(Since the maximum frequency allowed by the first stage is 440Hz).

• I am afraid, this won`t work. Without a buffer between both stages we have a damping (referred to the midband gain) at both band edges (220, 440Hz) of only 0.14dB. With a decoupling buffer we have 0.36 dB. This does not fulfill the requiremts of (as I assume) -3dB at these frequencies (see my detailed answer regarding the quality factor). – LvW Oct 15 '14 at 12:20

I like this question, If it's just an RC's with no buffer/ opamp in between, then you need to think about the impedances. You'd like the low impedance in front.