Cascaded RC Bandpass filter loading problem?

Question

So I calculated the Highpass stage output impedance to be 22kohm and the input impedance of the Lowpass stage is 430kohm when f=5khz, still in the output I am getting half of my supply voltage despite the input impedance of the second stage being much larger than the output impedance of the 1st stage, I even tried putting a buffer op-amp in between the two stages and that didn't change anything. other than the two impedance of the cascaded stages, what am I forgetting to consider ?

*Note that the bandpass is between 5khz and 5.5khz and on the oscilloscope it's 5v/div.

Your design is good. But your bandpass Q requirement is ten...a single-stage RC filter like this has a Q less than one - not good enough as you have discovered. Look for an active op-amp RC bandpass filter to achieve your desired Q of ten. OR if you must use passive components (no op-amp), then LCR filter is required to achieve decently narrow passband. — glen_geek, Mar 08 '19 at 00:20

score 0 · Accepted Answer · answered Mar 08 '19 at 05:25

What you're forgetting is the -3dB cutoff point for the two filter stages. Recall that $$ f_c = {1 \over {2 \pi R C}} $$ Reference 1, Reference 2

For both your first stage and second stage we calculate a cutoff frequency of right around 5kHz (4974 and 5488, if I'm doing my math correctly) which means we'd expect each stage to knock the voltage down by about 3dB (0.707 times its original value), and both stages together to knock down the voltage by 6dB (0.707 * 0.707 = 0.5, approximately). And that's exactly what we're seeing.

My math checks out with yours; the second stage has a much higher impedance than the first, so it makes perfect sense that adding buffers would do nothing to change the situation.

Uhhh makes perfect sense, thanks. – BigRedMachine Mar 08 '19 at 12:29 — BigRedMachine, Mar 08 '19 at 12:29

Cascaded RC Bandpass filter loading problem?

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