# Determining Circuit for Active Band-Pass Filter

Discussion in 'General Electronics Chat' started by TrevorP, Dec 26, 2009.

1. ### TrevorP Thread Starter Active Member

Dec 8, 2006
55
0
Hey all,

I am working on a little hobby circuit and I've already designed a low-pass Butterworth filter (fc = 150Hz), and a high-pass filter (fc = 500Hz) using Sallen-Key configurations. They both are simulating exactly as I want them too. Now, I'd like to design a band-pass filter with corner-frequencies at 150Hz and 500Hz.

Computing the log-average I found that my center frequency should be at 273.9Hz.

Do I still assume that my Q-factor will be 1/sqrt(2)?
If so, will this be equivalent as if I was to cascade a high-pass filter with fc = 150Hz with a low-pass filter with fc = 500Hz?

I'm ideally looking for a solution using only one op-amp. Also, I'm using a single sided supply, but I do have Vcc/2 available but the opamp will have to be powered with Vcc and 0V (for it's Vcc+ and Vcc-). The signal has already been biased.

Feb 4, 2008
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3. ### electrojim Member

Jun 3, 2009
11
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What's your application? If you need a band-pass filter with flat response across the passband, then you will need cascaded HP and LP stages, and probably high-order filters at that. This necessarily means more than one op-amp. If, however, you need a simple "tuned amplifier" (like a tuned L/C circuit with a given "Q"), a one-op-amp circuit ought to be just fine.

4. ### TrevorP Thread Starter Active Member

Dec 8, 2006
55
0
Yeah basically a crossover for a guitar. I do intend to use LR filters (cascaded HP and LP stages). I managed to create the mid-band filter by cascading an HP and LP then inverting the signal about Vcc/2 so the phase shift isn't destructive apon adding the signals again. There is about 3dB gain at the cut-off frequencies as would be expected, but cascading should solve that problem quite nicely.