Band Pass Filter Calculator

This calculator is a more specialized version of the Bandpass Filter Calculator. I found that I was constantly going back to this design, even though it adds a resistor. Having the Ro and RQ1 resistors separate allows the Q and gain of the filter to be adjusted independently from the center frequency, which is an advantage under some circumstances.

The Q and gain are linked together - adjusting one affects the other. RQ1 and RQ2 resistors are adjusted proportionally. To increase the gain and Q, multiply Ro by a factor to get RQ1, and divide Ro by the same factor to get RQ2. The responses below were obtained for various values of Q and gain:

Notes:

• Practical limits are a Q of about 50 on the high end, and 1 on the low end.
• The open loop gain of the op amp selected must be at least 40 dB greater than the gain of the op amp at the resonant frequency.
• The input impedance of the op amp becomes important as RQ1 increases in value, for large values of Q you may need an FET input type op amp. You can combat this by scaling the R's and C's, but remember you need to use an NPO type capacitor for stability.