A band-pass filter passes a specific range of frequencies while preventing the passage of all others. Passive LC band-pass filters have been used for many years but the tuning procedures are difficult. These difficulties include removing or adding turns to the inductor and the need for specific capacitor values. Although a passive LC filter uses only inductors and capacitors, an active band-pass filter uses an op amp plus a few resistors and capacitors, but it is physically
smaller and requires less PC board space. The primary disadvantage of using an active filter is that it requires a power supply for the op amp.
The circuit in Figure 3-15 is a two-pole active filter using a TL081 op amp. This type of circuit is usable only for Qs less than 10. The gain of this stage is nominally or slightly larger than the square root of the Q. For example, with a Q of five, the gain chosen would be slightly over two.
The component values for this filter are easily calculated from the following equations. Assume you want to build a filter with a center frequency of 800 Hz. R2 is a potentiometer about twice the calculated value, which is then adjusted to set the resonant frequency precisely. This larger than calculated resistance value is used to compensate for the tolerance of the other resistor and capacitor values. A capacitor value of 0.01 µF to 0.1µF is often used for filters in the audio range.