There are couple of methods of designing band-pass filter. Some of them are cascaded HPF LPF band-pass filter, multiple feedback band-pass filter, biquad filter and state variable filter. Here state variable band pass filter design is illustrated with online state variable band-pass filter calculator.
The state variable filter schematic is shown below.
As shown above, a state variable filter consist of a summing amplifier and two integrators made using operational amplifiers. The two op-amp integrators each of which acts as single-pole low-pass filter combined forms a second-order filter. A state variable filter is primarily used as a band-pass filter but the filter also provides low-pass and high-pass outputs as shown in the above circuit diagram. For band-pass filter operation, the center frequency set by the two integrators(C1,R4 by first integrator and C2 and R7 by the second integrator) are usually made equal. This equal center frequency then is the center frequency in the passband.
Differences from other filter topologies:
- Sallen-Key Filter: The Sallen-Key filter is a simpler active filter configuration that uses operational amplifiers and capacitors to achieve low-pass, high-pass, band-pass, and notch filter responses. It typically employs two active components and offers good performance in a compact design but might have limitations in adjusting multiple parameters simultaneously.
- Biquad Filter: The biquad filter is a second-order filter topology that can be implemented using various configurations like Sallen-Key, state variable, etc. It's called "biquad" because it has two poles and two zeros in its transfer function. It provides flexibility in designing different filter responses and can be implemented using various circuit configurations.
- Twin-T Filter: The Twin-T filter is a passive filter topology that uses a combination of resistors, capacitors, and sometimes operational amplifiers to create a notch or band-stop filter. It's different from the State Variable filter, which is an active filter topology providing simultaneous outputs for low-pass, high-pass, and band-pass responses.
The State Variable filter stands out for its versatility in providing multiple filter responses from a single circuit, allowing for adjustment of various parameters, and offering flexibility in applications where different types of filtering are needed simultaneously.
