A generalized circuit diagram of the two-pole state-variable active filter is shown. The state-variable filter can be inverting or noninverting and can simultaneously provide three outputs: low-pass, bandpass, and high-pass. A notch filter can be realized by adding one summing op amp. In the state-variable filter circuit, one amplifier performs a summing function and the other two act as integrators.
State-variable-active-filter - schematic

The choice of passive component values is arbitrary, but must be consistent with the amplifier operating range and input signal characteristics. The values shown for Cl, C2, R4, R5, and R6 are arbitrary. Preselecting their values will simplify the filter tuning procedures, but other values can be used if necessary.


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