Active Filters Operating below 1 Hz

Second-order RC filters are synthesized using two voltage amplifiers with gains A1 and A2. As the natural frequency depends on A1 only, and the damping factor depends on A1 and A2, these factors can be chosen consecutively by varying the two gains. The passive components need not have variable or accurate values. This feature is particularly useful in the realization of filters operating below 1 Hz, and cutoff frequencies down to at least 10-3 Hz can be obtained by exploiting the high input resistance of MOS transistors.