A New Technique for Programmable Very Low Frequency Filtering

A new technique for obtaining a very large time-constant integrated filter is proposed. The time-constant of the filter is made proportional to r0C, where r0 is the output impedance of a transistor. Since 1/r0 is proportional to the bias current, the filter cut-off frequency is linearly programmable. The technique is demonstrated ex- perimentally using a simple circuit.