- Published in issue 6/1994 on page 22
RC active filters
Could the authors of Practical Method of Designing RC Active Filters which appeared in the March 1955 issue of IRE Transactions on Circuit Theory) have expected that the unity-gain low-pass and high-pass sections would still be highly popular 40 years later? Perhaps. What is certain is that this popularity is not misplaced: only two resistors, two capacitors and an operational amplifier are needed to construct a unity-gain, second-order filter with a high input impedance and low output impedance. Several of such second-order sectians can be cascaded to obtain a higher even-order filter. Formulas for calculating the component values to obtain given cut-off frequencies and filter types abound. Unfortunately, almost without exception, these are intended for calculating the capacitor values and assume equivalent resistors.