A Universal Low-Q Active-Filter Building Block Suitable For Hybrid-Integrated Circuit Implementation

A highly versatile second-order active-filter building block for low-Q applications (Q125) is described. The versatility is obtained by combining features inherent in the Sallen-Key circuit topology and those pertaining to hybrid-integrated circuits (HIC’s). A high degree of circuit stability is obtained by designing individual network functions to depend primarily on the characteristics of stable thin-film resistors and capacitors. Frequencyand Q-sensitivity are examined in detail. Methods of minimizing Q-variation with respect to the active and passive elements are discussed. Lower bounds on the Q-variation obtainable with the hybrid-integrated filter building blocks are given. All possible second-order network functions, including the allpass function, can be obtained with the building block. The resulting universality and standardization permit the complete design of active’ filters consisting of cascaded building blocks by computer.