An Active RC Filter Based on Simulation of LCR Filters with a Reduced Number of Operational Amplifiers


IEICE TRANSACTIONS (1976-1990)   Vol.E72   No.12   pp.1417-1424
Publication Date: 1989/12/25
Online ISSN: 
Print ISSN: 0000-0000
Type of Manuscript: PAPER
Category: Analog Signal Processing

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It is well known that the RC active filter simulating the conventional LC ladder filter has a lower sensitivity than the cascade RC active filter. A number of synthesis methods for this kind of filters have recently been proposed. A new class of simulation-type RC active filter with a reduced number of operational amplifiers is proposed in this paper. First, as a prototype filter to be simulated, an LCR filter is proposed, which is obtained by connecting resistors in series with capacitors in the conventional LC filter. As this type of filter has zeros on the negative real axis of s-plane, the conventional transfer function can not be used. Then, the transfer function for this type of filter is derived. The synthesis method of an LCR ladder filter with lossy capacitors is described. The relations between the voltages and currents of the above-mentioned LCR filter are derived, and circuits for realizing these relations and method for synthesizing an odd-order filter are shown. Finally, fifth-order low-pass filter is designed and it is shown that the measured response is in a good agreement with the theoretical value. Futhermore, the element sensitivity is estimated by a computer and it is shown that the sensitivity of the proposed filter is slightly higher than that of the conventional leapfrog filter. The number of required operational amplifiers becomes lower than half that of the conventional leapfrog realization.