Tunable Active Ladder Filters Using Operational Amplifiers

摘要

A form of active filter is described in which electronic integrators replace the reactive elements in the equivalent passive ladder networks. Some additional amplifiers are required in order to obtain the correct sign relationships between the various feedback and feedforward connections, but the filters may be m-derived (i.e. zeroes are introduced in the stop band to give a steeper cut off) simply by introducing additional interconnections. The apparent extravagance in the use of operational amplifiers is offset by the recent appearance of relatively cheap integrated circuits, and the fact that the filters may be tuned continuously over, say, a 10:1 frequency range by means of a simple switching technique. An unusual advantage of the method of realisation is that existing design data for passive filters may be used directly. Methods of compensating for finite amplifier gain and input resistance are described although this is not expected to be necessary in most practical cases. Design data for a range of one and two-section symmetrical Darlington filters and for Butterworth filters is given and also the frequency responses of some resistor-terminated constant-k filters. (Author)