Talk:Active filter

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It seems to me that the diagram shown here is less an active filter than an ac coupled inverting amplifier because the filter component is inactive. That is, it doesn't rely on the amplifier for its function in any way except for the virtual ground the amplifier provides. Its filtering function could be duplicated by a passive RC filter except for the buffering effect of the amplifier. The more common active filter would be in the form of a Sallen-Key filter (non-inverting) or a multiple feedback filter (inverting) in which the amplifier provides positive feedback to shape the filter's response and make it roll off more quickly into the stop-band. If I can figure out how to upload a diagram, I'll substitute a better one. Anoneditor 23:01, 11 July 2006 (UTC)

But it's also an impedance buffer, which is a defining characteristic of active filters. — Omegatron 23:21, 12 July 2006 (UTC)

Maybe, but the same result (minus the signal inversion) would occur using a voltage follower after a simple RC filter, and that combination would hardly be characterized as an active filter. I think the defining characteristic of an active filter is the use of amplifier feedback to control the shape of the filter's response. To me, the buffering action is more ancillary (though certainly useful) than defining. In any event, don't you agree that the original figure wasn't ideal for the purpose of showing an example of an active filter? Anoneditor 05:41, 13 July 2006 (UTC)

You seem to forget the R1 resistor which is connected to the output ... it is a feedback. Majorkell 15:33, 24 August 2007 (UTC)

No, I haven't forgotten it. The discussion with Omegatron was over the diagram that pre-existed the current one, i.e., the one that existed prior to July 11, 2006.Anoneditor 20:14, 24 August 2007 (UTC)