Originally posted by: Born2bwire
Originally posted by: TecHNooB
How come Sallen Key filters use additional circuit elements to enhance/attenuate the gain. Couldn't you use magnitude scaling and get the same result? Or does magnitude scaling not work. I'm currently under the impression that magnitude/frequency scaling works for any linear circuit (given a transfer function).
Taking a quick look at the wikipedia article, they show the Sallen Key filter having a unit-gain amplifier. The reason for a unity-gain amplifier, at least in the circuit I see on wikipedia, that I can think of is that it isolates the filter from the load. If you place a load on a filter, the load's impedance can affect the filter's characteristics. A unity-gain buffer will show the filter it's infinite input impedance and it will isolate the properties of any load. Thus, the filter is stable across any load that you place on it and you can also assume that the load will be presented a zero impedance from the buffer's output.
It seems TI has an article discussing the filter, why don't you take a read through that? I think though the answer to your question is that the amplifier circuit gives rise to the benefits that I mentioned above.
http://focus.ti.com/lit/an/sloa024b/sloa024b.pdf