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Isolating RC filter from circuit..

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Newbie level 4
Jun 7, 2015
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5 order active filters are generally illustrated as in picture with sallen-key toplogy..

the first order part, RC circuit, added the beginning or the end of the filter with a voltage follower..

in both case, for example in picture, we do not isolate the RC circuit from previous stage, so RC can

consume power, right ? than what should we do ? add another voltage follower before RC?


and why do we add that voltage follower at first ? is it so important that isolate 1st order RC circuit from second order part ?

and finally,

if i want to isolate the filter completely from previous and following stages,
what about hiding first order part in feedback loop? like this:

i will be happy if you correct me where i am wrong..

The voltage follower does two things:
1) It isolates the 1st order RC circuit at trhe input from the second order part.
2) It provides an important extremely low output impedance to drive the second order part.

If you put the input RC stage in the feedback of the opamp then the (inverting) opamp inverts the phase shift of the entire circuit.

As the name "active filter" suggests, the topology needs an active element, e.g. an amplifier to achieve the intended function. If you bother yourself with circuit analysis, you'll notice that the sallen key transfer function can be only implemented with a voltage controlled voltage source respectively a buffer.

Without active elements, it's impossible to realize complex poles by using only R and C elements.

If you only refer to the first order filter, the buffer between first order low-pass and second order sallen key can be in fact "absorbed" by redimensioning the RC values. You'll still need a low impedance source to drive the filter input.

I think, everything you wrote is correct.
Isolating the first RC stage from the source is necessary only if it is no real voltage source (low source resistor).
However, in any case the second solution as described (active 1st order filter) is preferred (one opamp for filtering and buffering).

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