# Problem! Need solving!

Discussion in 'Homework Help' started by neo1111, Jan 11, 2014.

1. ### neo1111 Thread Starter New Member

Jan 11, 2014
28
1
What can you tell me about an active R-C filter band-pass??I mean theory.What this circuit actually does??

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2. ### DerStrom8 Well-Known Member

Feb 20, 2011
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Is this for a class? If so, you should post in the homework forum.

What do you think the circuit actually does?

3. ### #12 Expert

Nov 30, 2010
18,076
9,678
It's called, "Sallen-Key". I have a whole book about them, but I'm not going to type it out here. You'll have to be more specific.

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4. ### shteii01 AAC Fanatic!

Feb 19, 2010
4,529
718
It passes through signals that have frequency in the bandpass of the filter.

If the signal has frequency that is not in the bandpass of the filter, the signal is blocked.

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5. ### crutschow Expert

Mar 14, 2008
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6,137
The signal outside the bandpass is attenuated by some factor depending upon the order of the filter and the frequency distance from the passband. It's not "blocked" if you mean no signal at all.

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6. ### crutschow Expert

Mar 14, 2008
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7. ### LvW Well-Known Member

Jun 13, 2013
779
105
The effect as described above can be easily verified:
* For low frequencies R4 provides nearly 100% feedback, and
* for very large frequencies 100% feedback is provided by C1 and C2.
* Somewhere in the mid frequency range (around the center frequency) we have a fixed feedback factor which determines the gain at the center frequency (depending on the selected parts values).

More than that, note that the circuit has inverting characteristics.

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8. ### LvW Well-Known Member

Jun 13, 2013
779
105
Since the OP (Neo1111) has requested some theoretical explanations, here are some relevant aspects:

Contrary to the well-known Sallen-Key topologies which have an active element with a fixed and finite gain (very low), the actual structure (multiple-feedback) is based on an infinite gain amplifier (opamp).
Such an amplifier allows inversion of the feedback characteristic.
Thus, if the feedback network provides a pair of conjugate-complex zeros the resulting closed-loop function will have a pair of conjugate-complex poles (which are a basic requirement for large-Q filter circuits).
Such a frequency-dependent RC-network with a pair of zeros can be realized using the "bridged-T principle".
In the circuit under discussion the "bridge" is provided by R4.
This describes the working principle of the filter under discussion.

Last edited: Jan 12, 2014
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9. ### JoeJester AAC Fanatic!

Apr 26, 2005
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His circuit won't work as drawn.

10. ### LvW Well-Known Member

Jun 13, 2013
779
105
I know - you are talking about the dc supply, correct?