Problem! Need solving!

#12

Joined Nov 30, 2010
18,224
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.
 

shteii01

Joined Feb 19, 2010
4,644
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.
 

crutschow

Joined Mar 14, 2008
34,285
.....................

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

LvW

Joined Jun 13, 2013
1,752
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.
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.
 

LvW

Joined Jun 13, 2013
1,752
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.
 
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