12 db HF with inverted Buffer?

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jayam000

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Joined 2009
#1
I want to add Hf filter to amplifier. I am thinking of using just one dual opamp. The first stage for -12 db HF filter & second for buffer but inverted. Is this ok?
-12db filter will reverse phase by 180 degrees, hence this design.
 

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#3
You could use an MFB filter.
Unfortunately D.Self found that the High Pass version of the MFB added a bit of distortion, whereas he said the Low Pass version was distortion free.
Only one opamp needed for a 2pole filter that inverts the signal. Worth trying to find out if you can detect the added distortion.
 
#4
jayam000 said:
-12db filter will reverse phase by 180 degrees, hence this design.
Click to expand...
That's below the cut-off frequency, above your left hand circuit is non-inverting. If you want to do something with the extra opamp half, you could put a non-inverting buffer before the filter, that way you have more freedom to select the input impedance with the biasing resistor from the + input to ground (missing in scottjoplin's link above) as shown here. This could also be useful if for some reason the circuit feeding yours isn't happy driving an impedance that varies significantly with frequency.

Edit: this will also free you to use different / better values for the components of the filter, e.g. you could make the resistors 11k and the caps 0.2 and 0.1uF, improving the noise performance of the circuit but, thanks to the buffer, not resulting in a potentially too low input impedance.

Cheers,

Cabirio
 
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