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# Non-Inverting Opamp Butterworth Filter

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#### hkBattousai

##### Member
I have drawn a butterworth filter in the picture below:
VCC_OP = +5V DC
VEE_OP = 0V = GND

I would like to learn if this circuit will invert the input signal or not. The opamp is single supplied, and supply voltage is 5V. The input signal is varying between 0V and 5V.

What happens the the output voltage if the signal is inverted? I think, if you invert a positive signal you obtain a negative one, and since the opamp cannot give negative output, it will simply give 0V, am I right?

If this circuit is inverting, how can I convert it to a non-inverting one? There are only two opamps available in the circuit. Source of the input signal has high impedance (around 560kΩ, arrives from a resistive voltage divider), so I feel like I must have a buffer at the input stage of the amplifier.

I found this circuit schematics in Wikipedia:

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hi,
The left side OPA in the dual circuit is incorrect, check the feedback

EDIT:
The single OPA circuit is not inverting

Last edited:
hi,
The left side OPA in the dual circuit is incorrect, check the feedback
1) What is wrong with the feed-back? Do you mean that the feedback should be connected to "-" input instead of "+" one? I thought that didn't matter in voltage follower opamp circuits.
2) Can you please confirm that the right side opamp circuit is inverting in this first circuit?

The single OPA circuit is not inverting
Would it be all right if I replace the right side opamp circuit in the first schematics with the one in the second schematics?

hi,
The rightside OPA has no feedback from output to input.??

EDIT:
Have you downloaded the TI Filter Pro from Texas [ free] its great for filters

Last edited:
I think you want the first filter to act like a buffer, then the output has to be connected with the inverting input and the input signal to the not-inverting. The first opamp is not necessary because you only have a 2nd order filter, if you have a 3rd order youll have to place a buffer in between. The filters are VCVS. I think the first one is connected incorrectly, but the second one is surely a VCVS, I thought they do not invert the signal ( input is +), multiple feedback filters do .

cheers

Is my circuit OK now?

Well the feedbacks are OK now, I don't know if your component values are OK, that's up to you. You don't need the first opamp its just a buffer, make sure you have a single supply opamp that works between vee_op and vcc_op!

hi,

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The corrected circuit is a Sallen and Key Butterworth second-order lowpass filter, not a notch filter.
It does not invert. Add an inverter opamp to its output if you want it inverted.
It MUST be fed from a low impedance that is less than about 500 ohms and the buffer opamp has a nice low output impedance to drive it.

Have you downloaded the TI Filter Pro from Texas [ free] its great for filters

This FilterPro is a really nice tool. Thank you for suggesting it. Is there any good tools like this, not for filter designing only, anything that is good to know?

You don't need the first opamp its just a buffer.
It MUST be fed from a low impedance that is less than about 500 ohms and the buffer opamp has a nice low output impedance to drive it.

You two are saying opposite things. I'm confused about using buffer or not.

You two are saying opposite things. I'm confused about using buffer or not.

SneaKSz is wrong.
The input resistor on the filter is 10 k ohms so to avoid increasing its effective value it must be fed from an impedance that is much lower than 10k ohms, like the extremely low output impedance of the buffer opamp.

SneaKSz is wrong.
The input resistor on the filter is 10 k ohms so to avoid increasing its effective value it must be fed from an impedance that is much lower than 10k ohms, like the extremely low output impedance of the buffer opamp.

You're right, forgot the section about the source impedance!

1) What is wrong with the feed-back? Do you mean that the feedback should be connected to "-" input instead of "+" one? I thought that didn't matter in voltage follower opamp circuits.
I trust you now realize that it does matter. In general, feedback to the "+" input is positive and will result in the circuit latching at a rail voltage.

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