Hi everyone,

I'm trying to implement this active low-pass filter using a TL072CP.


I have simulated it in LTspice, and its behavior is pretty much what I expected.


However, when I assemble it on the breadboard and analyze its Bode plot, it appears more like a high-pass filter with poor gain.


I'm quite sure the issue lies with the TL072, as I have built the same circuit using a LM741, and its behavior matched the simulation exactly.
Since both the TL072 and LM741 are operational amplifiers, I expect them to produce approximately the same result, given that the transfer function is identical. However, this is not happening.

What am I missing?

 

Looking at the bode plot it should work nicely to favor base.

 

What are the op amp supply voltages?

 

Link to the source of the schematic?

ak

 

My invisible shoulder squirrel thinks that the filter elements should be higher impedance.

 

It looks very much like an inverting first-order filter with the feedback to the wrong pin of the op-amp.

 

It looks very much like an inverting first-order filter with the feedback to the wrong pin of the op-amp.

In other words, swap pins 2 and 3 and you should be better off.

 

What are the op amp supply voltages?

+15V vcc
-15V -vcc

 

My invisible shoulder squirrel thinks that the filter elements should be higher impedance.

how do I assemble the filter considering these properties?

 

It looks very much like an inverting first-order filter with the feedback to the wrong pin of the op-amp.

In other words, swap pins 2 and 3 and you should be better off.

Thank you! I’m going to try it. However, I thought it wouldn’t matter, considering that the op-amp follows the virtual short circuit principle.

 

I thought it wouldn’t matter, considering that the op-amp follows the virtual short circuit principle.

Not sure what that is, but whatever it is - no. If you are referring to a consequence of the typical inverting amplifier configuration called a "virtual ground", then, again, no.

You are taking a characteristic of one node in one particular type of circuit and generalizing it to other circuit types. The input pins of an opamp are not freely interchangeable. Instead of a linear amplifier, your schematic shows a comparator with hysteresis.

Please post links to the source of the schematic, a tutorial page, or whatever you used, and we can comment further.

ak

 

I thought it wouldn’t matter, considering that the op-amp follows the virtual short circuit principle.

As noted, absolutely not.
If it didn't make any difference, why do you think there are polarity symbols at all op amp inputs?

It's the virtual ground principle, not virtual short, and it requires negative feedback to the minus input.
Feedback to the positive input will cause it to latch at near one of the voltage rail voltages.

 

It looks very much like an inverting first-order filter with the feedback to the wrong pin of the op-amp.

Good eye. I had not spotted that the first time I looked.
Absolutely correct, the input pins are swapped.

 

To paraphrase the Spice Girls, if you want, if you really, really want a low pass filler, use a tried and true filter topology.

For instance…Sallen Key are simple topologies, easy to calculate or you may even use one of the many online filter calculators which will do the job for you.

 

It worked!
I swapped pin 2 with pin 3. Thank you a lot guys!

 

Here is the circuit configuration of a 2nd-order low pass filter.

 

For even a faster rolloff (18dB/octave), you can use a single op amp, 3rd-order Sallen-Key type filter as shown here.

 

Not sure if this link will work, but this is my favorite filter article. A 3-pole Butterworth with equal value resistors ***and capacitors***. All that plus a gain of 2 - what's not to love.

EDN Magazine, March 17, 1983: https://annas-archive.org/slow_download/91e80d7c5231d7c711d007f75ff6754d/0/0

Another good article is Electronic Design, August 7, 1995, Ideas For Design

ak

 

A 3-pole Butterworth with equal value resistors ***and capacitors***. All that plus a gain of 2 - what's not to love.

That it requires two op amps for a 3-pole filter.

 

Hello,

Analog has a tool for filter calculation:
https://tools.analog.com/en/filterwizard/

Bertus