Active BP filter problem

[auhunter]

bp filter

Hi,
I really need some help/suggestions. I am trying to construct a standard Multiple feedback active filter (MFB). I have used the forumlas, purchased several programs, used online calculators, to come up with the values to make an MFB bandpass filter. The latest attempt today I used an online calculator to get the resistor and capacitor values and I used the formulas to verify the same values are correct. However, when I build the circuit, it doesn't work. I've tried LMH6624, TL072, TLC2272 opamps. It doesn't seem to matter whatever I try, it doesn't seem to work correctly. In fact, they don't seem close to the paramters which are Fc=2100Hz, BW=300Hz, gain=1, utilizing 113KΩ input, 226KΩ feedback resistor, and 1164Ω for the 3rd resistor, the caps are equal at 4.7nF and 5V for Vcc. I've tried changing to other parameters with corresponding component values, filter topologies, etc but it doesn't seem to matter, I can't get one to work as it should. I'm using a Wavetek 288 signal generator and a Tektronix 2246 scope. I have been playing with this for quite some time but obviously I'm missing something because the results aren't coming close to the desired Fc and BW I'm trying for. Maybe someone here has suggestions as to what I might be doing wrong or what further I can try. Thanks for any help.
auhunter

 

[Audioguru]

lmh6624 ads

You forgot to post your schematic.

 

[auhunter]

tl072 single supply active filter

Hi,
I didn't post a schematic because this is a standard circuit available everywhere. But here is a link to it. Thanks for any help.



https://www.captain.at/electronics/active-filter/

 

[Audioguru]

captainactive filter

The circuit does not show its dual-polarity supply. Didn't you use one?
Which opamp did you use?
Is the filter fed from the very low output impedance of another opamp?
What is its load?

 

[alberto1683]

tl072 single supply filters

Hi ,
Of course , the dual polarity power supply is your problem.
Try with +12V and -12V in the correct pins. Perhaps yousupply the circuit with 5V and GND or something like that.

Regards,

 

[auhunter]

www.captain.at mfb filter

"The circuit does not show its dual-polarity supply. Didn't you use one? "
No, I didn't use a dual polarity power supply. I've biased it to mid supply on the non-inverting input and I've just connected it to ground and let it clip the negative portion of the sinewave...no difference in circuit performance either way.

"Which opamp did you use?"
I tried LMH6624, TLC2272, TL072. While the LMH6624 has the best specs, it behaves the worst. The other 2 perform very similar except that the TL072 does not chop the negative portion of the sinewave as the TLC2272 does. I'm guessing it has something to do with the TL072 being and FET input amp but I'm out of my knowledge zone and am not sure. Either way, those 2 perform similarly, if I sweep the signal generator, the outputs increase when I increase the frequency instead of behaving as though center frequency is at 2100hz as expected.

"Is the filter fed from the very low output impedance of another opamp?"
No, it is fed into the 113K input resistor from a signal generator. I switched it from its output selections of 50Ω, 75Ω, and 600Ω outputs, no change in performance.

"What is its load?"
The output is simply connected to the probe of a scope.

Thanks

Added after 28 minutes:

Hi,
"Of course , the dual polarity power supply is your problem.
Try with +12V and -12V in the correct pins. Perhaps yousupply the circuit with 5V and GND or something like that."

Can you explain why you say this (of course I will try it because I'm desperate to figure this out) but I have read all I can find on this subject the last several weeks. I have found several articles on single supply op-amps showing op amps such as the TI TLC2272 used in this application using a single supply and biasing the signal to mid supply. That's why I've not suspected a bipolar supply as my issue. If you look at this document, on page 17 is the same circuit shown implemented using a single supply opamp.
https://www.ti.com/sc/docs/psheets/abstract/apps/sloa058.htm

Audioguru has me thinking about a load...
Thanks,
auhunter

 

[LvW]

wrong active bp

Hi AUHUNTER;

Question 1: What about the bias point ? Is the dc output voltage app. at Vdd/2 ?
Question 2: What are your results ? Describe it a bit more in detail. No BP behaviour at all ?

 

[auhunter]

the filter input should be biased to midsupply

"Question 1: What about the bias point ? Is the dc output voltage app. at Vdd/2 ?"
Yes, if I bias the circuit with a voltage divider for mid supply. If I don't then it chops of the bottom half of the sinewave.

"Question 2: What are your results ? Describe it a bit more in detail. No BP behaviour at all ?"
For example, I built a circuit where Fc=2100Hz and BW=300Hz. I have full output at 2100Hz and below (I went down to about 500Hz). I swept the generator up and it stays at maximum output until about 50Khz where it begins to attenuate with a very slow drop off. So it acts more like a 50Khz low pass filter with a very slow roll-off.
Thanks,
auhunter

 

[LvW]

single supply tl072

Hard to find an error based on this description.
Perhaps one part has not the proper value ? Mikrofarads instead of nano ?
Or one part is damaged ?
Did you simulate the circuit already ?

Added after 38 minutes:

I just have simulated your circuit. I couldn´t find any error. That means that all part values are correct and result in the wanted band pass response.

 

[Audioguru]

mfb bandpass calc

With a single polarity supply, the resistor that connects to the virtual ground must be at a low impedance to 0V. The virtual ground must be decoupled with a pretty big capacitor to ground. The source and input capacitor must be low impedances. Then it might work properly as a bandpass filter.

The negative swings of the opamp should not be clipped.
The minimum allowed supply for a TL072 is 7V when it has a very small input and output swing.

I don't have time to sketch the filter and I already asked for a detailed schematic.

 

[LvW]

mfb active filter

With a single polarity supply, the resistor that connects to the virtual ground must be at a low impedance to 0V. The virtual ground must be decoupled with a pretty big capacitor to ground. The source and input capacitor must be low impedances.

Why low impedance ? Normally, the + input is biased using a simple voltage divider with a medium or high impedance level (in order not to consume to much power).
As no (FET input) or only a very small current (nanoamperes) goes into this input the voltage drop along these resistors can always be kept small.
Of course, coupling capacitors at input/output are to be large (small impedance) - otherwise they do not couple !

Auhunter: Check all resistors - perhaps there is an error with the colour code ?
Good luck

 

[Audioguru]

I repeat, The resistor that connects to the virtual ground must be at a low impedance to 0V. A pretty big capacitor to ground can do it. The resistor can be directly connected to ground instead. Try it.

 

[LvW]

I repeat, The resistor that connects to the virtual ground must be at a low impedance to 0V. A pretty big capacitor to ground can do it. The resistor can be directly connected to ground instead. Try it.

Hi AUDIOGURU !

OK, now I see what you mean. You are going to reference R3 to the same rail (Vdd/2) as the + input. But the question is: Why ? Why so complicated with an additional element with a "low impedance to ground" ?
This will introduce errors which can be avoided because:
The normal and simple solution is to connect R3 directly to ground (like for split supply) and to bias the + input with Vdd2 (via a voltage divider which should have medium range resistors). That´s all ! Don´t you agree ?

 

[Audioguru]

If The resistor is not grounded (or is not decoupled to ground) then the bandpass filter doesn't work. Try grounding it or adding a pretty big capacitor to ground.

 

[LvW]

If The resistor is not grounded (or is not decoupled to ground) then the bandpass filter doesn't work. Try grounding it or adding a pretty big capacitor to ground.

Who has proposed not to ground the resistor R3 ? Read the last two lines of my last reply.

 

[Audioguru]

Did it work?

 

[LvW]

Of course, as it is a very popular circuit for a single supply BP stage.
Remenber: R3 connected directly to ground and the opamp input connected at the middle of the 1:1 voltage divider. This is in contrast to your circuit with a common rail for R3 and the + input.