2 x TL071 = TL072??

[transistance]

I am trying to build a 3rd order low pass filter. The design that im following uses 1 TL072. Since I could only find TL071 in my area i decided to use 2 of those to replace the dual opamp. My circuit is still not behaving properly though. When I'm doing this conversion am I missing something? The TL072 has no offset pins while the TL071 has 2 for each. I just connected these offsets to ground both on both chips. Am I doing something wrong there?

This is my first time building a filter on my own after college, so I'm trying to relearn some stuff. I am thinking that the cap and res tolerances when combined might be throwing my filter off.

Thanks in advance,

 

[ericgibbs]

I am trying to build a 3rd order low pass filter. The design that im following uses 1 TL072. Since I could only find TL071 in my area i decided to use 2 of those to replace the dual opamp. My circuit is still not behaving properly though. When I'm doing this conversion am I missing something? The TL072 has no offset pins while the TL071 has 2 for each. I just connected these offsets to ground both on both chips. Am I doing something wrong there?
Thanks in advance,

hi,
You dont connect the 'offset pins' to 0V.
If you dont need any offset correction, just leave them floating.

EDIT: the attached image shows the typical connection for a CA3140, for ref only.

 

[Nigel Goodwin]

Just ignore the offset pins entirely, there's no need to connect them to ground.

Two TL071's should replace a TL072 fine.

What problem are you having?, and perhaps you might try posting the circuit?.

 

[transistance]

I'm going to draw and post it as soon as i get out of work..

should i then carry it to electronic project section though? just for the sake of keeping the forum organized?

 

[ericgibbs]

I'm going to draw and post it as soon as i get out of work..

should i then carry it to electronic project section though? just for the sake of keeping the forum organized?

hi,
That would be best, members have different forum interests, so you may get the correct support..

 

[Speakerguy]

I answer everything everywhere (well, the top two forums and uC forum are where I mostly hang out). What is the lowpass filter feeding? You may not need a second opamp if the Zout is not important. Just follow the 2nd order active lowpass of the first opamp with a passive RC into whatever you are feeding, depending on the Zout it may not matter.

 

[audioguru]

A 3rd order Butterworth Sallen and Key lowpass or highpass filter is made with a single opamp. It has the very low ouput impedance of an opamp.

A TL072 dual opamp costs less than s single TL071 opamp.

But I can't attach the schematic because I have exceeded ny attachments quota.

 

[ericgibbs]

A
But I can't attach the schematic because I have exceeded ny attachments quota.

hi,
You have to goto your CP, bottom left of the CP window, attachments and delete some of your earlier files.

Its a pain going thru which is which...

 

[audioguru]

Don't delete any of your attachments. The software for the forum is sick and it won't make any difference except mess up old threads.

This forum had a quota a few years ago but when I exceeded it then the admin removed the quota.

 

[Nigel Goodwin]

Like I said in the other threads, PM Electromaster and tell him, it's probably an inadvertent default setting on the lateast software.

 

[audioguru]

ElectroMaster fixed the attachments quota problem.
Here is the circuit of a 3rd-order Sallen and Key Butterworth lowpass filter. The gain resistors are not needed then the opamp can be a simple follower if the RC values are different.

 

[transistance]

I'm moving my questions to under electronics project forum as appropriate.

 

[MD MUBDIUL HASAN]

ElectroMaster fixed the attachments quota problem.
Here is the circuit of a 3rd-order Sallen and Key Butterworth lowpass filter. The gain resistors are not needed then the opamp can be a simple follower if the RC values are different.

Could you tell me how to simulate this circuit ?
Any mathematical conditioning ?

 

[jpanhalt]

Here is an excellent application note from TI: https://www.ti.com/lit/an/sloa049b/sloa049b.pdf

It does not describe the 3rd order filter given above, but it includes step by step instructions on how to use filter tables.

In order to simulate it, you will need to add values for the capacitors and resistors.

 

[audioguru]

Here is a simulation, use the ratios of its RC parts to change its frequency:

 

[MD MUBDIUL HASAN]

Thank you sir Audio guru and jpanhalt for nice theory and simulation.

I have been played with LT spice before, trying to put things in Multisim or circuit lab.

Let me read the theory well, then I will make pcb for test.

Keep in touch.