Simple TL072 Op Amp buffer issues - Page 1
Forum section: electronics / beginners

author: ldkimball
author_rank: Contributor
timestamp: September 15, 2024, 07:07:46 pm
content: Hi all,I'm using a simple voltage divider/ op amp buffer circuit to make a 2.5V reference voltage for the rest of my circuit. The VCC+ is connected to +12V and the VCC- is connected to -6V. The voltage divider by itself outputs a voltage of 2.503V but when it's connected to the buffer, the output gives something closer to 2.67V. When I connect the non inverting input to ground instead of 2.5V, it gives around 170mV. When I connect it to 5V it gives 5.17V. So it seems to be tacking on an extra 170 ish mV seemingly from out of nowhere. I've read through the data sheet several times and I'm confident I'm working well within the recommended operating conditions of the op amp. I really don't understand what I'm doing wrong here. It goes without saying that I'm an electronics beginner. I appreciate any and all help!

author: Benta
author_rank: Super Contributor
timestamp: September 15, 2024, 07:14:59 pm
content: I don't see any power supply decoupling cap.And solderless breadboards are notoriously unreliable.I only use them for the pattern when baking biscuits.

author: ldkimball
author_rank: Contributor
timestamp: September 15, 2024, 07:17:35 pm
content: I haven't added a decoupling cap yet. Do you think that could be the problem?Update: I added decoupling caps and I'm still encountering the same issue.

author: Xena E
author_rank: Frequent Contributor
timestamp: September 15, 2024, 07:45:37 pm
content: Quote from: ldkimball on September 15, 2024, 07:17:35 pm... also the long wiring, and leakage in the piece ofsoggy toast, bread board that you built your circuit on, possibly.

author: ldkimball
author_rank: Contributor
timestamp: September 15, 2024, 08:01:42 pm
content: Measuring the voltage supplies at the VCC+ and VCC- terminals of the op amp, they are exactly what they should be. Also, when I connect the VCC- terminal to ground instead of -6V and then switch the inverting input connection to +5V to comply with the input voltage range indicated in the data sheet I get an output voltage of exactly 5V. To me this seems to rule out the breadboard as the main culprit here.

author: magic
author_rank: Super Contributor
timestamp: September 15, 2024, 08:07:26 pm
content: IMO this should work, so maybe it oscillates.Try moving the power cables further from the chip and place 100nF capacitor directly above the chip, plugging it into holes next to the power pins.Add 100Ω between the OUT pin and DMM probe.

author: ldkimball
author_rank: Contributor
timestamp: September 15, 2024, 08:19:32 pm
content: Still no luck. It seems the issue has to do with the negative power rail. When I switch VCC- to ground and change the input to something within the acceptable range (VCC- + 4V) it works just fine. I would just run with that but unfortunately 2.5V is under that range when VCC- is connected to ground.

author: ldkimball
author_rank: Contributor
timestamp: September 15, 2024, 08:34:59 pm
content: I got them from digikey so they should be legit. When I connect to 5/-6V I get the same issue with it tacking on 170mV.I'm using a CMR1215S3C DC to DC converter connected to a 7906 linear regulator to create the -6V supply. When I measure it it shows up as -6V on my multimeter. Could it be something to do with its current output?

author: RoGeorge
author_rank: Super Contributor
timestamp: September 15, 2024, 08:37:28 pm
content: Attach a picture that shows the power supply wiring to the opamp, and/or add a hand drawing/schematic of what is connected where. In the existing picture it is not clear, the resistors look like they are with one terminal floating.Also, for the unused opamp, connect together the inverting input with the output (to make a voltage repeater with amplification 1), then connect the remaining unused non-inverting input to the GND (at the 0V between the two series power supplies).

author: ldkimball
author_rank: Contributor
timestamp: September 15, 2024, 08:53:02 pm
content: Apologies for the messy wiring. I had a larger circuit connected and I decided to take it out to simplify things. Bottom voltage regulator is the 7905 and top one is the 7906. Black box on the top left is the DC to DC converter.

author: ArdWar
author_rank: Frequent Contributor
timestamp: September 16, 2024, 02:05:04 am
content: Check if your circuit is oscillating, including your 78xx/79xx output. Using them without decoupling caps may happen to be fine in 90% of non-demanding cases, but that's asking for trouble otherwise.

author: magic
author_rank: Super Contributor
timestamp: September 16, 2024, 06:40:28 am
content: Yes, 79xx in particular may need a cap on the output. Consult your datasheet.And I still see no 100nF across the TL072?

author: Filip SP9GMD
author_rank: Newbie
timestamp: September 16, 2024, 07:18:20 am
content: I think you should use feedback resistor instead of just jumper.Please read this article:https://e2e.ti.com/blogs_/archives/b/precisionhub/posts/resistors-in-the-feedback-of-a-buffer-ask-whyThe feedback resistor should match the resistive part of source impedance seen by the non-inverting input, so you can use superposition method to calculate resistance seen by input.You can give it a shot.Do you have access to oscilloscope?

author: Xena E
author_rank: Frequent Contributor
timestamp: September 16, 2024, 07:41:32 am
content: Quote from: Filip SP9GMD on September 16, 2024, 07:18:20 amYeah, that thing is hooting with all that wiring hanging about and no bypass caps.

author: magic
author_rank: Super Contributor
timestamp: September 16, 2024, 07:46:57 am
content: Quote from: Filip SP9GMD on September 16, 2024, 07:18:20 amJFET opamps don't have enough bias current for that to make a difference, certainly not 170mV difference. And it wouldn't depend on supply voltage.With JFET, you only worry about resistor matching if you care about high frequency distortion.

author: Xena E
author_rank: Frequent Contributor
timestamp: September 16, 2024, 08:18:34 am
content: Quote from: magic on September 16, 2024, 06:40:28 amNope.Don't decouple across the two supplies, decouple each to common (regulators common which is incoming negative in the OP case)... only decouple across the supply pins to an op amp chip if running it single supply.

author: Xena E
author_rank: Frequent Contributor
timestamp: September 16, 2024, 08:25:26 am
content: Quote from: magic on September 16, 2024, 07:46:57 amAdding those bias current balance resistors can also act as anti parasitic components which may help snub a tendency to input instability, which may be what Filip SP9GMD was suggesting it for.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 16, 2024, 09:54:55 am
content: If decouling is towards ground or just from positive supply to negative supply is a debateable point. It depends and with just a buffer both versions should work.There are also no capacitors at the supply regulators. The regulator may osciallate under these conditions, especially a 7906. These are even picky about the capacitor type / ESR.The 2nd, not used half of the TL072 should be also wired up, e.g. as a 2nd buffer to not interfere / capture hum.

author: ldkimball
author_rank: Contributor
timestamp: September 16, 2024, 01:06:51 pm
content: Turns out the problem was my lack of a bypass capacitor to smooth out the negative supply. Added a 0.1UF cap here and the problem is fixed! Thanks again for all of the help

author: floobydust
author_rank: Super Contributor
timestamp: September 16, 2024, 06:55:53 pm
content: cross-postedhttps://forum.allaboutcircuits.com/threads/tl072-op-amp-buffer-issues.202967