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I bought these TL072 (maybe TL072CD) IC amps, for a circuit I was trying to build, but the circuit wasn't working at all so I tried to troubleshoot the amp.

I built a voltage divider ~1/1000 (on the left of the pictures) and different amplifier circuits (inverting and non inverting with an amplification of roughly 100) for both op amps in the IC, but I always get the same 8.7V reading (supply is 9V).

I also tried switching the resistors around to create ~1/100 amplification, but as soon as I connect the power source, the output voltage is 8.7V. Did I break the amp?

This is the rough overview for the amps internals:

This is the rough overview for the amp

Here's some of the circuits I have tried:

Inverting circuit on the second internal opamp:

Inverting circuit on the second internal opamp

Non Inverting circuit on the second internal opamp:

Non Inverting circuit on the second internal opamp

Inverting circuit on the first internal opamp:

Inverting circuit on the first internal opamp

(Hard to see but the bevel is at the bottom side of the IC)

All three give the same output voltage (between opamp output and minus of battery) of 8.7V.

EDIT: I must have used the wrong resistor R3 in the photos (10K instead of 1K) so gain must have been more ~10 Here is a circuit diagram:

schematic

simulate this circuit – Schematic created using CircuitLab

I think by drawing it, i get it now (supply voltage was always just a number for me up to this point i never worked with it). I think that, if i change the voltage divider to this it should probably work (pls feel free correct me on this):

schematic

simulate this circuit

FINAL EDIT: Thanks again for all the suggestions, by creating a virtual ground the amp behaves now in a range where i expect it to behave. Your advice was valuable and now I know what to change on my main project :)

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  • \$\begingroup\$ Even if the opamp were ideal, your inverting test circuits could never function since both your input voltage and opamp negative supply are the same point. It can't give you a negative output without a split supply. \$\endgroup\$
    – brhans
    Aug 10, 2023 at 13:43
  • \$\begingroup\$ Can you provide a real circuit diagram, as well as every resistor value you've used?? I suspect your issue is as already described in the answer, but you may have more problems, too. \$\endgroup\$ Aug 10, 2023 at 15:27
  • \$\begingroup\$ nice spider ... \$\endgroup\$
    – jsotola
    Aug 10, 2023 at 17:54
  • \$\begingroup\$ Now that you've drawn the circuit diagram using CircuitLab built into this site, you can measure the DC operating point of the thing, and see if it matches the specifications. To evaluate the effect of offset, add a DC voltage source set for 5mV in series with either the (+) or the (-) input. Flip that source around (select with mouse, press X or Y as needed) to see what happens when the offset swings between its extreme values. "This should be ...": the circuit you got is a live DC simulation. You can actually read what it is :) \$\endgroup\$ Aug 10, 2023 at 20:41
  • \$\begingroup\$ What do you want the circuit to do? Be specific. You built something. Why did you build it? What do you want it to do? Otherwise I have no idea what you're trying to achieve. Please perhaps draw (on paper, then take a photo) a DC transfer function you'd like to get, i.e. the input-to-output voltage relationship, with input voltage on X axis, output on Y axis. The slope of any non-horizontal line segments in the response determines the gain in that region. I built a voltage divider ~1/1000 Cool. Why? \$\endgroup\$ Aug 10, 2023 at 20:44

1 Answer 1

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The TL072 has a common-mode voltage range that does not extend anywhere near the negative rail.

enter image description here

If you use a +/-12V supply it should work, though with a gain of 1,000 the input offset voltage of typically +/-1mV (maximum +/-5mV) will mean that the output could be a few volts off of 0V with no input.

Even expensive op-amps are not perfect and this is a good time to get acquainted with the imperfections. The TL072 is not a bad op-amp for its time for amplifying AC signals, and it has the unusual (for its time) ability to be able to handle input signals at the positive rail.

Also note the recommended minimum total supply voltage of 4.5V or 10V depending on type.

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  • \$\begingroup\$ Thanks for the informative answer. My TL072 has a minimum supply voltage of 6V with a single supply. I actually got different readings with different voltage dividers with the +12V setup (I glued the 9V battery to 2 AA). Thank you very much :) So the main problem was the high gain (should be ~100) together with the offset voltage which always outputed max voltage regardless of the very small voltage of the voltage divider (~8mV), or am I still not getting it? \$\endgroup\$
    – Jochen K
    Aug 10, 2023 at 14:25
  • \$\begingroup\$ Maybe your problem is caused by the input voltage is too close (within 4V) to the negative supply which causes the output of a TL072 to go as high as it can go. It is difficult to see what connects to what on your breadboard photos. Please attach a schematic of your circuits with resistors, capacitors and the supply voltage connections. \$\endgroup\$
    – Audioguru
    Aug 10, 2023 at 15:12
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    \$\begingroup\$ You need the negative supply to be at least 2V (or 4V depending on type) more negative than the lowest input potential. You have it at the same potential, so the op-amp can (and probably will) do something undesirable. It won't cause damage (unless the inputs go more negative than V-) but the op-amp won't function as you might expect. \$\endgroup\$ Aug 10, 2023 at 17:03

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