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schematic

simulate this circuit – Schematic created using CircuitLab

I can't nullify the output offset voltage of an LM741CN op amp.

I tried the circuit mentioned in the datasheet, but with no luck.

I connected the V- pin to ground of a 15V power supply and used a 10k pot as mentioned and the output is just stuck at +rail and doesn't even move a bit.

I also tried to use a 9V battery on a potentiometer to just compensate for the input offset voltage with the wiper end on the + pin. It worked and I got 30mv on the output but it also reads a 50 mA on the power supply and the op amp gets really hot.

I edited the circuit to bias the inputs to half of the supply voltage and made it my virtual ground now should be ok that the -v terminal is 7.5V below the inputs which allow for maximum swings on both sides like the circuit below. Are there any other cautions to take into account about this circuit? If there's any suggestions about any modifications or pitfalls in this circuit would be appreciated.

schematic

simulate this circuit

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    \$\begingroup\$ Show your circuit. If it's stuck at one of the output rails you have something else wrong. You can add one in using the CircuitLab button on the editor toolbar. Double-click a component to edit its properties. 'R' = rotate, 'H' = horizontal flip. 'V' = vertical flip. Note that when you use the CircuitLab button on the editor toolbar and "Save and Insert" on the editor an editable schematic is saved in your post. That makes it easy for us to copy and edit in our answers. You don't need a CircuitLab account, no screengrabs, no image uploads, no background grid. \$\endgroup\$
    – Transistor
    Oct 10, 2020 at 22:13
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    \$\begingroup\$ Where's the -15V supply? A 741 is not a single supply opamp. (You CAN use it as one, but not like this!) Read that datassheet again, noting how close the output can swing to each supply rail .. not very close! \$\endgroup\$ Oct 10, 2020 at 22:33
  • \$\begingroup\$ Very few parts of the 741 work properly. \$\endgroup\$
    – Hearth
    Oct 10, 2020 at 23:12
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    \$\begingroup\$ 741s are rudimentary op-amps but all I've used work as designed. Maybe you're not using them correctly ? \$\endgroup\$ Oct 11, 2020 at 1:37
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    \$\begingroup\$ The 741 IS NOT a single supply rail op-amp. a 'single supply op-amp'is universally defined as requiring the output to be capable of swinging to negative supply AND requiring both inputs to function down to negative supply (often lower using PNP input devices). The LM741 fails BOTH tests by a ~3V margin ! Not surprising as it's a 52 year old design. \$\endgroup\$ Oct 11, 2020 at 21:03

3 Answers 3

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enter image description here

Figure 1. The original circuit.

You are running the op-amp from a single supply and have the inputs tied to ground. You can't. The inputs must be a few volts above the negative rail. You need to supply a negative voltage on pin 4.


Edit after question updated.

enter image description here

Figure 2. The modified circuit.

This is better and you should see some ability to modify the output voltage. Note that you have a gain of 1M/1k = 1000 which is very ambitious for a 741. Try a gain of ten or twenty and continue your experimentation.

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  • \$\begingroup\$ I made a biasing circuit for inputs from the + terminal using 2- 10k resistors. to split the supply in half and connected the inputs to the 7.5 volt of this biasing and managed to get the pot to calibrate the output to 7.5 volts also any comments on this configuration? \$\endgroup\$ Oct 10, 2020 at 23:02
  • \$\begingroup\$ Which doesn't fix the fact that the 741 wasn't designed for a single supply. You've effectively made one. Do you even understand the importance of split supplies for analog circuits ? A 741's output can't swing much less than 3 volts above the negative supply rail. I'll bet that the output pin is around 3V above ground. Where did you get the idea that you could use a 741 like this ? I knew better in 1976 ! \$\endgroup\$ Oct 11, 2020 at 1:48
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    \$\begingroup\$ @Graham, "the 741 wasn't designed for a single supply" isn't quite correct. Op-amps don't have a 0 V reference so they know nothing about where GND is relative to their power pins. What we knew in 1976 doesn't really help the OP today! \$\endgroup\$
    – Transistor
    Oct 11, 2020 at 8:45
  • \$\begingroup\$ When the 741 was introduced in 1968, no-one imagined single supply rail operation for op-amps. That's a relatively new concept and requires the output to swing (close to) ground. A 741 won't swing much closer than ~3 volts to the negative supply (or positive). In the circuit shown, the output needs to swing to ground and a 741 won't, no matter how much you fiddle with the offset pins ! \$\endgroup\$ Oct 11, 2020 at 8:57
  • \$\begingroup\$ I'm aware of all that and addressed it lightly in the answer. \$\endgroup\$
    – Transistor
    Oct 11, 2020 at 8:58
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Try the chip in the below circuit. It's set up for a gain of 1000 so 1mV of input offset will result 1V at the output.

If it does not work in the below circuit the chip is probably damaged.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ I don't have a split supply actually But I can use the 9V battery and reduce the power supply voltage to 9V if it's okay. \$\endgroup\$ Oct 10, 2020 at 22:28
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    \$\begingroup\$ It's not okay. You can use two 9V batteries. The inputs have to be within the input common mode range so a split supply or equivalent is required. For now, assume you need a split supply. \$\endgroup\$ Oct 10, 2020 at 22:29
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    \$\begingroup\$ @GrahamStevenson Why not? What is the problem the 1-MegaOhm resistor introduce? I would like to know \$\endgroup\$ Oct 11, 2020 at 14:02
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    \$\begingroup\$ You know about the errors caused by input bias currents ? They make ALL the difference between real world designs and those that only simulate correctly I'd be somewhat reluctant to use even as high as 100k around a 741 to be honest. Input bias current TAMIN ≤ TA ≤ TAMAX max 1.5 μA let's see. 1.5ua x 1M = 1.5 V ! \$\endgroup\$ Oct 11, 2020 at 17:56
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    \$\begingroup\$ Typical offset current for a 741 is 20nA. That results in an input offset voltage of 20uV. Utterly meaningless in the context of this BENCH TEST of offset nulling. Typical offset voltage is 50x higher. So, don't worry about it, do the test. Offset null range is typically +/-15mV or almost 3 orders of magnitude higher. \$\endgroup\$ Oct 11, 2020 at 18:04
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That circuit is the way to go, but it's incomplete and you have been misled.

Don't try to operate an op amp with either of the inputs floating. Set up your op amp as a complete amplifier, inverting or noninverting, take your pick.

Set the input to ground, and now try adjusting the pot while monitoring the output.

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  • \$\begingroup\$ He's been misled that 741s are single supply rail op-amps. \$\endgroup\$ Oct 11, 2020 at 1:38
  • \$\begingroup\$ @GrahamStevenson - Oooh. You're right. my bad. \$\endgroup\$ Oct 11, 2020 at 17:36
  • \$\begingroup\$ No problem ! It could happen to anyone. \$\endgroup\$ Oct 12, 2020 at 1:02

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