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I am trying to use UA741CP operational amplifier. To test it I wanted to create a buffer circuit in which input voltage would be equal to output voltage. I use 5 V DC as my supply rail. A voltage divider is used to reduce the input voltage of OpAmp (100k 1st, then 10k) which gives 0.45 V (V_in).

But when I measure V_out I get 1.82 V. Moreover, I can completely disconnect the non-inverting input (+) and it remains the same. Also I noticed that when I connect the OpAmp into the circuit the output voltage goes from around 1.8 V to 1.82 V in a couple of seconds.

Do you have an idea what I might be doing wrong?

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ you have an OK circuit for your application, but a 741 op-amp is an incorrect choice. Also R1 and R2 serve no purpose since V_in dominates. Search for rail-to-rail op amps, and be sure that both inputs *and * outputs can swing from one power supply rail (+5v) to the other (gnd). \$\endgroup\$ – glen_geek Feb 15 '17 at 21:15
  • \$\begingroup\$ I hope that it is clear to you now that the 741 is a bad choice for this. A better choice would be the MCP601, it is designed for 5 V supplies. There are plenty of other suitable opamps, this MCP601 is just a suggestion. \$\endgroup\$ – Bimpelrekkie Feb 15 '17 at 21:28
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The datasheet is really quite clear about this:

This opamp isn't specified for only 5 V supply.

These parameters shown above are for ±15 V supplies. The common mode input range only goes to within 3 V of either supply. Nothing is said how that scales to lower supply voltages, so assume it is at least that much. That means there is no common input range left with only a 5 V supply.

Likewise, the output can't drive to within 3 V of the supply rails with a 10 kΩ load, and not to within 5 V with a 2 kΩ load.

You need to actually read the datasheet before designing a part into a circuit.

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The 741 is not specified to work with a single 5V rail.

Minimum recommended supply voltage is +/-10V. With +/-15V supplies, the input voltage range is only guaranteed to be within 3V of the rails, i.e. +/-12V. Similar issue with output voltage swing.

You have to make sure you read the datasheet, supply the part with adequate voltage rails and obey the common mode input range. The 741 is a very old part and a poor choice for any new design.

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  • \$\begingroup\$ Would be interesting to see how low of a supply you can go on a 741, data sheet aside. \$\endgroup\$ – dannyf Feb 15 '17 at 21:58
  • \$\begingroup\$ I regularly run 741s on a +/- 4.5V supply. It can be done. \$\endgroup\$ – TCassa Jan 22 '18 at 13:26
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The input voltage IS too close to ground. Reduce r1 or increase r2. You will need about 2v of headroom.

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