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schematic

simulate this circuit – Schematic created using CircuitLab

I am trying to connect my lm358 so that it acts like a buffer. I am connecting:

  • VCC to + 5 volts
  • Ground to Ground
  • Non-inverting input to +5 volts
  • Inverting input to output.

According to my research, connecting the output to the inverting input should cause the output to have the same voltage as the input (+5 volts.) However, I an getting 3.7 volts. Am I making a mistake here, or is the chip damaged or mislabeled? Should I be adding a resister or capacitor into this configuration somewhere?

Edit: I tested this design with +9 volts to the VCC and it was able to produce +5 volts on the output. Then, when I applied +9 volts to the non-inverting input, the IC produced 8.17 volts on the output.

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  • \$\begingroup\$ We love schematics. Please add one using the button on the editor toolbar. Add links to datasheets. \$\endgroup\$ – Transistor Jun 15 '16 at 5:13
  • \$\begingroup\$ @transistor The problem with simple schematics and CircuitLab is that the image then becomes huge and it really obfuscates the question. Hopefully, a workaround has been found. \$\endgroup\$ – dim Jun 15 '16 at 7:20
  • \$\begingroup\$ I know. I always add a little 't' (for transistor) at the bottom right of mine to scale them properly. It also helps me recognise my own work when I come back to it! \$\endgroup\$ – Transistor Jun 15 '16 at 7:37
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LM358 is not a rail-to-rail output opamp. So it means it is not able to drive its outputs hard enough to reach the supply voltage. There will always be a voltage drop even when it saturates. This exists both on the high side and, to a lesser extent, low side.

See the datasheet to know the magnitude of this drop (which depends on the current you need on the output - the more you draw, the bigger the drop):

For the high side:

Output characteristic current sourcing

For the low side:

Output characteristic current sinking

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  • \$\begingroup\$ Putput Op-Amp? I like it! (Though note needs to be made of the 358's nearly rail-reaching output when it comes to the low supply. To fractions of a volt.) \$\endgroup\$ – Asmyldof Jun 15 '16 at 6:43
  • \$\begingroup\$ @Asmyldof Argh, damn iPad... Anway, the answer wasn't detailed enough as I wasn't posting from my PC. I edited it. \$\endgroup\$ – dim Jun 15 '16 at 7:13
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An LM358's output cannot go up to the +ve rail, only within a couple of volts. If you ask it to buffer a 3v voltage you should be OK. If you want to buffer 5v, you need at least a 7v power supply.

... as you found with your 9v supply experiment.

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