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.

  • \$\begingroup\$ We love schematics. Please add one using the button on the editor toolbar. Add links to datasheets. \$\endgroup\$
    – Transistor
    Jun 15, 2016 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, 2016 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, 2016 at 7:37

2 Answers 2


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

  • \$\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, 2016 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, 2016 at 7:13

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.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.