lm358 arranged as a 5 volt Buffer

Question

^{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.

Answer 1

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:

For the low side:

Answer 2

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.