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I am running a LM358 in a simple non-inverting amplification circuit. I am using the single rail supply option, grounding V- and applying 24V to V+. The input voltage is 4V and I want to amplify it as much as possible. With no load, I do get around 22V (more is probably possible), however, when I attach a load (a solenoid valve), the output voltage drops down to around 16V. If I reduce the gain to output around 15V, it stays the same, with or without load attached. When I hook up the supply voltage directly to the solenoid valve, the voltage stays constant too. Is this normal or am I doing something wrong here?

jonen
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    How much current are you attempting to pull from the op-amp? You may have to add an emitter-follower buffer stage between the output of the op-amp and the load. Probably all you need is a single NPN transistor. – Dwayne Reid Jul 10 '23 at 19:33
  • @DwayneReid The solenoid valve draws 75mA. Thanks for the tip, so something like this, to achieve low impedance? https://www.electronics-notes.com/articles/analogue_circuits/transistor/transistor-common-collector-emitter-follower.php – jonen Jul 10 '23 at 19:43

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While different variants have different limits, the LM358 datasheet shows the order of magnitude for maximum output current (just search the document for "output current") is generally a few tens of milliamps or less. When you try to source or sink too much, weirdities like you're describing can happen. Solenoids can take a lot of current.

If this is your problem, you will need to include some sort of driver to provide the necessary current.

Scott Seidman
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