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When biasing an inverting op-amp input using a voltage regulator (say, L7805), at the non-inverting pin, can we use the same regulator to power other ICs or should we only use it on the op-amp?

Also, in the case of an inverting amplifier, does the op-amp needs an input resistor at the non-inverting pin or can directly connect the regulator output and the op-amp?

Thanks in advance.

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3 Answers 3

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I would say that this is not great practice, but is probably ok for non-critical applications. There will be applications where its not ok to do that, but it depends in what configuration you are using your opamp, and how clean the signal needs to be.

If you are using it in any sort of negative feedback situation (like an inverting amplifier), noise on the non-inverting input will lead to noise on the output. Connecting the non-inverting input to power other devices will likely introduce such noise. If you are doing this, you should filter that input, like this:

schematic

simulate this circuit – Schematic created using CircuitLab

The values are a bit arbitrary, and you might not need two capacitors, but I might do it just to be safe. If you have a specific opamp in mind, you should check the input current leakage specification, and you may be able to increase R1 to add more filtering.

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  • \$\begingroup\$ OP is asking about to use the opamp supply voltage, which you didn't draw. Also, you have added an additional supply voltage, now explain to the OP where he can get it. \$\endgroup\$
    – Marko Buršič
    Jan 17, 2021 at 22:26
  • \$\begingroup\$ Thanks BeB00, this is exactly what I was wanting to know. \$\endgroup\$
    – strange_bakery
    Jan 19, 2021 at 14:33
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You can use it to power other ICs (or other purposes) but keep in mind that the regulator has a non-zero output impedance (it's pretty good at DC but your other loads may have high frequency variations) and the regulator output voltage will thus likely vary more than if you had a dedicated regulator.

L7805 is based on a bandgap reference and is thus fairly noisy in itself, somewhere in the 300-400uVp-p range typically (10Hz-100kHz) with a 100nF capacitor on the output.

It's generally going to be superior to using a voltage divider off the power supply rail or a 5.1V zener diode, but not as good as using a reference IC.

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Of course you can use it, by using some resistor divider.

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ I dont think this is answering the question \$\endgroup\$
    – BeB00
    Jan 16, 2021 at 22:23
  • \$\begingroup\$ @BeB00 I think that your answer is even worse. \$\endgroup\$
    – Marko Buršič
    Jan 16, 2021 at 22:46
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    \$\begingroup\$ I'm not sure why, your answer doesnt really give any detail on the pros and cons of doing such a thing. It also seems like OP is asking about connecting the regulator directly, not through a resistor divider \$\endgroup\$
    – BeB00
    Jan 16, 2021 at 23:03
  • \$\begingroup\$ @BeB00 He is asking if he can connect opamp supply voltage directly or it needs a resistor. Now resistor or resistor divider is almost the same thing. \$\endgroup\$
    – Marko Buršič
    Jan 17, 2021 at 22:24
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    \$\begingroup\$ Are you sure? Thats not what I got from this question, you should check that with OP. That might add even more interesting effects that could be talked about, but the effects in my answer will still be present, and my strategy for reducing all the effects will still help. Your answer still doesn't really give any detail beyond a yes, and is still not what the question is asking \$\endgroup\$
    – BeB00
    Jan 18, 2021 at 0:25

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