Biasing an inverting Op-Amp using a voltage regulator

Question

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.

Answer 1

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:

^{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.

Answer 2

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.

Answer 3

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

^{simulate this circuit – Schematic created using CircuitLab}