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# Op-Amp question

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#### Electronman

##### New Member
I have seen they sometimes use a resistor in series with non inverting input of an opamp while it is operating at non inverting mode, what's the reason of using of that resistor?

Though I don't know the exact cause, sometimes a non-inverting opamp will oscillate if presented with too low an impedance at the non-inverting input. Adding some series resistance stabilizes things. It could also be to limit input current on transients or if common mode specs are exceeded.

The only time I have seen this is when the input is uncontrolled (you have no idea what input voltage the user is going to apply). there are internal diodes in the op amp that will conduct if the input goes outside the range of the power supply, so this resistor will limit that current (to save latching the part or smoking it clean off the board).

Sometimes the worst thing you can do to your circuit is to give it to a customer

If you are concerned about minimizing DC offset voltage due to the op amp input bias current, then you add an input resistor to the non-inverting input equal in value to the parallel resistance of the two resistors on the inverting input. That way the input offset due to the input resistors will be determined by the input offset current, not the bias current (which is typically more than the offset current).

But if the opamp is a modern one with FET transistors at its inputs then it does not have any bias current to fiddle around with like in your question about extremely old opamps that had a high input bias current.

Maybe your teacher should go back to school to learn about the parts that are used today.

But if the opamp is a modern one with FET transistors at its inputs then it does not have any bias current to fiddle around with like in your question about extremely old opamps that had a high input bias current.

Maybe your teacher should go back to school to learn about the parts that are used today.

Well, actually my teacher told me they use that resistor to perevent the circuit from the environment noises and mains distortions

Probably a bit unfair, as not all opamps are FET. For low noise with low source impedances one might use something like LT1028, a bipolar (I think) that runs a high first stage current. Still, input currents today are vastly lower than then used to be, no matter what recent parts you choose.

Most teachers who went to school a century or two ago base their questions on the antique lousy old 741 opamp. It had horrible performance and should be buried.

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