Feedback resistor in non inverting unity gain opamp?

For the scenario you have, you are correct.

It should be noted that setting the inputs to 'see' the same input impedance minimises input bias current errors (a difference in input impedance will mutiply the error by the amount of mismatch to give you an input offset voltage, which then appears at the output as an error). Input offset current errors are always going to exist (which is a good reason to have a low impedance driving source).

The error you will get (signal) is roughly I(bias) x Z(diff) x (Av) where Zdiff is the difference in the effective difference in input impedance seen at each input and Av the closed loop gain of the amp.

At DC, the output error (for this issue) would be I(bias) x R(diff ) x Av. Only resistive issues apply to the large signal model, of course.

As for the mismatch, it's usually more pronounced for FET input amps but it exists as an issue in all op-amps.

Note that for your case, you will have a signal error of I(offset) x

Apart from trimmers (which I won't use unless I must), the only simple way to minimise input offset current errors is to keep the source impedance low.

Cheers

PeteS

Does anyone have a schematic for a NAD3225PE? mine died last night :(

I bought it about 5 years ago when looking for a preamp for my 216THX. It has a dead amp channel - the company I bought it from had tried but failed to fix the fault, and had told the owner it was written off (one channel power stage + driver transistors removed). But the pre-amp worked fine, so I paid the owner $100 and was happy. until last night, when the preamp died.

I only really need to get the preamp working, but might as well fix the whole damn thing.

TIA

Cheers Terry

But fet opamps have picoampere bias currents, and they usually aren't guaranteed to have even the same polarity, much less the same magnitude.

John

I don't see how you addressed the issue of *distortion* that the OP mentioned. The issue re DC < damn I used *that* word again > conditions is well understood.

That's a new one on me for sure !

Graham