[sdiy] basic inverting op amp question 101

Gordonjcp gordonjcp at gjcp.net
Tue Dec 31 12:16:48 CET 2019

On Mon, Dec 30, 2019 at 02:03:34PM +0000, Tom Wiltshire wrote:
> At the simplest level, it’s a trade-off between current and noise. Lower value resistors generate less noise, but use more current (and provide lower input impedance) and higher value resistors do the reverse. So you tend to see “compromise” values around 10K-100K. 1Meg is getting “big” and 1K or less is getting “small”.
> Years ago I had the same question and since the internet didn’t exist yet, I did some experiments on my breadboard with a 741 and discovered that 1M upwards was noticeably noisier, but that anything between a few K and a few hundred K didn’t really seem to make much odds.
> Tom

And this is where it gets really weird - like Monty Hall Problem weird -
because despite what you'd think, two opamps cascaded for -10x gain will
be quieter than a single opamp with 100x gain.  Say you've got 10k input
resistors, for x10 gain you need 100k feedback resistors which are
rather better than ten times quieter than 1M - and you're only
amplifying the noise by 10x.  With a single stage the resistor is
noisier and you're multiplying that by 100.

This assumes not-terrible opamps, of course :-)


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