# [sdiy] basic inverting op amp question 101

Spiros Makris spirosmakris92 at gmail.com
Mon Dec 30 15:56:07 CET 2019

```On top of what Tom has mentioned, the input impedance of the inverting
stage is depended on one of those two resistors. Sometimes a high input
impedance is needed and thus you are forced to either use larger resistors
or more opamps, depending on the case.
Furthermore, the bias input current of the opamp is converted into a
voltage by passing through that input resistor- a higher resistor leads to
a proportionately higher offset. In FET input opamps the bias currents are
some pA and can usually be ignored, but BJT opamps will be prone to that

On Mon, Dec 30, 2019 at 4:07 PM Tom Wiltshire <tom at electricdruid.net> 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
>
> ==================
>        Electric Druid
> Synth & Stompbox DIY
> ==================
>
> On 30 Dec 2019, at 13:17, bbob <fluxmonk at gmail.com> wrote:
>
>
> in a basic inverting op-amp circuit, where the input and feedback
> resistors are equal, the gain will be -1... but what are the pros/cons that
> determine the choice resistor value?  i commonly see/have used 1k, 10k,
> 47k, 100k in synth circuits, but what design considerations drive those
> choices of values?   my immediate application is basic output buffers on a
> LFO, and 1k seems to be working fine, but it got me to thinking (uh oh).
>
> thx, b
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