[sdiy] Voltage Feedback Resistors and Circuit Stability

[Mattias Rickardsson]

Out of curiosity, what is your reasoning behind 100k being a good
compromise?
I thought that the widespread use of 100k was an unfortunate consequence of
having 100k input impedance as standard in eurorack. :-)

A bog-standard TL07x draws 1.4 mA per op-amp and has a noise around 18
nV/rtHz.

- Any resistor above 22k is noisier, so let's not go higher than 10k.

- Assuming a 5 V signal through 10k, the current is only 0.5 mA. Quite
small compared to what the op-amp eats constantly.

I'd say anything between 4k7 and 10k is a good compromise between noise and
power consumption for TL07x circuits. And perhaps 10k-22k for TL06x. For
low-noise op-amps 2k2 is often a good choice, since higher values increase
noise levels.

/mr

Den fre 13 nov. 2020 20:13Ian Fritz <ijfritz at comcast.net> skrev:

> R values around 100k or so give a good compromise between noise and power
> consumption.
>
> Ian
>
> On Nov 13, 2020, at 9:59 AM, tpuefke via Synth-diy <
> synth-diy at synth-diy.org> wrote:
>
> 
> I was hoping someone more experienced could help shine some light on this
> issue. This is still quite puzzling to me as a non-engineer.
>
> Looking through my collection of schematics from various places like MFOS
> (bless his soul), I see a lot of the standard 100k resistor in negative
> feedback op-amp configuration, for inverters, mixers, attenuators... For a
> long time i have been adapting these as my go-to values without giving it
> much thought and usually it works pretty well. The odd schematic here and
> there uses lower values.
>
> Considering that high feedback resistance implies a higher gain potential,
> shouldn't lower values (e.g. 10k, 1k even) usually be a better choice in
> terms of circuit stability?
> Or is this a non-issue for fractional gain setups?
>
> I've been wondering about this for a while now and recently stumbled
> across this post discussing Rf values and op-amp input capacitance:
>
> https://www.analog.com/en/analog-dialogue/raqs/raq-issue-122.html
>
>
> What are the pros and cons in your experience for using high resistances
> like these, in attenuator / unity inverter setups especially?
>
> Higher resistances simply to reduce current consumption in bigger
> circuits? What about resistor noise?
>
> Just trying to wrap my head around this to be able to make more informed
> decisions.
> Any feedback is HIGHly appreciated. :)
>
>
> thanks,
> Tom
>
>
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