[sdiy] Voltage Feedback Resistors and Circuit Stability

[Brian Willoughby]

I remember some articles about Johnson noise in resistors, and the consequences for op-amp circuits. I went looking around and found the following:

Analog Devices
AD549, AD797, AD820 & AD822 op-amp data sheets

Application Notes AN-581 & AN-937

Analog Devices also has a book on Basic Linear Design that's worth downloading.

Brian Willoughby


On Nov 13, 2020, at 08:56, 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