[sdiy] Voltage Feedback Resistors and Circuit Stability

[David G Dixon]

I concur with Ian.  However, I have had circuits where opamps with 100k feedback resistors screamed like banshees, while with 10k they didn't oscillate at all.  Hence, if stability is an issue, I often default to the lower value.  I also end up using 30k quite often.  This is often just a matter of convenience.  If I'm using a 2164 in my circuit, there will be 30k resistors, so I often just apply them to the opamp circuits as well.  They always work just fine.

  _____  

From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Ian Fritz
Sent: Friday, November 13, 2020 11:13 AM
To: tpuefke
Cc: Synth-Diy mailing list
Subject: Re: [sdiy] Voltage Feedback Resistors and Circuit Stability


[CAUTION: Non-UBC Email] 
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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