[sdiy] Voltage Feedback Resistors and Circuit Stability

[Adrian Corston]

Most of the video DIYers using LM 6172 op-amps (favorited for its high bandwidth, through-hole package and euro-friendly supply option) use a 1K feedback resistor, as recommended by the datasheet to avoid ringing.

Cheers,
A.


> On Nov 14, 2020, at 9:47 AM, David G Dixon <dixon at mail.ubc.ca> wrote:
> 
> 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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