<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div dir="ltr"></div><div dir="ltr">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.</div><div dir="ltr"><br></div><div dir="ltr">Cheers,</div><div dir="ltr">A.</div><div dir="ltr"><br></div><div dir="ltr"><br>On Nov 14, 2020, at 9:47 AM, David G Dixon <<a href="mailto:dixon@mail.ubc.ca">dixon@mail.ubc.ca</a>> wrote:<br><br></div><blockquote type="cite"><div dir="ltr">
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<div dir="ltr" align="left"><span class="951560623-13112020"><font color="#0000ff" size="2" face="Arial">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.</font></span></div><br>
<div dir="ltr" lang="en-us" class="OutlookMessageHeader" align="left">
<hr tabindex="-1">
<font size="2" face="Tahoma"><b>From:</b> Synth-diy
[<a href="mailto:synth-diy-bounces@synth-diy.org">mailto:synth-diy-bounces@synth-diy.org</a>] <b>On Behalf Of </b>Ian
Fritz<br><b>Sent:</b> Friday, November 13, 2020 11:13 AM<br><b>To:</b>
tpuefke<br><b>Cc:</b> Synth-Diy mailing list<br><b>Subject:</b> Re: [sdiy]
Voltage Feedback Resistors and Circuit Stability<br></font><br></div>
<div></div><span style="BACKGROUND-COLOR: #ffecb3; COLOR: #000000; FONT-SIZE: 12px"><span style="BACKGROUND-COLOR: #ffecb3; COLOR: #000000; FONT-SIZE: 12px"><span style="PADDING-BOTTOM: 3px; LINE-HEIGHT: 1.6; BACKGROUND-COLOR: #ffecb3; FONT-STYLE: normal; PADDING-LEFT: 3px; PADDING-RIGHT: 3px; COLOR: #000000; FONT-SIZE: 12px; FONT-WEIGHT: normal; PADDING-TOP: 3px">[<strong>CAUTION:</strong>
Non-UBC Email]</span></span></span>
<div dir="ltr">R values around 100k or so give a good compromise between noise and
power consumption.</div>
<div dir="ltr"><br></div>
<div dir="ltr">Ian</div>
<div dir="ltr"><br>
<blockquote type="cite">On Nov 13, 2020, at 9:59 AM, tpuefke via Synth-diy
<<a href="mailto:synth-diy@synth-diy.org">synth-diy@synth-diy.org</a>> wrote:<br><br></blockquote></div>
<blockquote type="cite">
<div dir="ltr">
<div style="TEXT-TRANSFORM: none; TEXT-INDENT: 0px; FONT: 15px monospace; WORD-WRAP: break-word; WHITE-SPACE: normal; LETTER-SPACING: normal; COLOR: rgb(60,60,60); WORD-SPACING: 0px; -webkit-text-stroke-width: 0px">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.<br></div>
<div style="TEXT-TRANSFORM: none; TEXT-INDENT: 0px; FONT: 15px monospace; WORD-WRAP: break-word; WHITE-SPACE: normal; LETTER-SPACING: normal; COLOR: rgb(60,60,60); WORD-SPACING: 0px; -webkit-text-stroke-width: 0px"><br></div>
<div style="TEXT-TRANSFORM: none; TEXT-INDENT: 0px; FONT: 15px monospace; WORD-WRAP: break-word; WHITE-SPACE: normal; LETTER-SPACING: normal; COLOR: rgb(60,60,60); WORD-SPACING: 0px; -webkit-text-stroke-width: 0px">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.<br></div>
<div style="TEXT-TRANSFORM: none; TEXT-INDENT: 0px; FONT: 15px monospace; WORD-WRAP: break-word; WHITE-SPACE: normal; LETTER-SPACING: normal; COLOR: rgb(60,60,60); WORD-SPACING: 0px; -webkit-text-stroke-width: 0px"><br></div>
<div style="TEXT-TRANSFORM: none; TEXT-INDENT: 0px; FONT: 15px monospace; WORD-WRAP: break-word; WHITE-SPACE: normal; LETTER-SPACING: normal; COLOR: rgb(60,60,60); WORD-SPACING: 0px; -webkit-text-stroke-width: 0px">
<div style="WORD-WRAP: break-word">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? <br></div>
<div style="WORD-WRAP: break-word">Or is this a non-issue for fractional gain
setups? <br></div>
<div><br></div></div>
<div style="TEXT-TRANSFORM: none; TEXT-INDENT: 0px; FONT: 15px monospace; WORD-WRAP: break-word; WHITE-SPACE: normal; LETTER-SPACING: normal; COLOR: rgb(60,60,60); WORD-SPACING: 0px; -webkit-text-stroke-width: 0px">I've
been wondering about this for a while now and recently stumbled across this
post discussing Rf values and op-amp input capacitance:<br></div>
<div style="TEXT-TRANSFORM: none; TEXT-INDENT: 0px; FONT: 15px monospace; WORD-WRAP: break-word; WHITE-SPACE: normal; LETTER-SPACING: normal; COLOR: rgb(60,60,60); WORD-SPACING: 0px; -webkit-text-stroke-width: 0px">
<div style="WORD-WRAP: break-word">
<div style="WORD-WRAP: break-word"><br></div>
<div style="WORD-WRAP: break-word"><a style="COLOR: rgb(45,113,184)" href="https://www.analog.com/en/analog-dialogue/raqs/raq-issue-122.html">https://www.analog.com/en/analog-dialogue/raqs/raq-issue-122.html</a><br></div>
<div><br></div>
<div><br></div>
<div>What are the pros and cons in your experience for using high resistances
like these, in attenuator / unity inverter setups
especially? <br></div></div></div>
<div style="TEXT-TRANSFORM: none; TEXT-INDENT: 0px; FONT: 15px monospace; WORD-WRAP: break-word; WHITE-SPACE: normal; LETTER-SPACING: normal; COLOR: rgb(60,60,60); WORD-SPACING: 0px; -webkit-text-stroke-width: 0px"><br></div>
<div style="TEXT-TRANSFORM: none; TEXT-INDENT: 0px; FONT: 15px monospace; WORD-WRAP: break-word; WHITE-SPACE: normal; LETTER-SPACING: normal; COLOR: rgb(60,60,60); WORD-SPACING: 0px; -webkit-text-stroke-width: 0px">Higher
resistances simply to reduce current consumption in bigger circuits? What
about resistor noise?<br></div>
<div style="TEXT-TRANSFORM: none; TEXT-INDENT: 0px; FONT: 15px monospace; WORD-WRAP: break-word; WHITE-SPACE: normal; LETTER-SPACING: normal; COLOR: rgb(60,60,60); WORD-SPACING: 0px; -webkit-text-stroke-width: 0px"><br></div>
<div style="TEXT-TRANSFORM: none; TEXT-INDENT: 0px; FONT: 15px monospace; WORD-WRAP: break-word; WHITE-SPACE: normal; LETTER-SPACING: normal; COLOR: rgb(60,60,60); WORD-SPACING: 0px; -webkit-text-stroke-width: 0px">Just
trying to wrap my head around this to be able to make more informed decisions.
<br></div>
<div style="TEXT-TRANSFORM: none; TEXT-INDENT: 0px; FONT: 15px monospace; WORD-WRAP: break-word; WHITE-SPACE: normal; LETTER-SPACING: normal; COLOR: rgb(60,60,60); WORD-SPACING: 0px; -webkit-text-stroke-width: 0px">Any
feedback is HIGHly appreciated. :)<br></div>
<div style="TEXT-TRANSFORM: none; TEXT-INDENT: 0px; FONT: 15px monospace; WORD-WRAP: break-word; WHITE-SPACE: normal; LETTER-SPACING: normal; COLOR: rgb(60,60,60); WORD-SPACING: 0px; -webkit-text-stroke-width: 0px"><br></div>
<div style="TEXT-TRANSFORM: none; TEXT-INDENT: 0px; FONT: 15px monospace; WORD-WRAP: break-word; WHITE-SPACE: normal; LETTER-SPACING: normal; COLOR: rgb(60,60,60); WORD-SPACING: 0px; -webkit-text-stroke-width: 0px"><br></div>
<div style="TEXT-TRANSFORM: none; TEXT-INDENT: 0px; FONT: 15px monospace; WORD-WRAP: break-word; WHITE-SPACE: normal; LETTER-SPACING: normal; COLOR: rgb(60,60,60); WORD-SPACING: 0px; -webkit-text-stroke-width: 0px">thanks,<br></div>
<div style="TEXT-TRANSFORM: none; TEXT-INDENT: 0px; FONT: 15px monospace; WORD-WRAP: break-word; WHITE-SPACE: normal; LETTER-SPACING: normal; COLOR: rgb(60,60,60); WORD-SPACING: 0px; -webkit-text-stroke-width: 0px">Tom<br></div>
<div style="TEXT-TRANSFORM: none; TEXT-INDENT: 0px; FONT: 15px monospace; WORD-WRAP: break-word; WHITE-SPACE: normal; LETTER-SPACING: normal; COLOR: rgb(60,60,60); WORD-SPACING: 0px; -webkit-text-stroke-width: 0px"><br></div>
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