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<span style="font-family:"Arial",sans-serif">First, the question was about : </span>
<a href="https://synth-diy.org/pipermail/synth-diy/2020-November/174435.html"><span style="font-size:13.5pt;font-family:"Arial",sans-serif;color:black;mso-color-alt:windowtext;background:#FAFAFA">[sdiy] Voltage Feedback Resistors and Circuit Stability</span></a><span style="font-family:"Arial",sans-serif">.<span style="mso-spacerun:yes">
</span>“Feedback resistance“ makes almost no sense.<span style="mso-spacerun:yes">
</span>It is a feedback RATIO that maters, as a design objective (e.g., setting gain) and in any (if any at all) op-amp stability issues.<span style="mso-spacerun:yes">
</span>Typically, this ratio is set by a series resistive voltage divider Vout/Vin=R2/(R1+R2) where R1 is connected to Vin, and R2 goes (usually) to ground.<span style="mso-spacerun:yes">
</span>A ratio of 1/11 (0.090909…) is obtained by R2 = R1/10.<span style="mso-spacerun:yes">
</span>R2 might be 10k with R1=100k, or R2 might be 1k with R1 = 10k, etc.<span style="mso-spacerun:yes">
</span>If Vin is the output of an op-amp, good practice suggests that the SUM of R1 and R2 should be at least several k (for the op-amp to drive) and less than about a meg (to avoid stray signal pickup). ELSE which values do you have the most of.
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<span style="font-family: Arial, sans-serif;">All this freedom goes out the door IF THERE ARE CAPACITORS IN THE DIVIDER LOOP!</span><span style="font-family: Arial, sans-serif;">
</span><span style="font-family: Arial, sans-serif;">In this case, if you scale the resistors by a factor B, you must scale the capacitors by 1/B. I can’t recall a familiar example of this in a feedback case, but in an input attenuator case, one is very familiar:
the case of an R1 =100k, R2= 220 ohm attenuator into our original OTA integrators</span><span style="font-family: Arial, sans-serif;">
</span><span style="font-family: Arial, sans-serif;">(S-V VCFs).</span><span style="font-family: Arial, sans-serif;">
</span><span style="font-family: Arial, sans-serif;">Originally no capacitors were used.</span><span style="font-family: Arial, sans-serif;">
</span><span style="font-family: Arial, sans-serif;">Then we started to use shunting “phase-lead” capacitors across the 100k R1.</span><span style="font-family: Arial, sans-serif;">
</span><span style="font-family: Arial, sans-serif;">The needed shunting capacitor was an inconveniently small (rare, and comparable to stray) 3pf or so. This is why we changed R1 down to 10k, R2 down to 22 ohms, and C up to a more agreeable 30 pfd.</span><o:p style="color: inherit; font-family: Arial, sans-serif; font-size: inherit; font-style: inherit; font-variant-ligatures: inherit; font-variant-caps: inherit; font-weight: inherit; background-color: ;"> </o:p></p>
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<span style="font-family:"Arial",sans-serif">Finally, keep in mind that “instability, in general, is intuitively associate with high gain (like positive feedback in PA systems).<span style="mso-spacerun:yes">
</span>In the case of linear op-amp applications, NEGATIVE feedback is used to restrain the extremely large gain of otherwise open-loop devices.<span style="mso-spacerun:yes">
</span>Low gain circuits have MORE (presumed negative) feedback.<span style="mso-spacerun:yes">
</span>To the extent that actual feedback slides slightly less negative, a high-frequency oscillation may kick in.<span style="mso-spacerun:yes">
</span>Maximum (negative) feedback (100%) is at unity gain – hence the near universal unity-gain internal compensation.
<span style="mso-spacerun:yes"> </span>Such an op-amp “follower” may oscillate if asked to drive a long scope cable (capacitor) for example (phase shift inside the<span style="mso-spacerun:yes">
</span>loop) while being perfectly well-behaved if the gain is perhaps 4!<span style="mso-spacerun:yes">
</span><span style="mso-spacerun:yes"> - Bernie </span><o:p> </o:p></span></p>
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