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<DIV dir=ltr align=left><SPAN class=246495800-15112020><FONT color=#0000ff
size=2 face=Arial>Yes, adding feedback capacitance is the standard way, but if
you've already got all the PCBs for a design made without these capacitors in
place, then it is nice to know that decreasing the values of the feedback (and
input) resistors is one way to eliminate ringing.</FONT></SPAN></DIV><BR>
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<FONT size=2 face=Tahoma><B>From:</B> Synth-diy
[mailto:synth-diy-bounces@synth-diy.org] <B>On Behalf Of </B>Richie
Burnett<BR><B>Sent:</B> Saturday, November 14, 2020 2:29 AM<BR><B>To:</B>
synth-diy@synth-diy.org; Bernard Arthur Hutchins, Jr<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>In my experience smaller feedback resistors
generally give better op-amp stability. With a large feedback resistance there
is more potential for stray capacitance between the virtual earth (inverting
input) node and ground to cause a phase lag. Too much phase lag degrades phase
margin and turns negative feedback into positive feedback, ultimately leading to
oscillation. Just like the case of capacitive loading on the output that Bernie
mentioned. However the virtual earth node can be much more sensitive to stay
capacitance than the op-amp output which is generally a stiffer voltage
source.<BR><BR>Of course the proper solution to the problem described above is
to add some capacitance across the feedback resistance. This introduces some
phase-lead into the feedback path to compensate for the undesirable lag from the
stray capacitance. This improves the phase margin and moves the op-amp further
away from instability/oscillation. This process is commonly used in things like
photodiode amplifiers to prevent the capacitance of the photodiode from
destabilising the op-amp.<BR><BR>-Richie,<BR><BR>Sent from my Xperia SP on
O2<BR><BR>---- Bernard Arthur Hutchins, Jr wrote ----<BR><BR>
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<P
style="LINE-HEIGHT: 107%; MARGIN: 0in 0in 8pt; FONT-FAMILY: Calibri, sans-serif; FONT-SIZE: 11pt"
class=MsoNormal><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-FAMILY: 'Arial',sans-serif; BACKGROUND: #fafafa; COLOR: black; FONT-SIZE: 13.5pt; mso-color-alt: windowtext">[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. <O:P> </O:P></SPAN></P>
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class=MsoNormal><SPAN style="FONT-FAMILY: Arial, sans-serif"><BR></SPAN></P>
<P
style="LINE-HEIGHT: 107%; MARGIN: 0in 0in 8pt; FONT-FAMILY: Calibri, sans-serif; FONT-SIZE: 11pt"
class=MsoNormal><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="FONT-FAMILY: Arial, sans-serif; font-variant-ligatures: inherit; font-variant-caps: inherit"> </O:P></P>
<P
style="LINE-HEIGHT: 107%; MARGIN: 0in 0in 8pt; FONT-FAMILY: Calibri, sans-serif; FONT-SIZE: 11pt"
class=MsoNormal><O:P
style="FONT-FAMILY: Arial, sans-serif; font-variant-ligatures: inherit; font-variant-caps: inherit"><BR></O:P></P>
<P
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class=MsoNormal><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><BR></DIV></BODY></HTML>