<div dir="auto">Let's not worry about blame and instead focus on a group effort to reduce inbox clutter.</div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Sat, Nov 14, 2020, 11:42 AM BrightBoy, <<a href="mailto:jdec@mindspring.com">jdec@mindspring.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">You can blame BAHjr<br>
<br>
Jeff<br>
<br>
-----Original Message-----<br>
>From: Quincas Moreira <<a href="mailto:quincas@gmail.com" target="_blank" rel="noreferrer">quincas@gmail.com</a>><br>
>Sent: Nov 14, 2020 12:53 PM<br>
>To: tpuefke <<a href="mailto:tpuefke@protonmail.com" target="_blank" rel="noreferrer">tpuefke@protonmail.com</a>><br>
>Cc: Richie Burnett <<a href="mailto:rburnett@richieburnett.co.uk" target="_blank" rel="noreferrer">rburnett@richieburnett.co.uk</a>>, "<a href="mailto:synth-diy@synth-diy.org" target="_blank" rel="noreferrer">synth-diy@synth-diy.org</a>" <<a href="mailto:synth-diy@synth-diy.org" target="_blank" rel="noreferrer">synth-diy@synth-diy.org</a>>, "Bernard Arthur Hutchins, Jr" <<a href="mailto:bah13@cornell.edu" target="_blank" rel="noreferrer">bah13@cornell.edu</a>><br>
>Subject: Re: [sdiy] Voltage Feedback Resistors and Circuit Stability<br>
><br>
>Hey folks, why is it that pretty much every SDIY thread recently gets<br>
>split up into multiples? This one is now on two separate threads on my<br>
>gmail inbox, and it's been happening a lot!<br>
><br>
>On Sat, Nov 14, 2020 at 6:11 AM tpuefke via Synth-diy<br>
><<a href="mailto:synth-diy@synth-diy.org" target="_blank" rel="noreferrer">synth-diy@synth-diy.org</a>> wrote:<br>
>><br>
>> Thanks guys.<br>
>><br>
>> Very informative replies and i think i'm beginning to see the light here.<br>
>><br>
>> So a good rule of thumb might be using 100k in input stages for impedance matching and lower values in buffered stages, as far as your power budget allows...<br>
>><br>
>> I sure have got a lot of reading-up to do on some of these subjects, like stray capacitance and Johnson noise (and are there any prescriptions for that?).<br>
>><br>
>> As mentioned i don't have a formal engineering background, so nomenclature and some advanced concepts can be a struggle. It's the worst of both worlds really, hobbyist musician without formal education delves into EE. What could possibly go wrong?<br>
>> I'm surprised i haven't really managed to blow anything up yet.<br>
>><br>
>> The main sources of knowledge that helped me a great deal, besides this list and MFOS, were MIT's open courseware (6002 specifically), Malvino&Bates, the odd book or application note where i can follow some of the content, Aaron Lanterman's lectures (Hi Aaron!), and naturally some of the schematics that are out there.<br>
>><br>
>> But deep practical knowlege (and a good scope) remains for now just an aspiration.<br>
>><br>
>><br>
>> Thank you good people, stay safe.<br>
>> Tom<br>
>><br>
>><br>
>> ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐<br>
>> On Saturday, 14. November 2020 11:29, Richie Burnett <<a href="mailto:rburnett@richieburnett.co.uk" target="_blank" rel="noreferrer">rburnett@richieburnett.co.uk</a>> wrote:<br>
>><br>
>> 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>
>><br>
>> First, the question was about : [sdiy] Voltage Feedback Resistors and Circuit Stability. “Feedback resistance“ makes almost no sense. 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. 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. A ratio of 1/11 (0.090909…) is obtained by R2 = R1/10. R2 might be 10k with R1=100k, or R2 might be 1k with R1 = 10k, etc. 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.<br>
>><br>
>><br>
>> All this freedom goes out the door IF THERE ARE CAPACITORS IN THE DIVIDER LOOP! 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 (S-V VCFs). Originally no capacitors were used. Then we started to use shunting “phase-lead” capacitors across the 100k R1. 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.<br>
>><br>
>><br>
>> Finally, keep in mind that “instability, in general, is intuitively associate with high gain (like positive feedback in PA systems). In the case of linear op-amp applications, NEGATIVE feedback is used to restrain the extremely large gain of otherwise open-loop devices. Low gain circuits have MORE (presumed negative) feedback. To the extent that actual feedback slides slightly less negative, a high-frequency oscillation may kick in. Maximum (negative) feedback (100%) is at unity gain – hence the near universal unity-gain internal compensation. Such an op-amp “follower” may oscillate if asked to drive a long scope cable (capacitor) for example (phase shift inside the loop) while being perfectly well-behaved if the gain is perhaps 4! - Bernie<br>
>><br>
>><br>
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><br>
><br>
><br>
>-- <br>
>Quincas Moreira<br>
>Director | QMA<br>
>mobile: 5534988825<br>
>site: <a href="http://quincasmoreira.com" rel="noreferrer noreferrer" target="_blank">quincasmoreira.com</a><br>
>email: <a href="mailto:quincas@gmail.com" target="_blank" rel="noreferrer">quincas@gmail.com</a><br>
><br>
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</blockquote></div>