[sdiy] Voltage Feedback Resistors and Circuit Stability

Sat Nov 14 20:24:24 CET 2020

Let's not worry about blame and instead focus on a group effort to reduce
inbox clutter.

On Sat, Nov 14, 2020, 11:42 AM BrightBoy, <jdec at mindspring.com> wrote:

> You can blame BAHjr
>
> Jeff
>
> -----Original Message-----
> >From: Quincas Moreira <quincas at gmail.com>
> >Sent: Nov 14, 2020 12:53 PM
> >To: tpuefke <tpuefke at protonmail.com>
> >Cc: Richie Burnett <rburnett at richieburnett.co.uk>, "
> synth-diy at synth-diy.org" <synth-diy at synth-diy.org>, "Bernard Arthur
> Hutchins, Jr" <bah13 at cornell.edu>
> >Subject: Re: [sdiy] Voltage Feedback Resistors and Circuit Stability
> >
> >Hey folks, why is it that pretty much every SDIY thread recently gets
> >split up into multiples? This one is now on two separate threads on my
> >gmail inbox, and it's been happening a lot!
> >
> >On Sat, Nov 14, 2020 at 6:11 AM tpuefke via Synth-diy
> ><synth-diy at synth-diy.org> wrote:
> >>
> >> Thanks guys.
> >>
> >> Very informative replies and i think i'm beginning to see the light
> here.
> >>
> >> 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...
> >>
> >> 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?).
> >>
> >> 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?
> >> I'm surprised i haven't really managed to blow anything up yet.
> >>
> >> 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.
> >>
> >> But deep practical knowlege (and a good scope) remains for now just an
> aspiration.
> >>
> >>
> >> Thank you good people, stay safe.
> >> Tom
> >>
> >>
> >> ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
> >> On Saturday, 14. November 2020 11:29, Richie Burnett <
> rburnett at richieburnett.co.uk> wrote:
> >>
> >> 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.
> >>
> >> 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.
> >>
> >> -Richie,
> >>
> >> Sent from my Xperia SP on O2
> >>
> >> ---- Bernard Arthur Hutchins, Jr wrote ----
> >>
> >>
> >> 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.
> >>
> >>
> >> 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.
> >>
> >>
> >> 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
> >>
> >>
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> >
> >
> >
> >--
> >Quincas Moreira
> >Director | QMA
> >mobile:  5534988825
> >site:  quincasmoreira.com
> >email:  quincas at gmail.com
> >
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