[sdiy] Voltage Feedback Resistors and Circuit Stability

[Bernard Arthur Hutchins, Jr]

David said, Sun Nov 15 06:33:27 CET 2020:

“After many years of building circuits, one acquires a basic sense of where stability caps may be required.  Often, I discover that an opamp circuit is RINGING A BIT WHEN I GO PROBING AROUND WITH THE SCOPE.  I tend to lay out the pads for stability caps on most inverting opamp circuits, but often don't stuff them unless I find that I have to.  I keep a big stash of ceramic 101s for just that purpose.” [Emphasis added]

Is this “ringing” (ringing being a DAMPED oscillation following a discontinuity) or is it rather an actual oscillation (usually low level and HF looking like an out-of-focus trace ----caused by the scope cable capacitance? The problem is caused by the scope cable and goes away when you stop looking!  Then how can you know!  A series 1k resistor on the scope cable is all you need.  See my MW comments of Oct 20:

“The second useful function of the 1k series resistors is that they “decouple” capacitive (typically shielded cables at perhaps C=100 pfd/meter) loads from op-amps thus preventing high-frequency (MHz) oscillations WITH ASSOCIATED DC SHIFTS. Connecting a cable (even to a scope) directly to an op-amp output forms an RC low-pass (R being the inherent op-amp output resistance of perhaps 100 ohms). Such an oscillation is HF but low level (slew limited) and non-symmetric (non-symmetric slew limiting). The result is a fuzzy looking scope trace (looks out of focus) and is only there when a cable is attached, and as a VCO control can cause a small but noticeable pitch shift.

The oscillation occurs because the R is internal to the op-amp and the RC is INSIDE any feedback loop and contributes excessive phase shift. With the 1k series the RC (R now 1k) is OUTSIDE the op-amp’s feedback loop.  Fuzzy trace and pitch shift gone.”


-Bernie
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