Resistor opinion

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#142
DPH said:
It's still 4x better thermally, so the tracking delta is notably smaller between Rf and Rg (and, IIRC any voltage terms are usually V^2, so that 1/2 peak voltage is great) and much easier to package with less stray capacitance.

They're all along a spectrum of compromises vs spec needs. 🙂 This is a case of gooder vs gooder-er in terms of matching tempcos.
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Yes, but if you want to fix that, why not fix it completely instead of only for 1/4?

DPH said:
Also the thermal gradients from everything else on the PCB are going to swamp out the Rf vs Rg imbalance at some point -- obviously depending on layout/case/etc
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The thermal stuff within the amplifier doesn't impact because it is inside the feedback loop. That is why the feedback resistors are the critical ones, and effort there will directly make the amp more linear.

Jan
 
#143
johnego said:
Usually tempco spec is similar for the same resistor type. So 20k or 1k they may have the same spec, say 1%/K, so no different, the gain is still the same either one 20k or 20x1k..
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No this is incorrect. The 20k gets 20 times the dissipation than the 1k so it gets much hotter and it changes its value much more, percentage wise, than the 1k.
Distortion will result.

Jan
 
#144
jan.didden said:
The thermal stuff within the amplifier doesn't impact because it is inside the feedback loop. That is why the feedback resistors are the critical ones, and effort there will directly make the amp more linear.
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While this is admittedly fairies on the pinhead we're discussing, I think my point was missed. Let's say layout-wise the Rg is closer to a heat source on the PCB than the Rf resistor stack such that Rg is nominally 1-2 °C warmer than the rest of the feedback resistors -- we now have a static gain offset from our target. Similarly, with a high signal, there will be a smaller deltaT from Rg to its surroundings vs Rf, such that *again* we're going to see that there's a temperature mismatch instead of perfect tracking. This could obviously happen anywhere in the feedback resistor stack, and the greater the number of resistors involved, the more likely this is to occur.

When does this matter? It would require a combination of careful modeling of the circuit and thermal space, backed up with heavy experimental validation to say. Intuition says that it'd be so infinitesimal that it's completely lost in the lowest of noise (even with synchronous sampling), but nothing on this planet is ever perfect. 🙂
 
#145
johnego said:
I thought Jan was questioning the 'sources' of the heat.
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He wasn't specific.


One example, emitter resistors, I have amps where they are perpendicular
to the 2sa1386 transistor (next to it) and it's heat sink. One end get warmer
than the other end, higher distortion etc.



If the resistor configuration was parallel to a s2a1386 transistor and it's heatsink the resistor would have more equal temperature across it.


I guess that's also the difference between consumer electronics (don't care)

and test and measurement/metrology (they care and everything counts) gear.



cheers,
 
#156
I just received a package from Mouser. It was mostly the resistors to complete my latest project. I decided to try Vishay/Beyschlag MBB 0207 series for the first time and found all but 2 values I needed. Ordered 11 various varieties MBBs and 2 values of Xicons.

Measuring the resistors still on their reel tape before soldering, ALL the Beyschlag measured open. The Xicons measured to spec.

WTF. Is this a common occurrence with Beyschlag?
 
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