5 ms·

Does the non-linearity come from the diode or does it come from the resistor?

by melony 4y ago

deleted 4y ago
[deleted]
klodolph 4y ago
Resistors are generally very linear. They're put in the same category as transformers, capacitors, and inductors, where we just ignore the non-linearities--they're usually too small to measure, although reverse-biased electrolytic capacitors are definitely nonlinear and I've heard you can be affected by nonlinearity on carbon-composition resistors if you put enough voltage across them--like, 100V--don't know if it's true, just "heard it somewhere". Diodes are incredibly nonlinear, that's the whole point of having diodes.
- melony 4y ago
  Isn't the diode linear only within a certain part of the VI graph?
  - jupp0r 4y ago
    No, it's exponential all around. For any infinitesimal small delta V you can assume it to be almost linear. This works for amplifying really small signals.
  - klodolph 4y ago
    It's not really linear anywhere, except in the sense that it's got a smooth VI graph, so you can pick a point on the curve and draw a tangent line if you want. That's not really linear, unless you have small enough signals or bad enough oscilloscopes. It's much closer to exponential, which is Shockley diode equation / "diode law" which is the simplest model you would use besides the "diode is a one-way switch with a voltage threshold" model. The diode law is I = Is[ exp(VD/nVT) - 1 ]. There are more sophisticated models of the diode that use multiple exponentials. Remember that these are all just approximations of reality based on some simplifying assumptions, the only real question is "how much do you want to simplify", and linear models are very rarely useful for modeling diodes.
    - mhh__ 4y ago
      This is one thing that's always annoyed me about reading EE books - just tell me the equation and I'll approximate it (thank you), don't tell me a million approximations before the actual behaviour.
      - klodolph 4y ago
        You're asking for "the equation", but "the equation" does not exist. There are always multiple approximations, and the only ground truth is experiments (with messy data that does not exactly fit the equations) and the underlying quantum mechanics (which is difficult to simulate). If you want something super accurate you can end up running a solid state physics simulation. If you want to model a circuit in SPICE, you get to pick from like a dozen different models for transistors, depending on what you care about--including execution time for your simulation! If you're designing a circuit, it might be easier to think of a transistor as an infinite gain transconductance amplifier, or something equally absurd--and yet useful. If you're designing for RF, then you have to abandon the lumped component model--there is no longer any such thing as a "wire", everything is instead connected by miniature antennas. Trying to teach this to freshmen EE students is insanity. The problem here is that actual insight into how something works is not something that you should conflate with accuracy. The model you choose is something that mediates your understanding of reality. By choosing the right simplifying assumptions, you get something that provides you with the right balance of insight and accuracy. Over the years, people have come up with many models at different points on that tradeoff between insight and accuracy, which allow you to understand circuits and components in different ways. It would be wise to "stand on these shoulders of giants", so to speak, and use these high-value models that balance insight with accuracy, unless you are really interested in studying solid-state-physics in which case, by all means, go as far into the deep end as you want. We still teach freshmen physics students that momentum is mass times velocity, or something like that, for the same reason. It doesn't make sense to explode someone's head with quantum physics and general relativity and then try to introduce Newtonian mechanics as a limiting case. Precious few students would survive.
        mhh__ 4y ago
- buescher 4y ago
  Some dude did a whole series of capacitor linearity articles in Electronics World a couple of decades ago, with measurements. I should revisit them, but I don't have all the issues... but the internet gives: https://linearaudio.nl/cyril-batemans-capacitor-sound-articles https://linearaudio.nl/cyril-batemans-capacitor-sound-articl...

Does the non-linearity come from the diode or does it come from the resistor?

by melony 4y ago

Does the non-linearity come from the diode or does it come from the resistor?

deleted 4y ago

[deleted]

klodolph 4y ago

Resistors are generally very linear. They're put in the same category as transformers, capacitors, and inductors, where we just ignore the non-linearities--they're usually too small to measure, although reverse-biased electrolytic capacitors are definitely nonlinear and I've heard you can be affected by nonlinearity on carbon-composition resistors if you put enough voltage across them--like, 100V--don't know if it's true, just "heard it somewhere". Diodes are incredibly nonlinear, that's the whole point of having diodes.

melony 4y ago
Isn't the diode linear only within a certain part of the VI graph?
- jupp0r 4y ago
  No, it's exponential all around. For any infinitesimal small delta V you can assume it to be almost linear. This works for amplifying really small signals.
- klodolph 4y ago
  It's not really linear anywhere, except in the sense that it's got a smooth VI graph, so you can pick a point on the curve and draw a tangent line if you want. That's not really linear, unless you have small enough signals or bad enough oscilloscopes. It's much closer to exponential, which is Shockley diode equation / "diode law" which is the simplest model you would use besides the "diode is a one-way switch with a voltage threshold" model. The diode law is I = Is[ exp(VD/nVT) - 1 ]. There are more sophisticated models of the diode that use multiple exponentials. Remember that these are all just approximations of reality based on some simplifying assumptions, the only real question is "how much do you want to simplify", and linear models are very rarely useful for modeling diodes.
  - mhh__ 4y ago
    This is one thing that's always annoyed me about reading EE books - just tell me the equation and I'll approximate it (thank you), don't tell me a million approximations before the actual behaviour.
    - klodolph 4y ago
      You're asking for "the equation", but "the equation" does not exist. There are always multiple approximations, and the only ground truth is experiments (with messy data that does not exactly fit the equations) and the underlying quantum mechanics (which is difficult to simulate). If you want something super accurate you can end up running a solid state physics simulation. If you want to model a circuit in SPICE, you get to pick from like a dozen different models for transistors, depending on what you care about--including execution time for your simulation! If you're designing a circuit, it might be easier to think of a transistor as an infinite gain transconductance amplifier, or something equally absurd--and yet useful. If you're designing for RF, then you have to abandon the lumped component model--there is no longer any such thing as a "wire", everything is instead connected by miniature antennas. Trying to teach this to freshmen EE students is insanity. The problem here is that actual insight into how something works is not something that you should conflate with accuracy. The model you choose is something that mediates your understanding of reality. By choosing the right simplifying assumptions, you get something that provides you with the right balance of insight and accuracy. Over the years, people have come up with many models at different points on that tradeoff between insight and accuracy, which allow you to understand circuits and components in different ways. It would be wise to "stand on these shoulders of giants", so to speak, and use these high-value models that balance insight with accuracy, unless you are really interested in studying solid-state-physics in which case, by all means, go as far into the deep end as you want. We still teach freshmen physics students that momentum is mass times velocity, or something like that, for the same reason. It doesn't make sense to explode someone's head with quantum physics and general relativity and then try to introduce Newtonian mechanics as a limiting case. Precious few students would survive.
      - mhh__ 4y ago
buescher 4y ago
Some dude did a whole series of capacitor linearity articles in Electronics World a couple of decades ago, with measurements. I should revisit them, but I don't have all the issues... but the internet gives: https://linearaudio.nl/cyril-batemans-capacitor-sound-articles https://linearaudio.nl/cyril-batemans-capacitor-sound-articl...