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In Reply to: RE: Feedback - Part 4 posted by morricab on February 16, 2024 at 07:24:38

Triodes are square law devices, like FETS. This is not linear. You could operate them only over a narrow range of their characteristic where they will approximate a linear characteristic but that is the same with any non-linear device - if you operate it only over a tiny portion of its range it will approximate to linear (whether that range is large enough to do anything practical is TBD). Degeneration and global feedback work in the same way, so if you think global feedback is bad you should not look at degeneration as good (a difference is that degeneration applies to one device so stability is not an issue, but then global feedback stability stability around many stages is not an issue if it is done right).

The topic of feedback translating low order harmonics to high order harmonics is an interesting one. It was raised decades ago in Wireless world magazine with an example of a single stage amplifier that had a large amount of purely second harmonic. After applying feedback the second harmonic was reduced considerably but also low levels of higher harmonics appeared. The F-Word article that I linked to in a previous post covers this case in the Section 'Storyline 2: Re-entrant distortion'. At worst, this is a case of going from no high order harmonics to some high order harmonics but never 'high order dominant'. And, there are no truly square-law devices, they all have some imperfections that make them not pure square law so they create high order harmonics on their own - I have never seen a single-ended triode power amp spectrum that did not already contain a spray of 3rd, 4th, 5th etc harmonics.

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