What causes BJT based amplifier distortion?
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 4th February 2021, 05:40 PM #11 N101N diyAudio Member   Join Date: May 2018 In simulation.
stanislav1957
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Join Date: Dec 2018
Location: НОВГОРОД
Quote:
 Originally Posted by N101N In simulation.
No.This is not true.
Although in the simulator, not every circuit will give low distortion.

PRR
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Join Date: Jun 2003
Location: Maine USA
Quote:
 Originally Posted by edbarx Mathematically, for global negative feedback to reduce the difference between the non-inverting and inverting input to zero, the open loop gain must be infinite....t.
You are trying to make this a Zeno Paradox. Have fun.

The true philosopher will understand this analogy:
"A mathematician, a physicist and an engineer were asked to answer the following question. A group of boys are lined up on one wall of a dance hall, and an equal number of girls are lined up on the opposite wall. Both groups are then instructed to advance toward each other by one quarter the distance separating them every ten seconds (i.e., if they are distance d apart at time 0, they are d/2 at t=10, d/4 at t=20, d/8 at t=30, and so on.) When do they meet at the center of the dance hall? The mathematician said they would never actually meet because the series is infinite. The physicist said they would meet when time equals infinity. The engineer said that within one minute they would be close enough for all practical purposes."

If one transistor has THD as high as 20% and raw gain of 10^3, and four cascade stages has gain of 10^12, and the amp needs gain of 10^2, then it may be possible to approach THD of 20%/10^10 or 0.000,000,002%. Not zero!! But like the dancers, close enough to grope.

 5th February 2021, 06:52 AM #14 googlyone   diyAudio Member   Join Date: Nov 2006 Location: Adelaide, South Australia I kind of wonder why the original question was asked given the response to answers. Which seems to be championed by some "Never-solid-state" denial of reality. Earlier answers point out the fundamentals of how solid state amplifiers achieve good performance. Given the denials that this evoked, I guess pointing toward Douglas Self's work would not help much. But I will anyway. Just out of interest, N101N, would you suggest valves will deliver lower distortion in a typical application?
 5th February 2021, 08:15 AM #15 N101N diyAudio Member   Join Date: May 2018 Valves have superior capabilities, but are struggling to cope with modern day loudspeaker loads and listening practices. JFETs and MOSFETs have a more favorable architecture than bipolars for signal handling, but are limited as a voltage amplifier.
 5th February 2021, 10:28 AM #16 googlyone   diyAudio Member   Join Date: Nov 2006 Location: Adelaide, South Australia And at this point any further discussion spears off into valve vs transistor dogma. The OP asked what approaches were taken to achieve a linear response. While there seemed to be some misinterpretation of transistor behaviour in the OP question, the basic answer is in the preceding posts, and pointers to texts that go into thorough detail as to how this works. Similar to design approaches taken to address transistor operating characteristics, there are a multitude of design approaches taken to deal with the characteristics of valves. (Output transformer anyone? Inter-stage coupling? Whole topologies built around the use of "N Type" valves... for want of a term for the complete absence of complementary valves...) It's not an issue of "better or worse" or borderline religious dogma, but basic design.
 5th February 2021, 11:14 AM #17 N101N diyAudio Member   Join Date: May 2018 What are you trying to say? What you can do is use devices in their most favorable operating region but not avoid or improve their basal characteristics.
 5th February 2021, 11:23 AM #18 DNi   diyAudio Member   Join Date: Jan 2013 Location: Winterthur, Switzerland edbarx: In his book "Audio power amplifier design handbook", Douglas Self discusses at saome length the causes of nonlinear distortion in transistor amplifiers. Regards, Braca
edbarx
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Join Date: May 2018
Location: Maltese Archipelago
Quote:
 Originally Posted by PRR You are trying to make this a Zeno Paradox.
The first step to understand a philosophical statement is to search for and understand what assumptions/axioms it is based on. The little reading I did, indicates, Zeno would reject all physics of matter, which is known to consist of atoms. There is no need to argue collections can be finite or infinite today. This is a known fact.
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Any fool can make things bigger, more complex, and more violent. It takes a touch of a genius, and a lot of courage, to move in the opposite direction. [Albert Einstein]

edbarx
diyAudio Member

Join Date: May 2018
Location: Maltese Archipelago
Quote:
 Originally Posted by googlyone The OP asked what approaches were taken to achieve a linear response. While there seemed to be some misinterpretation of transistor behaviour in the OP question, the basic answer is in the preceding posts, and pointers to texts that go into thorough detail as to how this works.
Since, the purpose of this thread is to understand, it makes a lot of sense to ask: What did you notice I did not understand?
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Any fool can make things bigger, more complex, and more violent. It takes a touch of a genius, and a lot of courage, to move in the opposite direction. [Albert Einstein]

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