What causes BJT based amplifier distortion?

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The obvious answer is active device non-linearity, but this is too generic and abstruct. BJTs have an exponential input characteristic, which means, the more current passes through the base, the easier it is to pass more. The output characteristic, that is, the collector current at a constant base current, is not a horizontal line. It is neither a line with a positive gradient, but a curve albeit with small positive gradients. As if these imperfections were not enough, there is also a voltage dependent capacitance formed by both depletion layers at the emitter-base interface and at the base-collector interface. Since, a transistor silicon chip is thin, the emitter-base depletion layer and the base-collector depletion layers are in close proximity, and the electric field at the latter affects the electric field at the former. All these unwanted characteristics of NPN and PNP doped silicon crystals contribute to a transistor operation that is not simply linear, but depends on device voltages and applied frequencies.

Since, this forum is intended for enthusiasts to build their own amplifiers, I would like those who have primarily the understanding of the concepts to comment. The question is how does amplifier topology circumvent these unwanted transistor properties? I would like the discussion to first consider simple three stage amplifiers, differential inputs, VAS and the power stage including its driver.

Please note, I am not asking to repair my amplifier. That, thanks to this forum, specifically, Mooly and mjona and other contributors, still works and is daily filling my home with beautiful music.
 
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nigelwright7557 said:
A transistor doesn't need to be linear as feedback forces it to be linear.
A typical LTP front end just matches output signal to input signal.
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. Besides, an infinite gain can never be achieved, if such a gain were possible, it would send the amplifier into self oscillation. Self oscillation is why an amplifier employing global negative feedback, has to have its open loop gain reduced, using Miller capacitor compensation, notwithstanding real amplifiers do not have an infinite open loop gain. The capacitor shunts the main audio signal to the VAS's collector, and this shunting becomes more and more pronounced with increasing frequency. So, an amplifier employing global negative feedback behaves differently for all frequencies in the audio range and beyond. At the lowest extreem, negative feedback compensation is most effective because of more open loop gain. However, this effectiveness, decays with increasing frequency, forcing an even bigger voltage difference between the inverting and non-inverting inputs for the same output voltage. This means, the distortion components, that is, the non-linear components that should be added to the distortionless signal to get the voltage difference signal, increase with increasing frequency.

Global negative feedback as used in audio amplifiers is a sword with two edges: it simplifies amplifier design and it introduces problems that would not be present without it.
 
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Bipolar transistors are fairly linear if used in common-base or emitter-follower configuration though - and not too bad if current-driven in common-emitter. Its only voltage drive that exposes the exponential characteristic, and good amp designs seek to minimize that.

For instance the standard 3-stage BJT amp uses a differential pair, then a current-driven VAS, then emitter-follower output devices.

The differential pair linearizes the characteristic to arctangent, and global feedback then reduces the signal swing over that arctangent region to a very low voltage indeed.

The VAS is linearized at higher frequencies by a capacitor, which also serves to swamp the non-linear capacitance. The capacitor means the global feedback decreases at higher frequencies though, but then there is less VAS distortion for it to cure due to the capacitor.

Basically clever design reduces non-linearities.


There are also some clever front-end designs using the translinear loop principle, where exponentials are used to advantage.
 

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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.
 
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?
 
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.
 
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.
 
googlyone said:
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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