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Old 28th December 2011, 09:56 PM   #321
wahab is offline wahab  Algeria
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RF amplifiers have less efficient local feedback due to group delay
being variable not only with frequency but also with the transistor s
moving parameters in function of temperature.

Negative feedback efficency is inherently dependant of the output s
phase value.

Idealy , it is 180 , but if phase delay increase , say to 270 ,
NFB efficency will be drasticaly reduce and linearity will decrease.

In audio apps nothing of the sort as the dynamic non linearity
evolution is way slower than the NFB loop , so the 10khz signal
has nothing to be modulated by/with.

Besides , one has to look closely at the phenomenon , as it was
too much rapidly branded as being of negative influence.

Let s admit than a pulse rise the device temperature.
Although with a significant delay , the transistor see its beta
increasing as well as the zero crossing standing current also
increased , assumed an eventual thermal tracking is dependant
of the main heatspreader.

Would such parameters variations be negative in respect
of the amp linearity ? I would find it unlikely.
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Old 28th December 2011, 10:04 PM   #322
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So based on some typical datasheets along with experiences and other references I dont see why there wouldn't be temperature modulation with signal.
I would expect to see some rectified droop type variation of the die temperature at 10Hz.

Not commenting on the audibility or if its a dominate source when such signals are present.

Might it be easier to just single pulse an amp while looking at the input to the output stage recovery?

Thanks
-Antonio
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Old 28th December 2011, 10:06 PM   #323
tvrgeek is offline tvrgeek  United States
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Sounds like a discussion best solved only on a test bench in a light box.
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Old 29th December 2011, 12:29 AM   #324
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Quote:
Originally Posted by DF96 View Post
What if the corner frequency for the LF distortion rise is significantly lower than the LF rolloff of the amplifier? You can't then measure it using the normal methods, but an envelope technique might still show it.

Of course, NFB would still reduce it as it is something in the output which is not in the input but NFB second-order effects might create problems.

So, what sort of junction thermal time constant are we talking about for a typical audio output BJT chip? I guess we can assume the case is at a constant temperature, so it is junction-case heat flow which matters. Case to ambient thermal time constants would be in the 10s secs-mins range?
No,no! Thermal feedback, something like Vbe multiplier, adds some additional details to the interesting picture!
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Old 29th December 2011, 12:39 AM   #325
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Quote:
Originally Posted by tvrgeek View Post
Sounds like a discussion best solved only on a test bench in a light box.
It would be great if we could test separately influences of Kirk, Early, thermal effects, base width, Ft modulation, beta dependence on current and temperature, Vcbo, capacitances, and so on, one by one on the surgeon bench. Then put a vacuum triode there and check do they exist at all.
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Old 29th December 2011, 12:51 AM   #326
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Quote:
Originally Posted by Pano View Post
Have you tried GIMP?. Not drawing software per se, but you can do a lot with a little practice. There isn't a good schematic capture software for Linux? None of the SPICE versions?
I am downloading gEDA now, let's see how convenient it is, after I build it for Scientific Linux (Fedora-like)
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Old 29th December 2011, 04:02 AM   #327
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Sy,

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Originally Posted by SY View Post
It's basic heat transfer physics which I had thought (from your earlier posts) you grasped.
I completely grasp it. And if you have IR equipment with a sufficient resolution and speed (some USB IR Cameras do) you can see thermal modulation in Plastic Transistors on heat sinks without opening them up.

BTW, the presence of thermal memory in typical Op-Amp circuits was confirmed previously by non other than Scott Wurcer (#769), though in his case (and in real circuits) the thermal modulation (or memory) mainly applied to Input and VAS transistors...

I find you continued denial of the actual existence of these effects, as well you making up experimental proof that never existed somewhat baffling, but I am sure you have your reasons...

Ciao T
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Old 29th December 2011, 04:18 AM   #328
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Hi,

Quote:
Originally Posted by Pano View Post
OK, but I'm still interested in brief signals that are 20dB or more greater than the average level.
Yes, in other words, music.

Quote:
Originally Posted by Pano View Post
Does running the output devices and drivers at room temperature or above the boiling point of water make any difference? How could you tell and how could you separate any difference found from other causes (PSU, or other)?
There are differences when running devices hot (e.g. hot transistors have higher beta, which reduces loading distortion in the circuit) so a circuit running at elevated temperature will likely measure different than one run cold. There are as remarked also other factors that are not obviously electronic in nature.

If we take these out of the equation however, the difference in thermal load is likely to introduce very similar levels of parameter modulation no matter at what absolute temperature you start, the relative load change and relative temperature change will be the same.

It is however non-trivial to separate out all these (and other) thermal effects and as remarked, in terms of measured distortion they are often swamped out by many factors...

Ciao T
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Old 29th December 2011, 04:20 AM   #329
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Hi,

Quote:
Originally Posted by ThorstenL View Post
by non other than Scott Wurcer (#769)
Appypollylogy.

There was 'sposed to be a link with this:

by non other than Scott Wurcer (#769)

Ciao T

Last edited by ThorstenL; 29th December 2011 at 04:25 AM.
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Old 29th December 2011, 04:50 AM   #330
Pano is offline Pano  United States
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