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Old 24th December 2012, 12:26 PM   #101
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Wonderful graphs!

What did you use to create them??
I presume simulations were the source, or is this real amplifiers?

As far as an increase in gm outside the xover region, and the second feedback loop being active when the device (side) is *on*, I am wondering if this is unavoidable? In other words there is no circuit design thus far that anyone has come up with that is in essence "off" while that device is amplifying?

Another related question, perhaps someone knows the answer - were all the "early" sliding bias designs concerned with making the output devices non-switching, or did they (as I always presumed) aim at getting "class A" performance at moderate levels and sliding *off* the bias to get headroom on peaks? (Class A with less heat, in essence) [EDIT: It seems to me that these early circuits acted on both polarity devices simultaneously, not separately.]] In which case, perhaps this should not be referred to as "sliding bias"...?

Perhaps "NSB" or non-switching bias??
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Last edited by bear; 24th December 2012 at 12:32 PM.
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Old 24th December 2012, 02:11 PM   #102
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Quote:
Originally Posted by bear View Post
Wonderful graphs!

What did you use to create them??
I presume simulations were the source, or is this real amplifiers?
I just simulated it with MicroCap.

Quote:
As far as an increase in gm outside the xover region, and the second feedback loop being active when the device (side) is *on*, I am wondering if this is unavoidable? In other words there is no circuit design thus far that anyone has come up with that is in essence "off" while that device is amplifying?
.......
Maybe I'm wrong, but considering that the voltage X-fer characteristic of an optimally biased class-AB BJT OPS is almost perfect, I'm afraid that any manipulation of the bias voltage will affect the THD figures. Not that it really matters, as this is where the 'other' loop comes at rescue.

Cheers,
E.
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Old 24th December 2012, 02:35 PM   #103
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You need to AFEC your amplifier. Keep the main amp forward gain LOW, but fast, and then use AFEC to take it to the low single digit ppm distortion levels. This approach works especially well with class A, but can also bring benefits with optimally biased class B.
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Old 24th December 2012, 09:02 PM   #104
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I wish you all a Merry Xmas.

Cheers,
E.
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Old 25th December 2012, 02:12 AM   #105
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Quote:
Originally Posted by Edmond Stuart View Post
I wish you all a Merry Xmas.

Cheers,
E.
Merry Xmas Edmond!

How is your distortion analyzer coming on my the way?
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Old 25th December 2012, 10:37 AM   #106
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Default /OT

Quote:
Originally Posted by Bonsai View Post
Merry Xmas Edmond!
How is your distortion analyzer coming on my the way?
Thx Andrew.
As for the distortion analyzer, I'm still struggling with the (software) IQ detector and frequency discriminator. The trouble is that if you don't (or can't) calculate the phase, amplitude, etc. based on exactly a whole number of cycles, you get erroneous answers. A well known trick is to use a window (Hamming, Hanning, Kaiser, etc), which, indeed, reduces the error, but not completely, only by a few orders. And that's is not enough. So I'm looking for an error function that exactly compensates for the errors of the detectors/discriminators. Regrettably, my knowledge of math fails short to figure this out analytically. So I hope to find it by trial and error. Not the most efficient way!

Cheers,
E.

PS: I also simmed an OPS based on MOSFETs, with and without sliding bias. And also in this case the THD increases with sliding bias turned on, but not always: with small signals, below 10V or so, THD decreases.
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Old 26th December 2012, 08:28 AM   #107
wahab is offline wahab  Algeria
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Quote:
Originally Posted by Waly View Post

In fact, it is gm doubling around the crossover largely responsible for the "crossover distortion".
That s correct..

Quote:
Originally Posted by Waly View Post
More overlapping -> more non-linear total transconductance -> more distortion.
Not systematicaly true according to your own explanations.

Quote:
Originally Posted by Waly View Post
Heavy bias towards class A and the transconductance hump flattens away, to the point to which the gm doubling effect is negligible compared to other distortion mechanisms.
Also correct , although "gm doubling" should be replaced by "gm variation".


Quote:
Originally Posted by Edmond Stuart View Post
In this particular case, I don't think gm doubling is the culprit. It is the modulation of the bias voltage that is responsible for the increase of THD.
Also my sims didn't reveal gm doubling.
Hi Edmond ,

I think Waly more or less explained it correctly but as you point it Vbe
modulation has a role in gm variation.

To summarize , all the evil comes from the external emitters resistors
as well as from the internal Re resistors , although from a lesser extent.

Assuming there s no external emitters R and using two pairs of devices to mitigate
the internal Re , sims show that the output stage will barely switch off even at high amplitude.

Moreover , the transition from one device to the other will be smoother ,
with less gm variation but more importantly with higher gm in the transition
area.

Adding external RE will only increase dramaticaly the gm variation , and
worse , will reduce the combined gm in the said crossover zone , inducing
higher distorsion since the non linear part of the output/input caracteristic
will be of greater absolute value.

Last edited by wahab; 26th December 2012 at 08:31 AM.
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Old 27th December 2012, 02:07 PM   #108
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Quote:
Originally Posted by wahab View Post

> Originally Posted by Waly
>In fact, it is gm doubling around the crossover largely responsible for the "crossover distortion".

That's correct..
Hi Wahab,

That is NOT correct. Gm doubling takes place at large output current, thus not within the crossover region. Please read (again) post #100 and look at the 4th picture.
.....
Quote:
Hi Edmond ,

I think Waly more or less explained correctly...
I would say just less explained it correctly...


Quote:
but as you point it Vbe modulation has a role in gm variation.
It's the bias modulation that flattens the tops and valleys of difference signal between input and output. As a result, this difference signal is heavily distorted. Since the bias voltage appears as series component in the signal path, the whole circuit also produce more distortion. For the same reason the effective gm is also distorted and doubles at large current excursions.

Has it something to do with widening of the crossover region? No. Suppose the sliding bias is arranged in such way, that when one of the OP tranies is hard turned on (say 10A) the other tranny draws only 10uA. Since the latter is almost turned off completely, it hardly can contribute to gm. Yet you will see gm doubling. So something else is responsible for gm doubling.

Quote:
To summarize , all the evil comes from the external emitters resistors
as well as from the internal Re resistors , although from a lesser extent.

Assuming there s no external emitters R and using two pairs of devices to mitigate the internal Re , sims show that the output stage will barely switch off even at high amplitude.
That's right. In addition, the product of top and bottom collector current will be (almost) constant. In this (hypothetical) case you don't need a sliding bias at all; you get it for free.

Quote:
Moreover , the transition from one device to the other will be smoother , with less gm variation but more importantly with higher gm in the transition area.

Adding external RE will only increase dramaticaly the gm variation , and
worse , will reduce the combined gm in the said crossover zone , inducing
higher distorsion since the non linear part of the output/input caracteristic
will be of greater absolute value.
Sorry, I disagree. Without RE and RE' you will get a huge gm dip at the crossover zone.

Cheers,
E.
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Old 27th December 2012, 03:07 PM   #109
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Damn it, I tried to post something and I still get this "Reported attack Site" msg.
Let's try again:

I realize that the whole discussion about gm doubling might be confusing, the more so as I previously stated that the distortion was not due to gm doubling, by which I meant, of course, gm doubling in the usual sense, i.e. the phenomenon which appears around the crossover zone of class-AB amps. So we better should make a distinction between small signal gm doubling (the familiar one) and large signal gm doubling, which occurs in class-i amps.

Cheers,
E.
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Last edited by Edmond Stuart; 27th December 2012 at 03:19 PM.
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Old 27th December 2012, 03:27 PM   #110
wahab is offline wahab  Algeria
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It is gm variation that cause distorsion whatever the level at wich it occurs.

As such , distorsion will be higher around points where the derivative
of the gm has the higher absolute value , that is , around the points
where the gm slope is the steppest.
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