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Old 16th December 2012, 12:27 AM   #81
Bonsai is offline Bonsai  Taiwan
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Old 17th December 2012, 08:56 PM   #82
Struth is offline Struth  Canada
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Hi Guys

Ed, what Self and Leech said was and is relevant to the discussion you participated in regarding parallel output devices and et al. No they did not mention class-i but then neither did you until your last post. Whereas most of my posts explicitly discussed class-i.

Andrew T, please reread.

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Old 17th December 2012, 09:31 PM   #83
wahab is offline wahab  Algeria
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Quote:
Originally Posted by Struth View Post
Hi Guys

Ed, what Self and Leech said was and is relevant to the discussion you participated in regarding parallel output devices and et al. No they did not mention class-i but then neither did you until your last post. Whereas most of my posts explicitly discussed class-i.

Andrew T, please reread.

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Class i and siblings

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Old 17th December 2012, 10:37 PM   #84
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Re #82 and prev.
This is obfuscation. You are clearly avoiding discussion of the OP but why distract those who want to continue?

The point is, that those reading this thread can expect to see discussion about class i, not just more flag-waving
for the conventional approaches, views and gurus, who already have their overwhelming share of airplay here.
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Old 21st December 2012, 07:31 PM   #85
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Quote:
Originally Posted by Struth View Post
Hi Guys
Ed, what Self and Leech said was and is relevant to the discussion you participated in regarding parallel output devices and et al. No they did not mention class-i but then neither did you until your last post. Whereas most of my posts explicitly discussed class-i.
Andrew T, please reread.
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First, my name is Edmond, while Ed is an abbreviation of Edward.
Second, >but then neither did you
Is that so? See post #8 and in particular this thread: Class i and siblings
And please, read this thread first before dropping any further comments.
Third, you participated in regarding parallel output devices
True. 'Some people are more equal'.

To all: To revitalize this thread I like to put a few more remarks about class-i and similar circuits (e.g. AB2):

1. The sliding bias does NOT lower the distortion. Quite the opposite: it increases the distortion by a factor two to three (at least in case of a MOSFET OPS, I don't how a BJT OPS behaves).
Why? Because the bias voltage is heavily modulated.
Why it is modulated that way? Because both legs of the OPS are never turned off completely.
And why is that? Because you can't control Iq if one of the OP devices is completely turned off. No current means no information to undertake some action.
Nevertheless, such 'non switching' amps are advertised as beneficial, which IMHO is just marketing blah blah.
The only advantage is that it copes far better with thermal transitions, as the bias voltage (and Iq) is immediately adjusted.

2. Despite above remarks, the distortion of class-i and siblings is far less then amps with a fixed (well, only temperature dependent) bias. This has nothing to do with the sliding bias. Instead, it's due to a second (differential) feedback loop, which locally provides error feedback to the OPS. As a result, the distortion increase from the sliding bias is more than outweighed by the second error correction FB loop.

Cheers,
E.
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Last edited by Edmond Stuart; 21st December 2012 at 07:52 PM.
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Old 23rd December 2012, 12:50 AM   #86
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Edmond,

trying to follow your (1.).

are you asserting that there is no distortion reduction by keeping the output devices from switching off (class AB) there is no distortion benefit per se?

Also, are you saying that the "tracking signal" that prevents the switch off is in fact a second feedback loop that effects each side of the output stage that is conducting??

It would seem that what one would want is for example if the postive rail devices that are being driven "off" and then the signal swinging "negative" that at some theshold at nearly cutoff, that what is wanted is a voltage - essentially the inverse of the signal driving negative subtracted from the negative going signal - applied to the positive transistor. But only when it is at the cutoff threshold and below. In other words never apply an AC signal that can drive either rail into cutoff, or more precisely, apply a signal that forces the output device to just stay on.

Now, what I am completely unclear on is if this in fact is possible and if it is being done, or if some other means is being used in Class i or other "non switching" amp. And, is this a distortion canceling feedback mechanism?

Sorry to admit to being unclear on this, I'm sure some time with these circuits and a simulator would make it much more clear...

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Old 23rd December 2012, 02:31 AM   #87
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trying to follow your (1.).
http://www.essex.ac.uk/csee/research...0amplifier.pdf
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Old 23rd December 2012, 04:26 PM   #88
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>"...the nonlinear switching bias circuitry although performing its primary function may actually accentuates crossover distortion."

Thx Walter.
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Old 23rd December 2012, 05:05 PM   #89
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Quote:
Originally Posted by bear View Post
Edmond,

trying to follow your (1.).

are you asserting that there is no distortion reduction by keeping the output devices from switching off (class AB) there is no distortion benefit per se?
Hi Bear,

Yes, that's correct.

Quote:
Also, are you saying that the "tracking signal" that prevents the switch off is in fact a second feedback loop that effects each side of the output stage that is conducting??
Yes. There are two FB loops: one differential loop that provides local error feedback and one common mode loop that controles the bias. It's the latter loop that may increase distortion (in case of class-i and also my AB2 circuit, for example).

Quote:
It would seem that what one would want is for example if the postive rail devices that are being driven "off" and then the signal swinging "negative" that at some theshold at nearly cutoff, that what is wanted is a voltage - essentially the inverse of the signal driving negative subtracted from the negative going signal - applied to the positive transistor. But only when it is at the cutoff threshold and below. In other words never apply an AC signal that can drive either rail into cutoff, or more precisely, apply a signal that forces the output device to just stay on.

Now, what I am completely unclear on is if this in fact is possible and if it is being done, or if some other means is being used in Class i or other "non switching" amp. And, is this a distortion canceling feedback mechanism?

Sorry to admit to being unclear on this, I'm sure some time with these circuits and a simulator would make it much more clear...
_-_-bear
To understand how class-i works, it helps to realize that that are two FB loops, as already mentioned above. As for the 2nd loop (bias control), your description above is correct. BUT... -quite unexpectedly!- the harmonic content of the difference between input and output voltage (of the OPS) is larger compared to a configuration with a fixed bias. At first instance I also didn't believe my eyes when I simulated these circuits. Yet it's true and confirmed by M.H.

BTW, I'm only talking about MOSFETs. As for BJTs the picture gets even more complicated because of the storage effect of the majority carriers, which also come into play.

Cheer,
E.
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Old 23rd December 2012, 05:24 PM   #90
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Edmond Stuart View Post
>"...the nonlinear switching bias circuitry although performing its primary function may actually accentuates crossover distortion."

Thx Walter.
There are exceptions: I can think of at least two, the Renardson scheme, and the Circlophone.

In the Circlophone, the servo circuit participates to the global linearization of the amplifier.
This is easily seen by disabling the servo, and replacing it by a fixed bias of identical average value:
Attached Images
File Type: png CircloServo.png (158.7 KB, 139 views)
File Type: png CircloUnservo.png (151.6 KB, 134 views)
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