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Old 7th January 2010, 05:41 PM   #521
andy_c is offline andy_c  United States
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Originally Posted by traderbam View Post
I'm not really sure what point you are trying to make. In the limit, when one scrapes the bottom of the noise floor one will find every possible frequency present so you will always be right in a sense.
See the link I posted previously. The point is that if the open-loop distortion is low enough, and for even moderate amounts of feedback for the square-law device, the feedback-generated distortion components are lower in amplitude than what one will see for any real-world open-loop amplifier that allegedly lacks these components to begin with.

But this "open-loop distortion is low enough" requirement is not met by SET power amps, so one might be better off without feedback in that case.

Stated another way, for the square-law open-loop case that Baxandall looked at, one must have high open-loop distortion for the feedback-generated higher-order products to exceed what one would see in a real-world open-loop amplifier.

Last edited by andy_c; 7th January 2010 at 06:08 PM.
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Old 7th January 2010, 07:16 PM   #522
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Originally Posted by andy_c View Post
Stated another way, for the square-law open-loop case that Baxandall looked at, one must have high open-loop distortion for the feedback-generated higher-order products to exceed what one would see in a real-world open-loop amplifier.
I don't have Baxandall's article in front of me. My understanding is that he analyzed a single-tone with a second order non-linearity. So if you are stating that the novel distortion products arising in a 2nd order non-linear system excited by a single tone with NFB are negligible when the OL distortion is moderate by current practical standards then I have no reason to disagree.
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Old 7th January 2010, 07:16 PM   #523
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Piercarlo
donīt be angry, stuff happens all the time in public conversations. Maybe we could climb out of the situation and even become friends.
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Old 7th January 2010, 07:25 PM   #524
blmn is offline blmn  Brazil
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Originally Posted by syn08 View Post
The main point of high Ft devices is to keep the loop stable and allow high loop gains at HF.

However, high Ft devices have low nonlinear parasitic capacitances. Therefore one could expect increased open loop linearity with high Ft devices.
Itīs a valid point IMO, but, a flat response until 2 or 3 octaves over 20kHz, with low distortion behaviour, must be enough, IMO too. I think it can be achieved using proper design and good,,but no soo "fast" devices at output.

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Old 7th January 2010, 07:28 PM   #525
andy_c is offline andy_c  United States
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I don't have Baxandall's article in front of me.
Baxandall's article can be found at Jan's site here. It is part 5 of the article (page 21 of the PDF). The BJT stuff is in part 6. Sounds like we are at least somewhat in agreement. Maybe it would be better to pick this up in the Cordell feedback thread? This is becoming OT to the original post about output devices.
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Old 7th January 2010, 07:39 PM   #526
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wahab,
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well, i m using mosfets for my main amp for a
good 20 years...
You should continue to do so. Maybe I could suggest a topology...
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Old 7th January 2010, 08:45 PM   #527
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Originally Posted by syn08 View Post
However, high Ft devices have low nonlinear parasitic capacitances. Therefore one could expect increased open loop linearity with high Ft devices.
This is not ever true. Many classic low Ft bipolar devices have lower internal capacitances than modern faster devices. Regrettably capacitances, Ft and speed are not so straightforwardly related among them. Judging internal behaviour of seminconductors on the assumptions that their equivalent circuits (with resistor, capacitors, dependent generators and so on) are "true" circuits may be heavily misleading.
The only thing we can safely assume about higher Ft devices is that they can still operate at frequencies where other devices are become just "dead loads" appended to the drivers. But if this assure better linearity and stability or not is a matter that should be evaluated only on the field. Mosfet are surely better provided than bipolars in high frequency features but this usually worsen stability problems instead of alleviating them (at least in physical circuits, where layout issues should be taken in account).

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Old 7th January 2010, 09:56 PM   #528
Bigun is online now Bigun  Canada
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Originally Posted by andy_c View Post
See the link I posted previously. The point is that if the open-loop distortion is low enough, and for even moderate amounts of feedback for the square-law device, the feedback-generated distortion components are lower in amplitude than what one will see for any real-world open-loop amplifier that allegedly lacks these components to begin with.

But this "open-loop distortion is low enough" requirement is not met by SET power amps, so one might be better off without feedback in that case.

Stated another way, for the square-law open-loop case that Baxandall looked at, one must have high open-loop distortion for the feedback-generated higher-order products to exceed what one would see in a real-world open-loop amplifier.
How good does the open-loop linearity have to be in order for low levels of NFB to be 'safe' ?

Baxadall left me with the perception that low to medium levels of NFB are the worse kind for harmonic generation because the feedback is enough to increase higher order harmonics relative to the lower order harmonics but is not enough to lower overall THD to unobjectionable levels. I know this is oversimplifying but anyhow....

It would be a useful guideline to know how good the open-loop linearity should be - and I note you mention for square-law so I assume we are talking about a level of H2 distortion that would be 'acceptable', a level of H2 that when NFB is added generates a H3, H4 etc. at very low levels that we wouldn't find objectionable.

I might apply this guideline to my next design.
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Old 7th January 2010, 09:59 PM   #529
Bigun is online now Bigun  Canada
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Originally Posted by Lumba Ogir View Post
wahab,

You should continue to do so. Maybe I could suggest a topology...
Please spill the beans and let us have a recommendation - especially if you have also tried it and listened to it ?

I'm still planning to use an all-BJT output in my next project, but perhaps I should consider alternatives if there is a good reason.
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Old 7th January 2010, 10:12 PM   #530
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How good does the open-loop linearity have to be in order for low levels of NFB to be 'safe' ?
I'm not sure there's a standard answer to this.

A plot identical to Baxandall's for 10 percent open-loop distortion (second-order only) is shown here.

The one for 1 percent open-loop distortion (second-order only, not shown by Baxandall) is shown here.

And for 0.1 percent open-loop distortion (second-order only, also not shown by Baxandall), the plot is shown here.

For 0.1 percent open-loop distortion (second-order only), any feedback level could be considered "safe" (all distortion components 3rd-order and higher < -120dB).
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