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Old 1st July 2012, 01:31 PM   #2661
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Well the whole idea that nfb improves a stage before or after is incorrect. Any improvement in gain within a loop will improve the performance of the loop as a whole. More Vas gain will improve the whole amp, it will not improve the output stage.

The important idea is that the amp ALWAYS works in open loop, whatever the nfb around it. Nfb does nothing to the amp or its stages, it only manipulates the input signal to make the overall performance better. But the stages inside the amp do not change.

In your example, the nfb and the Vas gain improve the performance of the loop. Since the i/p stage is outside the loop, the i/p stage performance impact on the whole amp will not change, I agree. But that is not because it is before the Vas, it is because it is outside the loop.

If you would have an extra inverting stage inside the loop and connect the nfb to the +input (parallel feedback) and the -input to ground, the i/p stage is now inside the loop and the total loop is improved, even when the i/p stage is before the vas.

jan
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Old 1st July 2012, 01:34 PM   #2662
FdW is offline FdW  Netherlands
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Quote:
Originally Posted by wahab View Post
...but it wont reduce distorsion from the preceding stage , let say ...
That does not sound right to me, if different stages in the amp would interact/help/make/influence the amount of 'error-signal-corrected' then why are these stages missing from all formulae that (I have seen) calculate THD in (op)amp circuits? http://www.analog.com/library/analog...0op%20amps.pdf
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Old 1st July 2012, 01:58 PM   #2663
wahab is offline wahab  Algeria
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An exemple to substantiate my understanding.

Let say an IPS with 20dB gain and 1% THD.

Whatever the following stages , in the best case there s only
20dB NFB available for the IPS , that is , it wont do lower than 0.1% THD
once the amp s global loop is closed and the amp s total THD will be no less than 0.1%.

As for the formulae above , i dont see what it is related to ,
in respect to the question on the same post...
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Old 1st July 2012, 02:28 PM   #2664
FdW is offline FdW  Netherlands
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Quote:
Originally Posted by wahab View Post
...As for the formulae above , i dont see what it is related to ,
in respect to the question on the same post...
The point is, there is no allowance for the number (or type) of stages inside the amp that are mentioned in the THD formulae (or any other formulae related to distortion or feedback). (maybe the selected formulae was not the best one )
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Old 1st July 2012, 02:37 PM   #2665
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Quote:
Originally Posted by wahab View Post
When you increase the gain of the VAS , the added NFB can
reduce distorsion of the VAS as well as the one of the following stages ,
but it wont reduce distorsion from the preceding stage , let say the IPS ,
other than due to byproducts of this higher VAS gain , i.e , the IPS
will be less loaded since it will provide less output voltage to drive the VAS.
Yes. I have tried to make the best out of the dual LTP with JFET input (a-la Goldmund) topology. The problem with such topology is that the JFET is biased relatively low (JFET wants high current) so that the distortion of the IPS is very high. Even tho this is in the end corrected by closed loop FB, I believe that the sound quality will suffer (only perceivable by trained ears). Now I think this might be the reason why I don't like JFET input, because the local stage distortion is probably very high in many circuits.

I have temporarily dropped the dual LTP topology project because it is not easy to design the whole amp when the input bias current is above 10mA.
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Old 1st July 2012, 02:41 PM   #2666
Jay is offline Jay  Indonesia
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Quote:
Originally Posted by FdW View Post
The point is, there is no allowance for the number (or type) of stages inside the amp that are mentioned in the THD formulae (or any other formulae related to distortion or feedback). (maybe the selected formulae was not the best one )
The THD formula you showed is related only with input versus output, whether it is one stage or a thousand stages doesn't matter, it only cares with the final THD out of the last stage.
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Old 1st July 2012, 02:52 PM   #2667
FdW is offline FdW  Netherlands
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Quote:
Originally Posted by Jay View Post
The THD formula you showed is related only with input versus output, whether it is one stage or a thousand stages doesn't matter, it only cares with the final THD out of the last stage.
And? is there anything else that matters? Maybe my point was that I totally agree to Jan's point.

Quote:
Originally Posted by janneman View Post
Well the whole idea that nfb improves a stage before or after is incorrect. Any improvement in gain within a loop will improve the performance of the loop as a whole. More Vas gain will improve the whole amp, it will not improve the output stage.

The important idea is that the amp ALWAYS works in open loop, whatever the nfb around it. Nfb does nothing to the amp or its stages, it only manipulates the input signal to make the overall performance better. But the stages inside the amp do not change.

In your example, the nfb and the Vas gain improve the performance of the loop. Since the i/p stage is outside the loop, the i/p stage performance impact on the whole amp will not change, I agree. But that is not because it is before the Vas, it is because it is outside the loop.

If you would have an extra inverting stage inside the loop and connect the nfb to the +input (parallel feedback) and the -input to ground, the i/p stage is now inside the loop and the total loop is improved, even when the i/p stage is before the vas.

jan
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Old 1st July 2012, 03:42 PM   #2668
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Quote:
Originally Posted by wahab View Post
An exemple to substantiate my understanding.

Let say an IPS with 20dB gain and 1% THD.

Whatever the following stages , in the best case there s only
20dB NFB available for the IPS , that is , it wont do lower than 0.1% THD
once the amp s global loop is closed and the amp s total THD will be no less than 0.1%.

As for the formulae above , i dont see what it is related to ,
in respect to the question on the same post...
Wahab, if that i/p stage is followed with a Vas with 40db gain (asssume for the discussion that the Vas is distortion free).
You want an amp closed loop gain of 20 dB you have 40dB feedback gain to throw away as nfb and then your amp distortion goes down to 1% -40dB which is 0.01% if the ni/p stage is within the loop.
If the i/p stage is not within the loop the total amp distortion will remain 1%.

As FdW said, the formula for the nfb distortion reduction says nothing about each stage distortion reduction. The reason is that there is NO change to each stage distortion. How can the i/p stage suddenly distort less, there is no change to it!

As I said, the whole of the amp, all stages, continues to work open loop even when you close the nfb loop. Nfb only manipulates the input signal to make the amp overall distort less (and less gain) but each stage is NOT changed at all.

jan
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Old 1st July 2012, 04:44 PM   #2669
wahab is offline wahab  Algeria
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Quote:
Originally Posted by janneman View Post
Wahab, if that i/p stage is followed with a Vas with 40db gain (asssume for the discussion that the Vas is distortion free).
You want an amp closed loop gain of 20 dB you have 40dB feedback gain to throw away as nfb and then your amp distortion goes down to 1% -40dB which is 0.01% if the ni/p stage is within the loop.
If the i/p stage is not within the loop the total amp distortion will remain 1%.
The distorsion will be 0.1% even if the VAS gain is increased to 100db.

Fact is that when increasing VAS gain , the IPS will be less sollicited ,
wich will reduce its intrinsical THD , wich will lead to the wrong
assumption that this THD reduction is due to increased NFB.



Quote:
Originally Posted by janneman View Post
Nfb only manipulates the input signal to make the amp overall distort less (and less gain) but each stage is NOT changed at all.

jan
More precisely , it modulate the IPS gain by instantly
adjusting its transconductance.

This is what happens with the 40db excess gain of your exemple ,
that will lead to an IPS CL gain of -20db , the VAS will have 40db
effective gain , hence the 20db CLG.
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Old 1st July 2012, 05:08 PM   #2670
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The feedback decreases the effective input voltage to the i/p stage, and the i/p stage distorts less with less input voltage.
The gain or transconductance looks like an S shape, so with lower levels you're more at the straight part in the middle.
If you want to be fancy and call that 'instantly adjusting the transconductance' that seems to me to make it needlessly more difficult to understand.
It suggests some mechanism to change the transconductance from the outside which is not the case.
There is nothing that changes in the i/p stage itself of course.

jan
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Last edited by jan.didden; 1st July 2012 at 05:18 PM.
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