On Sun, 18 May 2008 22:15:00 GMT, Paul <paulguy@[EMAIL PROTECTED]
> wrote:
>On Thu, 15 May 2008 10:31:03 -0500, flipper <flipper@[EMAIL PROTECTED]
> wrote:
>
>
>>On Fri, 9 May 2008 04:26:34 +0100, "Ian Iveson"
>><IanIveson.home@[EMAIL PROTECTED]
> wrote:
>.... snip!....
>>
>>It wouldn't do any good if it were over just one stage because PFB
>>increases distortion and the NFB would just get you back to where you
>>started. But it's quite useful with multiple stages where one stage,
>>(or more) is better than the other, as if the case with the gain
>>triode vs the PP output stage. It has the effect of '****fting' some of
>>that linearity from one to the other so there's a net overall
>>improvement even if NFB=PFB.
>>
>....snip!......
>
> I'm not sure about this... but.... isn't nonlinearity just an
>unwanted signal, and in a feedback amp with any number of internal
>loops, isn't that signal reduced by the "loop gain", which to my
>memory is the overall openloop gain divided by the overall closed loop
>gain? That would mean that as far as unwanted signals go, the internal
>loops would have no effect.
> I'd wonder about slewing problems being reduced, but after a bit of
>thought I don't think it would have a lot of effect, unless some of
>the internal gains are reduced to the point that the prior stages must
>overload, or can't drive input capacitances because of the high levels
>required.
> Am I all wet?
>
>-Paul
I don't know how you got the idea that local NFB has no effect. Local
FB works just like global with the difference being how long the loop
is.
Your basic gain vs distortion math is correct but it applies to any
FB, not just global. I.E. Apply local NFB around (or in) one stage
and the distortion of that stage is reduced in the same manner as
overall amp distortion is reduced by global NFB (or vice versa with
PFB). e.g. Take just a single triode. What would the gain be without
the Rk (using, say, fixed bias instead)? Now, what is it with the Rk?
That's how much NFB the Rk adds and distortion is reduced by that
amount. Or, conversely, that's how much increase in distortion there
is when you bypass Rk to get more gain on that stage.
However, it doesn't do you any good to have a squeaky clean front end
and then feed it into a heavily (by comparison) distorted power stage
because distortion over multiple stages is the square root of the sum
of the individual squares so if one section is significantly higher
than the other it dominates the overall distortion figure (and by more
than if it were a linear relation****p).
But you also have to have the gain in order to apply global NFB so
it's better to use some local PFB, to get the gain with some increase
in distortion of the squeaky clean stage and use that acquired gain to
apply global, thereby reducing the overall distortion.
The fallacy some engineers fall into is observing that equal PFB and
NFB cancels so they conclude you gain (pun) nothing. I.E. You add
10dB, then subtract 10dB, and you're right back where you started. Or,
when not equal, why add 10dB and subtract 18dB when just subtracting
8dB gets the same result? And that would be true if it was the same
loop but what they miss is the PFB is local, over the much better
stage, while the NFB is global and, as mentioned, the combined
distortion is not linear but the square root of the sum of the
squares.
Of course, there are other ways to achieve the same result but I get a
kick out of this one because, component wise, it's 'free' as the
difference (besides picking good component values) is simply
terminating the phase splitter into Rk rather than ground.
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