Post by EeyorePost by MooseFETPost by EeyorePost by MooseFETOne huge problem with including a lot of local NFB is that it makes
the overall system harder to close.
That's not my experience. Quite the reverse actually. But then I do tend to
incorporate internal lead-lag compensation. This results in a far BETTER phase margin.
This means that you have lowered the outter loop gain in the process.
Yes. As stated in my introduction, the pursuit of super-fabulously high DC (or AC) open loop
gain makes no sense for audio.
You do need merely "high gain" however. This high gain needs to be
true at the frequencies of interest so the GBP does have to be at
least some reasonable amount.
Very high values of loop gain makes for very large amounts of
reductions in the harmonics within the band. This can argue for much
more gain than it would normally appear you need if you only needed
enough gain to be sure that the feedback resistors were what was
setting the gain.
Post by EeyoreBesides, gain is cheap these days. I have no objection to the
introduction of another gain stage for example. I'd rather have a sensible amount of very
linear and well defined gain than oodles of 'poor quality' gain.
Adding stages adds to the phase shifts. This is another "no free
lunch situation". When you increase the number of stages you also
want to increase the bandwidths of most of the stages to keep the
phase shift near the gain cross over within reason.
Post by EeyorePost by MooseFET--!!-/\/\--
! !
---/\/\-+--!-\ !
! >------+---
!+/
[.. stuff we have covered and agree on ...
Post by EeyorePost by MooseFETThe local feedback has all of the problems a global feedback has with
creating upper harmonics. Your global feedback is now at a lower gain
and thus can't remove them. This is just a case of the lack of a free
lunch.
I'm not sure I 'get that' entirely. I see where you're coming from and that would lead one
to imagine that pursuit of linearity in individual stages was a pointless pursuit and you
might as well have tons of non-linear gain and I know that's not the case, not least because
the very hugh gain system has to be stable and that tends to lead to rolling off the gain
(and the advantage of NFB) from very low frequencies.
No what I am pointing out is that local feedback is not a good
substitute for a naturally more linear stage. Consider this sort of a
situation:
---------- ------ ---------- ------
Signal--->! Subtract !---! Gain !--->! Subtract !--->! Bad !--+----
---------- ------ ---------- ! gain ! !
^ ^ ------ !
! ! !
------------------------+---------------------
You can trade back and forth how much subtracting you do in the two
subtraction circuits but you can't really fix the "bad gain" section.
I am for not leaving out the idea that the good gain has a limited
bandwidth and assuming all of the bandwidth limiting happens in the
"bad gain". I think this makes the idea obvious in it simple form.
To take a bit of a real example, consider a dreadful output stage that
works like this:
----+--------+---
! !
\ \
R1 / / R2
\ \
! !
! !/e
+-------! PNP
! !\
!/ !
! NPN !
!\e ! R3
----------+----/\/\---
--- mirror for other half
R3 is providing a measure of local feedback. R2 also is doing so.
This stage will still be a horror story. Adding a diode in series
with R1 to match to the E-B drop of the PNP makes it much less so.
The diode makes the PNP act much more like a linear current mirror and
thus reduces the natural distortion.
Since the transistors used in power stages are usually slower than the
others in the design. The output is almost always where the pole you
didn't design in lives.
Post by EeyorePost by MooseFETThe pole and zero inside the loop is a good thing to do to improve the
phase margin when you have other poles in the system. It allows you
to determine where the gain crossover happens and the phase at the
crossover. It is a method of lowering the overall loop gain. It
doesn't however get rid of the harmonics issue. It also is something
that you can only do a few times inside the loop. When the system
starts to look like 3 of those in series, you are back in trouble.
3 of them would be a bit much. I've not used more than 2 inside the loop in fact.
Trust me on this: Don't put three inside the loop. Reconsider the
design if you find yourself going there. Two is ok. One plus a
feedforwards is ok but three always seems to mean trouble.
Post by EeyorePost by MooseFETIn audio stuff, you generally want to put the pole-zero thing near the
output, ideally enclosing the output. This makes the system apply a
low pass filter to any distortion products that the feedback can't get
rid of.
Interesting idea.
It only works up to a point. It also requires largish (mechanically)
parts be involved. You have a capacitor and a resistor with fairly
large swings on them and are working at lowish impedances.
