Mac said:
On Sun, 24 Oct 2004 01:00:56 +0000, Joerg wrote:
[snip]
This is why all linear audio amps
need a certain amount of quiescent current. Class D doesn't, of
course, but that's a whole other topic.
Have you ever read Douglas Self's book, _Audio Power Amplifier Design
Handbook_?
Some of this is apparantly here,
http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm
In it he argues that if the output transistors are not conducting
half the time for a sine wave, then it shouldn't be called "class B."
He reserves the term "class B" for when there is an essentially
seamless transition from the upper transistor to the lower one, but
no period where they both conduct. So if the bases of the output
transistors are connected together, it is NOT class B, according to
Self. This makes some sense.
Yes. There is some inconsistent terminology used here. The classic
non-biased push-pull pair might better be class C. No one does though.
But the interesting thing is that he measures the THD and residuals
for class B (his definition) and Class AB open loop output stages, and
the class B stage has lower THD than class AB.
Maybe. Why Self claims that this possibility may be a "vital fact is
little known" is beyond me:
5.3 DISTORTION 3.
"THD increases as the bias advances into AB operation. This is due to
so-called "gm-doubling" (ie the voltage-gain increase caused by both
devices conducting simultaneously in the centre of the output-voltage
range, in the Class-A region) putting edges into the distortion residual
that generate high-order harmonics much as under-biasing does. This
vital fact is little known, presumably because gm-doubling distortion is
at a relatively low level and is obscured in most amplifiers by other
distortions"
This is very well known in the industry. For example, it's a fundamental
issue in designing op-amps with rail to rail inputs. However, it a bit
more complicated than this. The increase in gm due to bias increase,
generally gives better linearity and can produce, in principle, a
*lower* THD. Its not clear from Self's paper how he actually determined
that the optimal point occurred when the transistors were actually on
the edge of switch over, which infact they cant do in such a manner
anyway. These measurements should really have been performed open loop
to make the effect more noticeable. For example, even if the thd was
initially higher, the higher gm might possibly make the system more
stable (less effect of re.Cload), allowing for more feedback which could
reduce the distortion. You simply don't want to operate output devices
at too low a current. Everything collapses.
Secondly, I have done quite a few measurements myself on this, and I can
state that for mosfets, the x-over distortion, by and large, can keep
going down as you move more and more into AB->A. In general, there is no
general claim that can be made here on the "best" bias point.
Anyway, he seems to make a habit of these "little known phenomena"
claims, like on the same paper,
"It seems to be little-known that electrolytic capacitors generate
distortion when they have a significant AC voltage across them. It is
even less well known that non-electrolytic show a similar effect in
applications like Sallen & Key high-pass filters."
Again, this is a fundamental issue in designing speaker x-overs. That's
why one often uses polypropylene caps. I can't imagine anyone engaged in
such matters not being aware of this.
This makes a certain
amount of sense, too, because when both output transistors with their
finite betas are on, the driving stage (op-amp or other) experiences
lower load impedance than when one of the transistors is off.
The effect is fundamentally due to gm (transconductance) doubling, not
hfe.
The voltage gain is:
Av = gm.Rl/(1 + gm.RL)
Change is gm effect the voltage gain.
This
non-linearity is bound to have repercussions. But if you can get the
bias just exactly right, you would never have both transistors on,
and you wouldn't have a dead zone, either.
Anyway, he concludes that class AB is not a good idea.
I dont see that he actually makes that conclusion in that paper.
Either go fully
class A, or just go class B (again, with his definition of class B).
I dont see this as being practical really. In general, you can't have
production amps tweaked up like this. For starters, bias current
temperature compensation never tracks perfectly. The effect he documents
is like "14b 0.00103%, and for 14c 0.00153%". A difference of 0.0005%,
with the speaker generating, say 1% is hardly a good argument for not
overbiasing at all. The main reason for not significantly overbising is
simply to avoid wasting power.
Kevin Aylward
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.