Don Pearce wrote:
Eeyore wrote:
Don Pearce wrote:
Eeyore wrote:
Don Pearce wrote:
Eeyore wrote:
Don't know laterals - haven't kept up. Gimme some numbers and I'll go
have a look.
You're a bit late now they've almost come and gone. Arrived ~ 1980 courtesy of
Hitachi. 2SJ56 and 2SK176 were a classic complementary pair but no longer
manufactured. Equivalent types now sourced by Semelab and Exicon.
http://www.profusionplc.com/pro/gex/...teral%20mosfet
Oh, I thought there was something new going on. They have exactly the Gm
characteristic I was talking about. Here's the thing. Bipolar Gm is
enormously bigger than mosfet, and you can use that in an output stage.
Oh sure. But look at the Hitachi data I posted a few mins later. Look how linear that
curve is especially beyond the typical 100mA quiescent operating current.. Id vs Vgs The
Exicon data sheet doesn't have the equivalent plot for some reason.
The high gm of bipolars is great until you get to a few mA or tens of mA of Ic when it's
crap and that's where crossover distortion comes from. You just can't get rid of it.
Yup, looking now. The transfer characteristic is the alomst-square-law
curve I was expecting; I don't think you can get anything else from a
mosfet. As for Gm, the 0.4V change in Vgs from -1.2 to -1.6 yields a
drain current change of 0.24A (-0.35 to -0.59A) at 75C. That is a Gm of
0.6!
It averages out including higher currents at about 1S.
You can cross over a bipolar output stage long before you hit that
kind of number.
And you'll still get crossover distortion.
Take a look at these two graphs I copied from Doug Self's power
amplifier book. They show the voltage gain of the output pair against
operating point (input volts)for a variety of bias conditions. This can
be used to select a best bias. For the bipolars at the top, the fourth
curve up is clearly the best with gain varying from 0.97 down to 0.963
across the range. This is easily tamed, and even a small error doesn't
do much damage.
Now contrast this with the fets (2SK135/2Sj50) below. First there is no
stable flat line - the gain goes on rising all the way out to the 15V
which is the maximum he measured. Secondly there is no decent bias
current that will control the crossover. I guess that again the fourth
from the bottom is as good as it gets but that gives a gain variation
from 0.83 down to 0.77, with a much sharper turnaround into the Gm
doubling region (spiky crossover products result).
http://89.174.169.10/odds/crossover.gif
That is why it is so much easier to control crossover distortion in
bipolars.
Nonsense. I've seen Doug Self's 'blameless amplifier' diagrams. The crossover 'pip' is clearly
visible in all of them. It's quite good but not that good. I can beat that standing on my head.
When I said the crossover of my big Mosfet amp was 'invisible' that IS what I meant. INVISIBLE
and a THD 14% above the AP analyser residual.
For some reason I can't reach your gif btw.
Graham