Stewart Pinkerton wrote:
On Thu, 16 Dec 2004 15:00:59 -0600, (John Byrns) wrote:
Which rules have I changed? You may not agree with me that triodes
incorporate an internal negative feedback loop, but that doesn't mean I
have changed any rules, we just don't agree on the rules as they relate to
the operation of triodes, I might just as well accuse you of changing the
rules regardless of reality.
Apologies due, it was not *your* position with which I was in
conflict, but Ivesons. Which is unfortunate, since he and I are both
C&I engineers, but he doesn't seem to understand what goes on inside a
triode. I do agree that triodes have a large amount of internal
feedback (hence their low gain, and hence the pentode, which breaks
the loop). My position is that certain transistors, such as the Sanken
2SC2922/2SA1216 pair, are as linear as triodes when used correctly,
and can be employed to produce low parts count 'zero NFB' amps in
similar manner.
Well all you have to do is proove your position to be valid.
Please set up a single transistor in common emmitter mode, ie, 8 ohm load in
the
collector circuit, idle current about 2.5 amps, grounded emitter.
The input is applied to the base from a very low impedance source.
with less than 0.001% thd.
This will require you to use a low thd amp with low Ro, because the base
input impedance is very low
Then measure the thd products and draw a graph of the level of each
harmonic product as it increases from 0.0 watts to
a maximum of about 10 watts.
Let us know how you get on.
But not before you have done the above experiment will any of us know if the
bjts you have chosen are anywhere near as linear as a triode, or even as linear
as a 6L6 set up SE, no FB, and in beam tetrode mode.
As previously discussed, the use of local degeneration does not in my
view count as loop feedback, as it does not to my knowledge result in
the kind of discordant intermodulation products previously noted to be
produced by global NFB loops under certain conditions.
But the emitter follower topology is a case of a high amount of NFB, and
its use completely negates the purpose of you quest, to proove that a bjt is as
naturally linear
as a triode.
If we were to accept you premise that EF topology is OK, then
we would then request that the 300B would also be allowed to have 30 dB of
applied global NFB around a second gain tube to make the playing field level
for the comparison.
Hence, this
should not be a problem for the 'zero NFB' purist.
It remains a real problem for everyone except you, and you cannot
accept that modern transistors are woefully non linear unless
large amounts of NFB are employed, and then there is the problem
of overcoming the naturally low base input impedance of the bjt.
Similarly, I don't
agree that cathode/emitter followers are the spawn of Satan, they seem
simply to be an excellent impedance transformer. Is there any
*logical* objection to them?
Some have no problem with follower stages. I don't have a problem with them
where I use them for the output of a preamp for example.
But in this case where you are floundering while you try to design a simple
bjt amp which is as simple as a 300B amp, follower topology is plainly
quite unacceptable.
But your aim and zeal is to proove bjts can be used as easily as a 300B and its
simple circuitry,
and with as little FB, zero feedback in fact, and still end up with a simple
amp, two devices max,
and have thd lower than 300B, and still have Rin above 10k, and able to be
powered from a
volume pot straight after a CD player..
So, unless you wish to be seen by all as the universe's greatest cheat, please
do not
keep insisting on the use of emitter follower in this case in this thread is
OK.
Patrick Turner.
--
Stewart Pinkerton | Music is Art - Audio is Engineering