Maybe it's time to post this link again :
http://laplaza.org/%7Efvuotto/mustage.html

Ronald .


"Ian Iveson" schreef in bericht
...
Tim, I think my explanations work for CC with CCS at anode. I
stated that is what my explanations were about.

If you took output from the bottom anode of a mu-follower, then the
same
argument applies because that anode sees the CCS *above* it. In
these
cases there is not constant current through the bottom cathode
resistor...constant anode current does not necessarily result in
constant cathode current because some is robbed by the load. Such a
configuration has a high output resistance anyway, and an unbypassed
Rk increases it even further. To find the output resistance you
just have to consider ra, since rl is infinite.

r'a = ra + (mu +1).rk

where r'a is anode resistance in case of unbypassed rk, and ra is
anode resistance with zero or bypassed rk

That is, unbypassing adds (mu + 1).rk to the output resistance.
That could easily be 100k or more.

That is why I drew Yves' attention to the output resistance of *his*
stage. It wasn't about a mu-follower then...he said CCS loaded but
had he meant mu-follower he would have said so, presumably. I
responded to his remark that rk sees a constant current, which is
not true when a signal and a finite output load is present.

Hence Patrick's remarks to me had nothing to do with what I said.

The mu-follower is not so straightforward. Lots of things going on.
Only ones I have in practice have mosfets on the top and transformer
loads, and behave in some surprising ways (to me...dunno what goes
on in the tiny mind of a mosfet), and struggle if I take out the
cathode bypass caps.

Your arguments seem perfectly correct to me though.

But wait...bootstrapping increases the output resistance of a
cathode follower, right? If the mu-follower has no signal at its
bottom grid, then the bottom valve's ra is more or less constant,
and forms the bootstrapped portion of the top cathode follower's rk.
If you increase ra of the bottom valve, then you increase the
series-derived feedback to the top valve's grid, and hence the
output resistance of the cathode follower. So it seems that the
bottom ra is still significant. And it would still be raised by
(mu+1).rk.

Now you should be kind to me because I don't know what bootstrapping
means really. Every explanation I find makes reference to mechanics
but the analogies are always nonsensical. I can't pull myself up by
my own bootstraps, it doesn't work.

And there are other issues to consider...such as hum and noise
rejection.

There must be some reason why nearly all the mu-followers I see have
bypass caps?

cheers, Ian

"Tim Williams" wrote in message
...
"Ian Iveson" wrote in message
...
Two arguments, take your pick. Easiest for Yves' simple CC with
CCS
at anode.

I'm most familiar (such that I am) with a mu-follower.. which I
presume
could be done with a CCS in place of the resistor between the two
tubes.

In the case where the load is placed on the top tube's cathode,
above
the R/CCS, the load develops a voltage across the resistor,
causing
a stronger voltage at the bottom tube's plate, which is coupled to
top's grid, which acts as a CF, bootstrapping out the load
impedance.
This is my understanding.

If you vary the load, you vary the current robbed from the CCS
and
therefore the current arriving at the cathode. This results in
series nfb from the unbypassed resistor.

But the CCS (top tube you mean?) ensures a CC into the bottom
tube, by
supplying additional current to the load, if loaded down.

Naturally there will be a bit of voltage change on the cathode
resistor,
but nothing is perfect.

The signal cannot go
up the CCS so for a low output impedance Ra and Rk must be kept
low.

Ah, but it does, for what else is that coupling cap doing?

Rk is effectively multiplied...can't remember the formula and
too
tired to work it out. You know it anyway.

Uhm... notes are downstairs. But yes I have it.

The problem is not so bad for the mu-follower, because a
returning
signal does not see a CCS in either direction. But still the
bottom
Ra is part of the path to ground, and the bottom valve is
responsible for the gain, so the resistor makes a significant
difference.

Are you referring to the CC w/ CCS load mentioned earlier? Or a
mu stage?
Your post didn't specify too well :-\

Tim

--
In the immortal words of Ned Flanders: "No foot longs!"
Website @ http://webpages.charter.net/dawill/tmoranwms