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Patrick Turner Patrick Turner is offline
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Default Determination of Maximum Positive Feedback in Bootstrapped Driver


Hope this all mkes sense. Cheers to all, John


Maybe I'm not thinking straight but I don't see that answering the
question because it depends on what appears to be an arbitrary claim
that gain cannot exceed mu. I mean, it begs the question by declaring
a limit, which is the essence of the question.


Its very easy to think bently instead of straightly.

Are you presuming that the PFB is designed to produce a unity voltage
feedback (relative to the anode signal swing) so that Rl 'appears'
infinite?


The RL which is infinite is a constant current source or constant
current sink.

But the bootstrapping can be arranged to make RL effectively increase
in ohms value positively until it indeed becomes infinite and no
current change occurs in the tube. But from there the load can become
negative.
On a load line graph with load sloping down as you move from right to
left, the bootstrapping will tip the load line to flat until it is a
CCS which is a horizontal line. Then the load can be made to be a
negative load value which slopes up as you more from right to left
across the Ra curves.
Try plotting the lines and current changes while considering different
vallues of bootstrap taps on the OPT.

As the amount of bootstrapping volage increases, the gain rises above
µ, but it takes some doing, and my experiments showed trying to boost
gain above µ was completely pointless because of such rapdidly
increasing N&D and instabilities. THes s.
Usually, applying PFB through an anode is the most useless way to
apply positive FB to boost gain in order to increase open loop gain of
an amp without much increasing open loop N&D, because the latter
cannot be achieved.

But McIntosh found bootstrapping to be a real boon for achieving low
N&D and low Rout, and methinks whatever McIntosh did agrees with RDH4
conclusion that PFB can be used for an overall outcome which gives
less IMD with PFB than without PFB.

Bootstrapping is a *small amount* of PFB, and if the OP stage is
already fairly linear as is the case with a class A triode or UL or
CFB amp then the amount of THD fed back to increase itself is not
huge.
One has to analyse just what happens to an existing measurable
distortion voltage at the OPT tap as it is fed back. Forget about
other signal voltages - just think about the distortion voltages and
what MUST happen with them.

If that's the case then it's trivial to show from the gain equation
that gain approaches, and does not exceed, mu but it's not intuitively
obvious from the schematic that's the amount of feedback being
applied.

If the amount of voltage feedback exceeds plate swing then gain can,
indeed, be above mu.


Indeed.

PFB is used all over the joint in complex filter circuits. I have used
it myself to boost the Q of a narrow band bandpass filter. Some times
the N&D just does not matter.

The wien bridge oscillator relies on much PFB and slightly less NFB so
that it will continue oscillating, and in a recent revision of my 1kHz
oscillator I was able to get 0.0044% with just one opamp with just the
right lamp.



To illustrate I spiced a simple circuit with a 250V source having
100Vrms on it (simulating an arbitrary transformer feedback signal)
applied to a 6SN7 plate through a 27k and cathode to ground through an
unbypassed 470 ohm. Opposite phase 1Vrms (for visual 'instant math'
convenience) was then applied to the grid.

Plate swing was 52Vrms for a gain of 52.- Hide quoted text -

- Show quoted text -


Google prevents me seeing any further...

But try to build real cicrcuits with PFB....

Patrick Turner.