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Re: What is really going on...?

0

>1. B+ supply of 320V or so
>2. Plate resistor of 3K
>3. In parallel, driver tube and shunt-mode constant voltage
>supply.
>4. Connection to output stage grids
>5. CCS.
>6. B-

The plate resistor is not there anymore. It's been replaced by a
source follower MOSFET that is biased to about 150V on the gate - and about
that on the source (which is going to the plate). The drain of this MOSFET
goes to B+, about 300V, the source to the tube's plate. A capacitor is
connected from the cathode end of the tube follower to the gate of that
MOSFET follower. (Yes, it's follower to follower, with AC coupling to
the MOSFET follower to allow its level shift back up to higher voltage.)

Any time the cathode moves it is impressed upon the gate of the MOSFET,
AC-coupled. That amount of shift is then translated back onto the plate
because the MOSFET is a source follower. Thus Vpk is constant and has no
direct effect on establishing plate current.

The constant current source is in the cathode to B- and is not in conflict
with whatever Vpk is. The cathode is free to move with the grid. The
grid then moves the MOSFET source follower at the plate the same amount.
Vp moves the same as Vk moves which is the same as Vg moves. Vgk is
thus constant, Vpk is constant, and Ip is constant.

This is like the tube follower floats in a power supply at all terminals
but is always biased up to one operating point and stays
there even under AC conditions. The whole tube moves up and down in
voltage by control of the grid voltage, all electrodes together.


Kurt


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