Did anyone figure out how to use a modern Op-Amp (not 748) in a mosfet
shunt stabilizator for a tetrode screen power supply?
This is what G3SEC said in his original article:
"This is a very high-gain feedback loop, so it requires stabilization over
a wide range of frequencies. An ordinary internally compensated op-amp is
not suitable – in fact it will oscillate. The simple trick, courtesy of
G4JZQ, is to use an uncompensated op-amp such as the 748 with heavy
external compensation from the network R4-C1. (If you’re not familiar with
the 748, it’s simply the good old 741 without its built-in compensation
capacitor.".
The closest to what I'm looking for is a F1FRV design that uses LM 358 that
drives a bipolar transistor that then drives a mosfet. It is described here
http://f1frv.free.fr/main1a_Tetrode_Linear_Amp.html
I'm not sure why is F1FRV not having the oscillation problem described by
G3SEK. Is it because the bipolar transistor he uses between the mosfet and
opamp's output presents more of a resistive load rather than capacitive? If
this is the case perhaps I'll have to use an external bjt transistor same
as F1FRV.
I like the simplicity of the original G3SEK design, but sourcing 748 op
amps is a pain. Also I have a drawer full of many different kinds of modern
op amps.
Perhaps one could achieve similar compensation as is done in 748 in an
internally compensated device? Miller compensation perhaps? Is this doable?
To clarify, I don't plan on replicating the original G3SEK tetrode board
design. I have different supply voltages. I have a different Mosfet. I have
to change component values anyway. But I would really like to use a single
mosfet and a single Op-Amp.
L.
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