[Amps] Modern Op-Amp shunt stabilizator power supply (for tetrode screen)

[John Lyles]

The LM748 is very obsolete. LM301AN is a similar decompensated old op 
amp that should substitute. Both have about the same open loop gain 
curve and the 301 has a flatter phase vs freq plot. It is not available 
from the major distributors either, in plastic DIP package. However 
Jameco has them for $2.95 each.

A more modern approach is a LT1037IN8 from Analog Devices. This Linear 
Technology single op amp also fits in the same socket and has better 
offset voltage among other improvements. Also a decompensated part.

John

K5PRO


> Message: 1
> Date: Mon, 28 Feb 2022 18:50:15 +0100
> From:flynth at gmail.com
> To:amps at contesting.com
> Subject: [Amps] Modern Op-Amp shunt stabilizator power supply (for
> 	tetrode screen).
>
> 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.
>