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Geo,

Now, for your last question below, the answer is yes, you can replace the BJT with just the HV Mosfet and the circuit will function. In fact that is exactly Peter's original circuit, well almost...... It evolved to the subsequent forms for specific reasons though, in this thread, to improve performance.

For example, in Peter's simplified schematic the reference voltage that appears at the negative feedback terminal of the "opamp" is Vz + Vbe (crude opamp which is really a BJT gainstage with a load resistor = 1M and gain = gm*Rload). Since a 6.2V Zener has a positive tempco of about 2mV/degC and a Vbe has a negative tempco of about -2mV/degC, Wallah! (voila!).

So the BJT was a pretty big improvement in that regard, There are more comments I could make about the circuit errors with the MosFets as the gainstage instead of as the cascodes, but it suffices to say that these are what motivated the odyssey to more complicated circuitry.

However engineering economics and "good enough" motivated Peter to step back to the simpler circuit.

(If you had a 1000V BJT you would not need the cascodes for mere functionality.)

rogerw

Take a look at Peter's simpler schematic he just posted yesterday, and ignore the cascode MosFet, since its main function is to pass the BJT collector current up to the high voltage levels. Imagine the BJT was a 1000V Vce device and did not need the cascode for high voltage reasons.

Now, take a look at my enclosure here....... Look familiar?

rogerw

On 4/5/2022 4:21 AM, Geo Dowell wrote:

Really the question is, skip being variable right now for a moment, what function does the transistor play and ultimately, could you theoretically use a zener on the gate(? or where?) of the FET without the transistor to achieve a regulated output, and what value would the Zener need to be to make say, 900V?

Geo

From: "Geo Dowell" <GEOelectronics@...>
To: "CDV700CLUB" <[email protected]>
Sent: Tuesday, April 5, 2022 1:04:13 AM
Subject: Re: [CDV700CLUB] Question and an interesting site for those that modify CDV700s and more.

Peter and Roger,?

P. your hand drawn schematic helps me understand more about this.? Here's my question, with either or all of these circuits, what happens if the divider string is fixed, even if it has the added adjustments available, then the ZENER voltage is changed? Would that give the same effect??

If either one could model that, show change of HV with a range of Zener V from say 3 to 30V. And while doing that change the load resistor on P's schematic to 1.8M.

George?

---

From: "peter via groups.io" <epkoncept@...>
To: "CDV700CLUB" <[email protected]>
Sent: Monday, April 4, 2022 10:33:55 PM
Subject: Re: [CDV700CLUB] Question and an interesting site for those that modify CDV700s and more.

[Edited Message Follows]
[Reason: typo's]

Roger:
?Back to the circuit you posted in message #300
Simple cascode, with only 1 npn tr + 2 mosfets.
Since I found the 2SK3265, I removed the 2nd mosfet so its a very basic circuit.
66Meg, two 1 Meg, 100k+ 2.2M trimmer , 5.1V zener, 2SK3265, 2SC3311
Set trimmer for 900V, Vary input from 950~ 1150V, output only varied 5V or less.
I heated the circuit with a hair dryer, got it up from 25C to about 60C, voltage rose may be 5 or 6 volts. Good enough for me!
Its time to make it small enough to stuff inside my CDV-700 . I think the quiescent current draw while shunting was about 35uA when set to 900V with? about a 950V input.
It on the LHS of the attached jpg, circuit in message #300 is on the RHS
P

-- 
rogerw
On the Catclaw
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The PanDemic is over, but the DemPanic goes on.
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Virus-free.

-- 
rogerw
On the Catclaw
ΜΟΛΩΝ ΛΑΒΕ

The PanDemic is over, but the DemPanic goes on.
MisDisMal is a dictator's tool of repression.