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TL431 + common base amplifier stability?

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Zero999:
Where did you get 4MHz from?

What range of currents/voltages do you plan to operate it over? If it's fairly narrow, a resistor can be used.

For example:

V = 85V
I = 2mA to 3mA

Cheat and use the calculator linked below to calculate R1 and R2:
https://www.random-science-tools.com/electronics/divider.htm
VIN = 85V
VOUT = VREF = 2.495V
Preferred value series = E24

R1 = 430k
R2 = 13k

IR1 = IR2
IR3 = I1- IR1

IR1 = V/(R1+R2) = 85/(430k+13k) = 85/443k = 0.192mA

When I1 = 2mA
IR3 = 2mA-0.192mA = 1.81mA

When I1 = 3mA
IR3 = 3mA-0.192mA = 2.81mA

The minimum acceptable voltage drop across the TL431 is 2.5V and the maximum is 36V. Therefore the voltage across R3 could vary from 85 - 36 = 49V to 85-2.5 = 82.5V and the TL431 will be fine.

If we set R3 so it drops 82.5V at 2.81mA.

R = V/I = 82.5/2.81mA = 29.36k.
Choose 29k, which can easily be made with a 16k and 13k in series.

Calculate the voltage on the TL431, when I = 1.81mA
V = 85 - 29k*1.81mA = 32.5V

Calculate the voltage on the TL431, when I = 2.81mA, to check the rounding hasn't caused any problems.
V = 85 - 29k*2.81mA = 3.5V

EDIT: The voltage at U1's anode, as the current is swept from 2mA to 3mA.

001:
Thanx fo Your equations!

Is it schematic usable as is? Need I add some diodes to protect it at on/off?
What currenrt range practically achived?  0A2 tube is ok up to 30mA




--- Quote from: Zero999 on October 15, 2019, 04:14:25 pm ---Where did you get 4MHz from?


--- End quote ---

From BU508 datasheet. Why Yansi named it "slow"?

Zero999:

--- Quote from: 001 on October 15, 2019, 08:38:10 pm ---Thanx fo Your equations!

Is it schematic usable as is? Need I add some diodes to protect it at on/off?
What currenrt range practically achived?  0A2 tube is ok up to 30mA
--- End quote ---
Whether it's usable, depends on your application. I had the 5651 in mind when I did it, with the idea the current will be nominally 2.5mA, 2mA minimum, 3mA maximum.
http://www.r-type.org/pdfs/5651.pdf

The circuit only works over fairly narrow current ranges, because the voltage across the resistor varies, as the current varies. If the current is increased, beyond what it's designed for, a greater voltage will be dropped across the resistor, causing the voltage across the TL431 to fall below 2.5V and at lower currents, the voltage across the TL431 will increase to above 36V, which will damage it.

A zener diode could be used to get a wider current range. You might ask, why not simply use a zener diode in the first place? Well you could, but the TL431 has a much more stable output voltage, which varies less, as the current changes. Another possibility is a circuit called a VBE multiplier, which again uses a BJT, but it's less stable than a zener.

schmitt trigger:
Thanks everyone for providing  so many valuable suggestions.

As I mentioned earlier, this was a while back and did not really kept records of exactly what cascode transistor I employed back then, other than it was a 60 volt, TO220 in a Radio Shack blister package.

I know, I know....... A generic power transistor from Radio Shack.  :palm: :palm: Embarrassing.
Edit, or perhaps it was 80 volt transistor? Can’t remember.

T3sl4co1l:

--- Quote from: schmitt trigger on October 16, 2019, 01:27:51 am ---I know, I know....... A generic power transistor from Radio Shack.  :palm: :palm: Embarrassing.
Edit, or perhaps it was 80 volt transistor? Can’t remember.

--- End quote ---

Probably TIP31C or something like that.

Tim

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