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1) I don't understand, can you post a schematic?

2) In the earlier current mirror configuration, the only purpose of the diode-connected BJT is to drop one Vbe so that the voltage on the resistor would be nominally equal to the Zener voltage. There are several sources of error in that occurs, and a significant one is BJT base current. The base current in the right-most BJT with the Zener can have its collector current go from less than 10uA to well over 100uA or 200uA depending on the value of UnRegHV, etc.So, yes, you could employ Darlingtons in the current mirror configuration and it would greatly alleviate the base current error.

But it will not alleviate the mismatch in the two Darlington current mirror BJTs.

(The error quantity, delta Vbe, is extremely well-defined math-wise, and used in other circuits to good effect. In a Band Gap Reference the negative tempco of a Vbe is compensated by the positive tempco of delta Vbe, and when this is done the physics of it is you have just achieved the physical bandgap voltage of the semiconductor. For silicon ~1.2V.)

By eliminating the diode-connected device, the Vbe with its negative tempco becomes part of the reference voltage seen by the base resistor. But base current when the collector current hits above 100uA is a significant error in the 66M+66M, etc, so going to a Darlington alleviates that.

Then, with two Vbe's giving -2mV/degC each of tempco, using two 6.3V zeners giving +2mV/degC each does temperature conpensation.

It works well, I think...... try it.

BTW, in the real application where the "dropper resistor" might already be in the HV generator output circuit, AND the HV generator itself is not capable of putting out 100's of uA even though it has an open circuit output of, say 1200V...... Well in reality what you have is a 1200V source with a nonlinear output resistance and in series with an external "dropper resistor." As the UnRegHV approaches higher voltages, the shunt regulator (either the present one OR a zener string) pulls more and more current until the current limits of the HV Generator are being approached.... and the effective "dropper resistance" is increasing to limit the UnRegHV...... So, just like with a series zener string, shunt regulation!

rogerw

1) any comments of using two resistors for the dropper and taking the mid point connections to feed the bias resistor string ? I found that it greatly reduces any voltage rise as the Vcc is raised.
2) in the earlier current mirror configuration with two bjt's, is there any advantage on using two darlingtons?

P
PS I will have to remember to get the 6.2V zeners today!

-- 
rogerw
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