Does anyone know how temperature stable the two 70k ohm collector pull-ups in the ltz1000 reference designs need to be?
The reference circuit without the heater stabilisation uses a single 30k, and the 3458A board chose two 75k from memory - so I wonder how critical the temp-stability of the current is across these transistors needs to be?
Edit - Another thought - will not the pn junction in the compensating bjt and the temp-sense bjt, be around 2mv/C - and mostly independent of current? On that basis the changes in current due to temperature affects on the pull-up resistance shouldn't really matter.
I was looking for 70k resistors from the Vishay S-series, but neither element14 nor mouser stock them. Then I wondered if these really needed to be tempco stable.
Don't waste your money on expensive Vishay MBF resistors, especially not for the 70k ones.
Several people measured their influence, consistently.
R2 influences by 1/250, R3 by < 1/1000, only. That's the reason, why HP in the 3458A used Thin Film.
R4 and R5 (12..15k / 1k) affect the most (+/- 1/75), but the overall T.C. can be trimmed to near zero by changing the compensation resistor.
Therefore, PWW resistors @ <= 5ppm/K are fully sufficient.
Also, do not waste any more time on re-thinking again the whole stuff, what we have already discussed many, many times.
This circuit really bears no miracle any more, i think.
Frank