Poul-Henning,
On 08/31/2015 08:28 AM, Poul-Henning Kamp wrote:
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In message <[email protected]>, Magnus Danielson writes:
Ehm, eh... that transistor pair you have there. How *tight* together is
really the transistors thermal connection? I bet not all that good.
They're in the same TO5, but I don't know if they're the same die.
I have about 45-50 microvolt difference between their bases.
Monitor that over time to see if there is a correlation.
The current will vary more than the voltage would illustrate, due to the
exponential properties.
Anyway, the take-away is that you should look at that discrete
op-amp and see if it is not causing you the thermal dependence you are
trying to locate. Maybe replace it with a more modern op-amp like the
741 or something (irony may have been used).
Yes, that is absolutely the plan.
The integrator and AC-amplifier in the HP5065 gets upgraded to
chip op-amps in later revs, but this one does not.
Indeed. Modern low-offset op-amps has temperature-gradient input stages,
and is a marvel of stability compared to what the original 5065
designers could do easily. Seems like their transistor pair choice
solved quite a bit of the issues, but as we look to shave of
disturbances it is worth looking at.
Tempted to mod up my 5065s now.
Cheers,
Magnus
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