Bob,
Indeed. No bashing of the designer, it is an OK design for its age and
intended stability vs complexity. My point was that after the 702
design, many things happen and one thing they learned was to handle the
thermal balance. Already the 741 had taken a number of these steps. The
get-away might be that caring a bit about the thermal properties of the
operational amplifier could be where we can improve the design.
Cheers,
Magnus
On 08/31/2015 04:09 AM, Bob Camp wrote:
Hi
As we go joyously bashing the poor guys that designed this beast, it’s worth
noting
just how old the design is. 741 op amps were indeed “modern” when they did much
of this and quite possibly to modern to be trusted. Most of the design would
have been
right at home in the late 1960’s at a conservative design house. As time has
shown, in
a lot of cases that mistrust of the early linear stuff was well founded ….The
741 only
was designed in 1968….The 5065 design dates to roughly that time.
Bob
On Aug 30, 2015, at 8:08 PM, Magnus Danielson <[email protected]>
wrote:
Poul-Henning,
On 08/30/2015 09:54 PM, Poul-Henning Kamp wrote:
--------
In message <[email protected]>, Bob Camp writes:
Since they needed to calibrate the voltage swing, there aren’t a lot of options
with the technology they were using at the time. Today there are a lot more
choices of how to get this sort of job done.
It's not just the swing, it's also the shape of the curve:
http://phk.freebsd.dk/hacks/HP5065A/20150828_c_pot/index.html
If it were just the range things would be a lot simpler.
Ehm, eh... that transistor pair you have there. How *tight* together is really
the transistors thermal connection? I bet not all that good.
The reason I got involved with counters and atomic references was originally my
interest in analog synthesizers, and there we use a transistor pair for
exponential conversion, which has scale and offset issues and thermal issues.
The use of a Q81 +3300 ppm/C resistor in the division network helped to
compensate the thermal properties of that transistor pair, and you wanted stuff
like MAT-01 where the two transistors is thermally ties to each other and the
put your tempco resistor to that for good performance. All this requires good
measurement and good reference, so that what motivated me towards that step.
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).
Cheers,
Magnus
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