Ideas for Design / tempco amp

[Rene Schmitz]

At 09:54 18.02.1998 -0500, you wrote:
>>  Hell yeah there (hopefully) is. I mean, has anybody (besides himself)
>>  tried Rene Schmitz's temperature compensation amplifier? If you're going
>>  to use these arrays anyway, I think that this would be the ideal
>>  solution, and indeed much more elegant than the heating approach.
>
>   I did try it and I'm not sure if I put it together wrong, but it
>didn't work for me.  All it did was lock up at the positive rail.  So I
>gave up on that solution.  However it may work for someone else.  I
>didn't have much time to try it out more thoroughly.
>
>   Tony
>

Sorry folks, I would like to have ommented earlier, but I'm having *little*
trouble getting into the net at the moment....

Bad luck, mine works fine but I had similar trouble, and headache!
When not driving the expo converter the circuit worked as intended, but when
I connected it, it latched up. 
It is a single child, so I don't know whether its luck or what.
It cleared up when I put a cap over the resistor of the opamp of the diffamp,
(I used a 1.3n and this works). 
And just in case you have not recently downloaded the schem,
 I've had a mistake in my schematics, but I updated it a while ago. (...sorry)

I suspect there was some oscillation 
in the loop, since the diffamps' miller cap of the trannies, the 
difference opamp and the opamp that forms the divider have lowpass
characteristic. (3poles!) This can be "cured" by this cap. Its up to you to
experiment a little. Maybe one other way to avoid this is choose a different
loop gain.
(I'm no expert on these things like compensation, and compensating this
weird amp is surely no trivial task....)
If anyone finds a better way, please tell me, i'll be including this in my
schems.

Bye Rene






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