Tempco Update (good news)

[Christopher List]

Hi List - 

True to their word, KRL sent me samples of the now infamous C2AQ tempcos, and 
they arrived yesterday. I got home and swapped them for my normal 2K'ers. The 
C2AQ's were 1K, so I stuck a 100K in parallel with my existing 100K input 
resistor for a 50K input impedance. Fired it up and....
...it worked like a charm. No funky stuff when I touched it, no funky stuff 
when I waved my hand around it. I don't own a hair drier so I couldn't really 
test the tempco. I was going to put the module in the fridge for a while, but I 
figured it would warm up too quickly when I took it out and the tempco and 
transistor would warm at different rates so it wouldn't be a good test....

So the C2AQ's work, but I need 2K versions for the standard 100K input 
impedance. I can only assume that the 2K versions of the same resisitors won't 
cause problems.

The other side of the coin is the 538Q's (I've got about 20 of these lying 
around). These guys won't work as op-amp feedback resistors because of 
inductance problems. They <<should>> work as part of the voltage divider 
between the op-amp and the base of the transistor pair. More specifically, it 
needs to be on the ground side of the voltage divider, so that the effect of 
the tempco is the same. I tried this really quickly but it didn't work because 
I need to add a resistor on the other side of the voltage divider (or increase 
the value of the trimmer, or change the value of Rfb) and didn't feel like 
rebuilding that whole part of the circuit. 
Gene's tried it in the ASM-1 and says it works, so I guess the increased 
impedence to ground at the base of the tranny is not a problem. 
The only thing I'm wondering is:
Is the final effect of the tempco on the voltage at the base of the transistor 
the same when the tempco is put in here as when it's in the feedback loop of 
the op-amp? If it's not is the difference negligible? I've been too lazy to 
figure this out. Seems to me like the final effect on Vout when the tempco is 
used as Rfb is only 16ppm/C. When it's used as half of a voltage divider, it's 
effect depends on the other half of the divider. So what do you use on the 
other half? I'll play with it tonight just to see if I can get it to work, but 
I don't know how to validate the tempco effects...

- Chris