[sdiy] AN299 VCO (was: SDIY list troubles)

[Ian Fritz]

At 05:32 AM 4/30/2008, Paul Perry wrote:
>Certainly it works, by many accounts.
>The thing is, it may not work, if one (for example) swaps the heating and
>the temperature sensing transistors.
>It's one of those things that one might think would not matter..

Well, yeah!  The thermal conductivity of Si is very large.

But I haven't tried it, so I don't know.  But why couldn't you just 
increase the servo loop time constant and/or damping if the conductivity is 
an issue.


>As for the best matched pair - I find it hard to believe that the
>transistors on the wafer are wired up differently according to where the
>best pair is. That would make the chips very expensive indeed!

Maybe, but Q1 and Q2 are always way better matched than any other pair.  I 
found this out trying to make discrete transconductors, at which time I did 
extensive measurements of matching among the transistors.  So if you are 
correct, then it seems to me they would have to cut the dies so the matched 
pairs are always adjacent and in the same position.  Wouldn't this be 
pretty difficult?

:-)

Ian