[sdiy] Matching trannies for triangle VCO

[jorgen.bergfors at idg.se]

Thanks for your input, Jürgen.

>And then there is the collector voltage. The Vce of the two transistors in
the mirror
>is *very* different in your VCO, which will unbalance the circuit as well.
This could
>be fixed with a cascode configuration. (I don't know how big this effect
is, compared
>to the Beta issue.)

I tried adding a transistor to the current mirror, after a suggestion from
Magnus Danielsson.
I could not notice any benefits from this. There is of course a possibility
that I missed some positive effect, but at least I don't think it affected
the symmetry issue.

>Absolutely. (Assuming HF tracking is fine, as well.)

Actually the HF tracking was so good that I omitted the Rbe compensation
network that was i  Buchla's original desing. Even with the largest
resistor I could find, the Rbe compensatoion overcompensated. I don't
remember with which PNP transistors I measured this though. I will recheck.
Also note that this statement is not valid for supersonic frequencies. The
VCO will not go over 32 kHz, by the way.

/Jörgen