[sdiy] Matching trannies for triangle VCO

[jorgen.bergfors at idg.se]

Hi all. For my triangle VCO
(http://hem.bredband.net/bersyn/VCO/VCO%20Buchla%20core.gif) I tested some
different dual PNP transistors last night. These transistors are connected
in a form of discrete OTA.
A friend, who work at Infineon, gave me samples of their BC856S dual smd
transistor. Measuring the Vbe with the Moog test circuit, the matching
seemed to be very good. The difference was no more than 0,5 mV for any of
the ten transistor pairs.
But when I tried them in the oscillator, the result was very disappointing.
On the other hand, two pairs of hand-selected BC560C gave almost perfect
results. A BC857BS, which gave more mismatch on the Moog tester, worked
better in the oscillator. How can this be?
Maybe Vbe isn't the most important parameter to mach for in this case?
Maybe transistors with higher current gain will make the circuit less
sensitive to mismatch?
Anyway, the selected BC560Cs yielded every bit as good result as the very
expensive MAT03. I was able to adjust the triangle for perfevt symmetry at
25 Hz. This means that the symmetry is perfect in the entire audible range.
And it is OK at LFO frequencies too. I tried with very low frequencies and
measured 18 seconds for the down ramp and 12 seconds for the up ramp. OK,
this is not perfect, but anyone who is familiar with the circuit in
question should be impressed with these figures, I think.

/Jorgen