expo-converter and other ramblings

[Tony Allgood]

JH > I think I've found a circuit for a fully compensated expo current
source (with pnp's, for OTAs !)

This sounds very good indeed. But has anyone verified just how good the
gm linearity and absolut gm is over temperature for the LM13700. My HF
VCO, appears OK at room temperature using 2220 pnp pair and 1k TC as the
expo, but I haven't done any proper temperature tests yet. My suspicions
are that although the expo curve may indeed be rock steady, the OTA
itself may be a little ropey. The data sheet is not helpful; only a
graph is given on the National sheet, and that does indeed show a
straight line, but the graph is the usual tiny affair, and shows only
typical results. Has anyone actually used a OTA VCO for some time, and
compared it to their other types?

My other point, other than for the sheer fascination of the subject, why
do we need a non-temp co approach? The temp co's that Paul S and Farnell
are selling seem to do the trick. Paul's MOTM uses a TC, albeit with
some other stuff as well, and this is 'ultra-stable'. I have to say that
I have not seen the schem for this, but he does have two sets of pairs
in there as well as a TC. Also, most people seem to be happy with the
ASM-1 VCO.

This is not a dig at semiconductor temperature stabilised designs, no
sir; more of a 'trying to find out whether my new VCOs really do need
something else apart from a TC' sort of enquiry.

Regards,

Tony Allgood  Penrith, Cumbria, UK

Walsh synthesiser, SuperLadder, TB303 clone and rack mounted filter

http://www.techrepairs.freeserve.co.uk/projects.htm