expo-converter and other ramblings

[jhaible]

>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?

I asume you're speaking of an OTA in the VCO core (not about an OTA
for compensating the expo converter). (?)

The gm of an OTA is not temperature independent at all. You can
either overdrive the OTA input in a VCO, or you can make the
thresholds of your schmitt trigger temperature dependent. (Using diodes.
This is good if you're driving a - guess what : temperature dependent - Sine
shaper.)


> 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.

You're completely right. Now that tempco resistors become standard products
again, and even 3340's magically surface from the void, it's really not
necessary to do things like that.
It's only that the muse appears when she likes, and I cannot choose not to
carry out the ideas she brings. (;->)

JH.