Fri May 23 12:12:18 EDT 2014

Biasing OTAs

I have a bunch of LM13700[2][3] here waiting to find their destiny in
a voltage/current controlled filter or oscillator.  The EIDAC
circuitry is done (compensation software still TODO) but at least the
road is open to start to really experiment.

I'd been thinking about using a current mirror to drive the 2 OTAs
necessary for a SVF or SK style filter, however looking at the Korg
schematic[1] it seems that driving both OTAs from the same control
current is possible: since the input behaves the same (same current
mirror circuit), they can be wired together.

Makes me think if they can be driven directly from the EIDAC as well.
I'll need to think about protection in that case.  Don't want to blow
up the OTAs one after the other.  Problem is also that the MCP4922 in
buffered mode has a 2x gain selectable in software.  I can see that
going wrong...  Maybe best to keep it unbuffered?

[1] http://www.timstinchcombe.co.uk/index.php?pge=korg
[2] http://en.wikipedia.org/wiki/LM13700
[3] http://www.idea2ic.com/LM13600/LM13700.html