[sdiy] THAT 2180 as current switch

[René Schmitz]

Hi Chad, Jürgen, Aaron and all,

> The nice thing with an overdriven _OTA_ is that you are (more or less)
> switching a constant current (i.e. the maximum possible current,
> that's fed into the emitters). I'm not sure if a similarly good
> precision is possible when a 2180 is _overdriven_. 

I have only experience with the 2164 (and only in filters and VCAs).
But the 2180 does also have that blackmer cell design. So they are not 
OTAs, since these are current in current out devices.

In short, I'm with Jürgen here: Don't overdrive, there is no tanh 
function there anyway. You need some other form of "clipping" to get 
defined currents/voltages running into the input of the chips.

> The 2180
> is good when it's _not_ overdriven, but for a triangle VCO
> this means you need precise switching voltage levels ...

A CMOS switch steered from a schmitt trigger can deliver these.
I'd use two SPDT switches: One driven by the comparator, with the output 
taken after the switch. (In essence let the switch form an extra 
push-pull output for the comparator.) Then take the feedback resistor 
network from that new output as well, so the schmitt trigger that is 
formed benefits from these precise voltage levels as well. (It might be 
handy to power the switches from some lower symmetrical supplies, say 
+-7.5V, so you can have everything sitting nicely arround GND.)

Then with the output of that I'd use a second switch to flip a resistor 
between V and -V. So you can make V controlled externally, and have 
linear FM.
Or else save this extra effort, and just use the precision schmitt 
triggers output to go via a resistor back to the input of the VCA.

> In short: Try how it beghaves when overdriven, but it wouldn't
> be my first choice for a triangle VCO.

Maybe not, but if you have to make a crossbreed, thats perfectly fine.

Its interesting to think of what one could do with the other cells in 
case of the SSM2164....

Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159