[sdiy] THAT 2180 as current switch

[jhaible at debitel.net]

I've used the 2180 in a VCO (output driving an inverting integrator).
Works fine. But I only tried a sinus VCO, not a triangle VCO.

The 2180 does *not* have a differential pair input like OTAs,
so I wouldn't try to overdrive it. The good thing is that it's
very linear and low-offset, which makes it a good choice for
continuous operation. I don't know how it behaves when overdriven.
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_. The 2180
is good when it's _not_ overdriven, but for a triangle VCO
this means you need precise switching voltage levels ...
In short: Try how it beghaves when overdriven, but it wouldn't
be my first choice for a triangle VCO.

JH.

> 
> On Sun, 23 Apr 2006, Chad Cosby wrote:
> 
> > I'm trying to make the make Buchla's triangle-core oscillator using a 
> > amp w/ exponential gain control (THAT 2180 or SSM2164) as the 
> > exponential converter and the current switch.  Does any one know if I 
> > can get one of these guys to behave like Buchla's voltage starved OTA?
> 
> Yay! I think this is the first post by an Aaron student on the board.
> 
> Chad: I wouldn't call it "voltage starved" - you'd want to call it 
> "overdriven," so you're into deep into the "tanh" function of the OTA - 
> but I'm not sure what the equivalent behavior of a SSM2164 or THAT2180 is.
> 
> Group: Chad's the only one trying anything like a traditional VCO design 
> (sadly, no other students took on such a project. A few are trying to make 
> some oscillators not traditionally used for VCOs into VCOs.)
> 
> I do know someone used an SSM2164 as an expo converter for a sawtooth 
> core, but I hadn't seen anyone do it with a triangle.
> 
> Chad has a SSM2164 and a THAT2180 that I gave him to play with, as well as 
> a fast LM319 comparitor.
> 
> Feel free to give Chad advice. In particular, if there a reason you see 
> ahead of time that this _won't_ work, that would be good to know!
> 
> Thanks!
> 
> - Aaron
> 
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