[sdiy] More about the Aries VCO

[Don Tillman]

   > Date: 17 Jun 2004 18:08:20 +0200
   > From: jhaible at debitel.net
   > 
   > my emphasis was on "this", not on "CA3080 based".

Hey JH,

Ahhh, I misunderstood...

But I hadn't thought about this much before... I always figured, "Hey,
if you have a comparator-like operation to do, use a comparator.
That's exactly what they were made for.  Comparators are great because
they don't need to be opamps."

But OTA's have some very nice features that make for good comparators.

As far as I can tell, OTA's don't have compensation capacitors.  So
they're fast running open loop.  

And the open collector output structure of comparators is often a drag
to work with.  

   > I've seen a lot of 3080 schmitt triggers, used them myself as
   > well, and mostly the bounding of the trigger output and the
   > amount of feedback is defined by Iabc and a resistor at the
   > 3080's (current) output.
   > 
   > The Aries trigger appears to be quite different, as the bouding
   > function is not accomplished by a combination of bias current and
   > load resistor, but - as in a "real" comparator - with rather high
   > open loop gain and a discrete bounding network, where the Vbe of
   > transistors are temp compensated by diodes. Few components, fast
   > and discrete, and can drive remarkable load currents.

Now that I look at it again, yes, I agree.

  -- Don

-- 
Don Tillman
Palo Alto, California
don at till.com
http://www.till.com