[sdiy] OTA VCO bad or not!

[René Schmitz]

Hi Karl,

karl dalen wrote:
> Was OTA's so bad for doing regular triangle VCOs?
> Is the Electronotes OTA VCO so bad?
> I cant remember but they cant be that bad or can they?
> I now the issues about OTA's but so bad cant they be?!

The main issue is the temperature dependancy of the transconductance.
You need to set the levels for the OTA to "clip" so that the output 
maxes out at +-Iabc, with a much reduced thermal coefficient.
Then also the problem is the OTA is very unsymmetrical internally, one 
side has one current mirror while the other has three. Since the current 
gain multiplies, you'll have always some difference between the source 
and sinking currents (unless you're lucky and all gains are exactly 1).
And you need a PNP expo convertor, which is usually inferior to an NPN 
based one, because of the poorer carrier mobility in P-Si.

This all doesn't say its impossible to build an OTA-VCO, but I wouldn't 
really recommend using them if you have high performance demands.

Besides, why base something on parts that are (or will be) unobtainable 
in the near future?

Cheers,
  René

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