[sdiy] Couple of OTA questions

[harrybissell at wowway.com]

If you want a linear slew limiting circuit, try the one from
the ProOne or Prophet V.  It uses an OTA driving an inverting
integrator. Running to the rails is a feature, not a bug.

I have not seen Batz's circuit. In general allowing an OTA to rail
at the output will not cause any damage. The inputs should be limited to
some small differential, using series resistors and/or back to back diodes
across them (the ProV does this).

H^) harry




On Mon, 5 Nov 2007 08:04:06 -0600, Cornutt, David K wrote
> > From: ASSI [mailto:Stromeko at compuserve.de] 
> > 
> > It tries to go against the rails, but the voltage output 
> > range is limited by the internal biasing network anyway.  If 
> > you want any sensible output response from an OTA, it must be 
> > presented with a suitably low impedance.
> 
> Thanks.  I was actually asking the question because I've been
> looking at a slew limiting circuit that Batz Goodfortune drew up
> (dated 1996).  Unlike the standard lag circuit, this one is linear.
> It works somewhat like a tri-core VCO; it uses an OTA as a
> variable constant-current supply to charge a cap.  The Iabc
> is arranged to limit the output current and hence set an upper
> bound on the rate at which the voltage across the cap can rise.
> 
> It occurred to me that I could produce more "interesting" results
> by switching in larger or smaller caps.  But too small a cap will
> in some cases result in the cap rising to the max voltage that
> the OTA is capable of driving it to, depending on the input.
> Can this damage the OTA?
> 
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Harry Bissell & Nora Abdullah 4eva