[sdiy] Instability in an OTA-C LP cell

[ASSI]

On Wednesday 14 July 2010, Olivier Gillet wrote:
> Has anybody any idea about what's going on? Why would the fix work
> (or appear to work)?

It's a minor mystery to me that this circuit works at all.  An OTA has 
a current output and your circuit shows no designed DC path for that 
current to go anywhere, with or without the 47k resistor.  I'd guess 
there are some leakages that would sort-of provide a current sink to 
someplace, but only for very small currents and certainly not 
providing the linear I-to-V conversion that you would normally want to 
have there.  The other questionable design choice (that however saves 
this circuit from locking up every time I'd guess) is to drive the 
opamp with the divided-down input needed for the OTA, giving a very 
low output signal that will require a lot of re-amplification (33dB in 
fact) later on.  So in your circuit, the output of the OTA will sooner 
or later head for one of the rails (it doesn't really get there 
luckily) and the TL072 will phase-reverse and lock up whenever you 
exceed the common mode input range.


Achim.
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