[sdiy] CA3080 VCF and Mysterious Op Amp Behaviour

[eq927 at freenet.carleton.ca]

Perhaps this is the source of the strange behavior:
Several members of this list (including myself) have seen problems with the OTA-based  filter circuits where 
the OTA outputs are buffered by the simple op amp unity-gain non-inverting amplifier (eg output and (-) input 
wired together, signal applied to (+) input). Latch-up seems to occur fairly easily in this configuration when 
BIFET op amps (such as the TL072 and TL082) are used. Filter may seem to behave normally for small input 
signals and low-Q - when signal gets larger or Q is turned up the circuit stops operating - the only way to get 
it going is to turn off the power momentarily. The common-mode input ratings are probably being exceeded here, 
contributing to latch-up. The problem does not occur when an op amp integrator is used instead - take out the 
capacitor and reconnect it between the op amp output and (-) input. Connect the (+) input to ground. Connect 
the OTA output directly to the op amp (-) input. If the OTA has the input signal applied to the (-) OTA input, 
reconnect it to the OTA (+) input to keep the signal polarity correct. This makes it a lot less likely that the 
common-mode ratings will be exceeded, while providing the same electrical performance.
Mike