[time-nuts] ? phase comparison or other device

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Jun 30 18:56:21 EDT 2007

Operating an opamp in saturation for extended periods in a precision 
circuit is a bad idea, as the changes in dissipation of the various 
devices in the opamp will change the opamp input offset voltage when it 
comes out of saturation. Even though modern opamps use cross coupled 
quads and other layout symmetry techniques to greatly reduce the thermal 
feedback on the chip, the effect is still finite. With input frequencies 
above a few kHz the thermal feedback isn't significant, however with the 
low frequencies typically used in this application thermal feedback is 
significant. Thermal feedback is reduced by keeping the opamp out of 
saturation and limiting the output swing to minimise the change in the 
power dissipation in the output stage transistors.


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