[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.
Bruce
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