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发表于 2008-4-25 14:25:10
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Op amp stability and input capacitance
By Ron Mancini (Email: rmancini@ti.com)
Staff Scientist, Advanced Analog Products
Introduction
Op amp instability is compensated out with the addition of
an external RC network to the circuit. There are thousands
of different op amps, but all of them fall into two categories:
uncompensated and internally compensated. Uncompensated
op amps always require external compensation
components to achieve stability; while internally compensated
op amps are stable, under limited conditions, with
no additional external components.
Internally compensated op amps can be made unstable
in several ways: by driving capacitive loads, by adding
capacitance to the inverting input lead, and by adding in
phase feedback with external components. Adding in phase
feedback is a popular method of making an oscillator that
is beyond the scope of this article. Input capacitance is
hard to avoid because the op amp leads have stray capacitance
and the printed circuit board contributes some stray
capacitance, so many internally compensated op amp
circuits require external compensation to restore stability.
Output capacitance comes in the form of some kind of
load—a cable, converter-input capacitance, or filter
capacitance—and reduces stability in buffer configurations. |
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