LMC662
SNOSC51C –APRIL 1998–REVISED MARCH 2013
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However, if the feedback pole is less than approximately 6 to 10 times the “ideal” −3 dB frequency, a feedback
capacitor, CF, should be connected between the output and the inverting input of the op amp. This condition can
also be stated in terms of the amplifier's low-frequency noise gain: To maintain stability, a feedback capacitor will
probably be needed if:
(2)
where:
(3)
is the amplifier's low-frequency noise gain and GBW is the amplifier's gain bandwidth product. An amplifier's low-
frequency noise gain is represented by the formula:
(4)
regardless of whether the amplifier is being used in an inverting or non-inverting mode. Note that a feedback
capacitor is more likely to be needed when the noise gain is low and/or the feedback resistor is large.
If the above condition is met (indicating a feedback capacitor will probably be needed), and the noise gain is
large enough that:
(5)
the following value of feedback capacitor is recommended:
(6)
If
(7)
the feedback capacitor should be:
(8)
Note that these capacitor values are usually significantly smaller than those given by the older, more
conservative formula:
(9)
CS consists of the amplifier's input capacitance plus any stray capacitance from the circuit board and socket. CF
compensates for the pole caused by CS and the feedback resistor.
Figure 16. General Operational Amplifier Circuit
8
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