[sdiy] Voltage Feedback Resistors and Circuit Stability

[Richie Burnett]

In my experience smaller feedback resistors generally give better op-amp stability. With a large feedback resistance there is more potential for stray capacitance between the virtual earth (inverting input) node and ground to cause a phase lag. Too much phase lag degrades phase margin and turns negative feedback into positive feedback, ultimately leading to oscillation. Just like the case of capacitive loading on the output that Bernie mentioned. However the virtual earth node can be much more sensitive to stay capacitance than the op-amp output which is generally a stiffer voltage source.

Of course the proper solution to the problem  described above is to add some capacitance across the feedback resistance. This introduces some phase-lead into the feedback path to compensate for the undesirable lag from the stray capacitance. This improves the phase margin and moves the op-amp further away from instability/oscillation. This process is commonly used in things like photodiode amplifiers to prevent the capacitance of the photodiode from destabilising the op-amp.

-Richie,

Sent from my Xperia SP on O2

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