English: An input (green) 100 Hz sine wave centered around 2.5 V is turned on suddenly at time 1ms. This induces instability in the output (red) of a negative group delay filter (presumably due to limited bandwidth of the opamp) manifesting as high frequency oscillations. However in the steady state, the output leads the input by a negative group delay of -1ms. When the sine wave is suddenly turned off after 10ms, the output again becomes unstable as a result of the unpredictable change.
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Captions
Illustration of negative group delay filter's affect on a sine wave turned on and off.