An operational amplifier with feedback becomes an active low pass filter by adding a capacitor in parallel to feedback resistor.
Amplifier Output = - Input times ( Feedback Resistor / Input Resistor)
So, 1 volt x (4 k / 1 k) = 4 volts amplifier output
Cutoff Frequency = 1 / (2 x pi x Feedback Resistor x Frequency Capacitor)
Then, 1 / ( 2 x 3.1459 x 4,000 x .0000000002 ) = 19,867.1286 cutoff frequency
When we reach frequency when capacitive resistance equals resistance of feedback resistor, the total reactance is 0.707 of the resistor and output is at -3-dB point known as the cut off frequency.
