In an audio system, the filter circuit processes the amplified audio signal from an amplifier, strongly attenuating the signal components with low frequencies. Consider the simplest filter circuit consisting of an op-amp in an inverting configuration coupled with resistors R1 and R2 and capacitor C1. What is the resultant output signal for a known input signal? The capacitor's impedance is calculated using the angular frequency derived from the time domain expression for input voltage. The frequency domain equivalent circuit represents the input signal in polar form and impedances Z1 and Z2. The impedance Z2 corresponds to the parallel combination of capacitor C1 and resistor R2. Applying Kirchoff's current law and Ohm's law at the node yields a nodal equation for an ideal op-amp. This equation, when rearranged, indicates that the ratio of the output to the input voltage equals the negative ratio of the impedances. By substituting the known and calculated values into this equation, the output voltage is determined in polar form, which is then converted into the time domain.