What is the phase relationship of the input signal versus the output signal?

The phase relationship between the input signal and the output signal depends on the specific system or circuit being analyzed. In linear systems, such as filters, the output may lead or lag the input by a certain phase angle, often measured in degrees or radians. For example, in a first-order low-pass filter, the output lags the input by a phase angle that varies with frequency. In contrast, in some systems, such as inverting amplifiers, the output can be 180 degrees out of phase with the input.