オペアンプの AC 回路

An inverting op-amp filter uses resistors R1 and R2 coupled with capacitor C1 to process amplified audio signals. The capacitor's impedance varies with frequency, causing the circuit to strongly attenuate low-frequency components while allowing higher frequencies to pass. This frequency-dependent behavior shapes the audio output by reducing unwanted bass frequencies.

Capacitor impedance is calculated using angular frequency derived from the input voltage's time-domain expression. In the frequency domain, this impedance combines with R2 to form impedance Z2. The frequency-dependent impedance determines how the circuit responds to different signal frequencies, enabling the analysis to predict output voltage behavior across the audio spectrum.

Applying Kirchhoff's current law and Ohm's law at the op-amp node yields a nodal equation. When rearranged, this equation shows that the output-to-input voltage ratio equals the negative ratio of impedances Z1 and Z2. Substituting known values into this relationship produces the output voltage in polar form, which converts to the time domain for practical interpretation.

Polar form representation simplifies AC circuit analysis by expressing sinusoidal signals as magnitude and phase components. This format aligns with impedance calculations and enables straightforward application of circuit laws using phasors. Converting between time domain and polar form allows engineers to analyze filter behavior systematically and predict output characteristics.

Ideal op-amp analysis assumes no current enters either input terminal and the voltage across input terminals remains zero. These assumptions simplify nodal equations and enable direct application of Kirchhoff's laws without accounting for input impedance effects. This idealization makes the mathematical analysis tractable while remaining accurate for practical high-gain operational amplifiers.

The parallel combination of capacitor C1 and resistor R2 creates impedance Z2, which varies with frequency. As frequency increases, capacitive reactance decreases, changing the impedance ratio and thus the voltage gain. This impedance combination directly determines which frequencies are attenuated and which pass through, shaping the filter's overall frequency response characteristic.

Converting from polar form to time domain transforms the mathematical solution into a practical sinusoidal expression that describes actual circuit behavior over time. This conversion reveals the output signal's amplitude and phase shift relative to the input, providing engineers with actionable information about how the filter modifies the audio signal in real-world applications.