What is an active first-order low-pass filter using an op-amp, and what is the role of the feedback network?

• A. It uses an op-amp with R and C in feedback to set a precise cutoff and gain; feedback improves impedance shaping and stabilizes the filter characteristics.
• B. It uses passive components only.
• C. It amplifies high frequencies with a shelf response.
• D. It uses an all-pass structure with a notch.

Answer: (A)

The important idea here is how an op-amp with a resistor and capacitor in the feedback path creates a single-pole, low-pass response while providing buffering. The op-amp keeps the inverting input at a virtual reference, so the feedback network largely controls the closed-loop behavior without loading the source.

The RC in the feedback sets the pole: at low frequencies the capacitor looks open, so the feedback impedance is essentially a resistor and the circuit has a defined, higher gain. As frequency rises, the capacitor conducts more, the feedback impedance drops, and the closed-loop gain moves toward unity. This reduction in gain with frequency produces the low-pass characteristic, attenuating higher frequencies beyond the cutoff.

Because the pole location is directly determined by the chosen R and C values, you get a precise cutoff and a predictable response. The op-amp also provides high input impedance and low output impedance, buffering the filter from source and load and stabilizing the overall behavior.

The other ideas described don’t match a first-order active low-pass: purely passive components wouldn’t provide the buffering and controllable gain, a shelf response amplifier would change the high-frequency behavior, and an all-pass or notch structure is a different type of filter altogether.