Why is proper power-supply decoupling essential in op-amp circuits?

Why is proper power-supply decoupling essential in op-amp circuits?

• A. To increase closed-loop gain.
• B. To reduce supply noise and transient fluctuations that can cause oscillations.
• C. To improve input impedance.
• D. To adjust the offset voltage.

Answer: (B)

Power-supply decoupling is about keeping the op-amp’s operating conditions stable by attenuating noise and fast transients on the supply rails. When the output slews or other parts of the circuit draw current, the supply lines can momentarily bounce. Those fluctuations can modulate the internal biasing and create unwanted feedback paths, potentially causing oscillations or degraded performance. Placing capacitors directly at the op-amp’s power pins provides a low-impedance path for high-frequency noise and quick current surges, while larger capacitors handle slower, larger variations. This keeps the op-amp's internal operating points steady and prevents instability. Decoupling isn’t about increasing the closed-loop gain, improving input impedance, or changing the offset voltage, so those effects don’t come from decoupling.

Power-supply decoupling is about keeping the op-amp’s operating conditions stable by attenuating noise and fast transients on the supply rails. When the output slews or other parts of the circuit draw current, the supply lines can momentarily bounce. Those fluctuations can modulate the internal biasing and create unwanted feedback paths, potentially causing oscillations or degraded performance. Placing capacitors directly at the op-amp’s power pins provides a low-impedance path for high-frequency noise and quick current surges, while larger capacitors handle slower, larger variations. This keeps the op-amp's internal operating points steady and prevents instability.

Decoupling isn’t about increasing the closed-loop gain, improving input impedance, or changing the offset voltage, so those effects don’t come from decoupling.