In op-amp noise analysis, which statement is correct?

In op-amp noise analysis, which statement is correct?

• A. Only voltage noise matters with any source impedance.
• B. Current noise dominates with very low source impedance.
• C. Voltage noise is a spectral density at the input; current noise times source impedance adds to total; high source impedance makes current noise more significant.
• D. Noise is independent of source impedance.

Answer: (C)

In op-amp noise analysis, the total noise seen at the input is the combination of the input voltage noise and the current noise flowing through the source impedance. The voltage noise is a spectral density specified at the input, while the current noise contributes a voltage term equal to i_n times the source impedance Z_s. This means the current-noise term grows as the source impedance increases, making current noise more significant for high Z_s and less significant for very low Z_s. A compact way to view it is e_total^2 ≈ e_n^2 + (i_n Z_s)^2, where e_n is the input voltage noise density and i_n is the input current noise density. That’s why the correct statement notes that voltage noise is a spectral density at the input, current noise times the source impedance adds to the total, and high source impedance makes current noise more significant. For very small Z_s, the i_n Z_s term is negligible and voltage noise dominates; for large Z_s, the current-noise contribution can become substantial.

In op-amp noise analysis, the total noise seen at the input is the combination of the input voltage noise and the current noise flowing through the source impedance. The voltage noise is a spectral density specified at the input, while the current noise contributes a voltage term equal to i_n times the source impedance Z_s. This means the current-noise term grows as the source impedance increases, making current noise more significant for high Z_s and less significant for very low Z_s. A compact way to view it is e_total^2 ≈ e_n^2 + (i_n Z_s)^2, where e_n is the input voltage noise density and i_n is the input current noise density. That’s why the correct statement notes that voltage noise is a spectral density at the input, current noise times the source impedance adds to the total, and high source impedance makes current noise more significant. For very small Z_s, the i_n Z_s term is negligible and voltage noise dominates; for large Z_s, the current-noise contribution can become substantial.