IEC 60195: Resistor Current Noise — The Ghost Signal Haunting High-Gain Amplifiers

Resistor Current Noise — The “Ghost Signal” in High-Gain Amplifiers

IEC 60195:2016 specifies current noise measurement in fixed resistors. In high-gain amplifiers, precision instruments, and audio equipment, resistor excess noise can be 100–1,000× larger than thermal noise — yet many circuit designers are unaware of this parameter.

Noise sources: Thermal (Johnson-Nyquist) noise is a physics mandate — white noise present in every resistor, proportional to temperature, resistance, and bandwidth. But current noise is additional low-frequency (1/f) noise generated when DC current flows through the resistor, arising from microstructural inhomogeneity — current “hopping” between conductive particles produces random voltage fluctuations. Carbon composition resistors exhibit the highest current noise (Noise Index typically 0–6 dB); metal-film resistors are far quieter (typically below −20 dB); wirewound resistors have virtually no current noise.

Quantification: IEC 60195 defines Noise Index — in μV/V or dB. Per unit bandwidth, noise voltage = Noise Index × DC voltage. A 10 kΩ resistor with Noise Index=0 dB (1 μV/V) at 10 V DC bias produces 10 μV of noise — in a preamplifier handling 1 mV signals, this degrades SNR to unacceptable levels.

TN Lab — Only checking resistance and power rating? In high-sensitivity circuits, noise index may be the most critical parameter.