When amplifying a weak signal, it is usually necessary to minimize electrical noise, especially during the first stage of the amplification. As dissipative elements, even ideal resistors naturally produce noise at their terminals that fluctuates randomly. The Johnson noise is the result of the temperature change of the resistor, which is the basic noise source of the resistor and can be predicted by the wave dissipation theorem. Using a larger resistance value will produce a larger voltage noise, while a smaller resistance value will produce a larger current noise at a given temperature.
The thermal noise of an actual resistor may be greater than theoretically predicted, and the increase is usually frequency dependent. The excess noise of the actual resistor is only observed when the current flows through it.Specifies in μV/ V/ DECADE -μV units per volt of noise applied to the resistor at a frequency ten times higher. Frequency is measured in dB, so a resistor with a noise index of 0 dB will exhibit an excess noise of 1μV (root mean square) per voltage on the resistor for each frequency decade. Thus, excessive noise is an example of 1 / f noise.Film and carbon combined resistors produce more noise at low frequencies than other types of resistors, and wire wound and film resistors are usually used for better noise characteristics. Carbon composite resistors have a noise index of 0dB, while foil resistors may have a noise index of -40dB, and usually the stray noise of foil resistors is not significant. Thin-film surface-mount resistors generally have lower noise and better thermal stability than thicker film surface-mount resistors.
Thermal noise of resistors is also size-dependent; in general, as the physical size of a resistor increases (or multiple resistors are used in parallel), the excess noise decreases as the independent wave resistance of smaller parts flattens out.