Noise in electronic circuits

In electronics, noise is the unwanted random fluctuation of voltage or current that accompanies any signal and sets a limit on how small a signal can be detected. Unlike interference from a specific external source, fundamental noise arises from the physics of the components themselves and cannot be entirely removed, only minimised. The quality of a measurement or communication channel is therefore often described by its signal-to-noise ratio (SNR), the ratio of wanted signal power to noise power; a higher ratio means a cleaner signal.

The most basic source is thermal noise, also called Johnson noise, named after the work of John Johnson and Harry Nyquist. It is produced by the random thermal motion of charge carriers in any resistive element and is present in every resistor at any temperature above absolute zero. Its power depends on temperature, the resistance and the bandwidth over which it is measured, so wider-bandwidth and warmer circuits collect more thermal noise. Because it spans all frequencies fairly evenly, it is often called white noise.

Other mechanisms add to the total. Shot noise comes from the discrete nature of charge as carriers cross a junction, and flicker noise rises at low frequencies in transistors and other devices. Switching circuits can also inject noise. Noise analysis aims to identify and quantify these contributions so designers can choose components, bandwidths and operating conditions that keep the signal comfortably above the noise floor.

Frequently asked questions

What is thermal or Johnson noise?

It is noise generated by the random thermal motion of charge carriers in a resistive element. It exists in every resistor above absolute zero and increases with temperature, resistance and bandwidth.

What does signal-to-noise ratio measure?

Signal-to-noise ratio compares the power of the wanted signal with the power of the noise. A higher ratio indicates a cleaner signal that is easier to detect and process accurately.

Can electronic noise be eliminated completely?

No. Fundamental noise such as thermal and shot noise arises from physics and cannot be removed entirely, though careful design can reduce it and keep the signal well above the noise floor.