Helena Joyce
Thermal noise, also known as Johnson-Nyquist noise, is the electrical noise generated by the thermal agitation of charge carriers (usually electrons) within a conductor. It is independent of applied voltage and is directly proportional to temperature. This type of noise is present in all electrical circuits and can be a limiting factor in the sensitivity of electrical measuring instruments, such as radio receivers. The impact of thermal noise can be mitigated by lowering the temperature or resistance in electrical circuits. In RF circuits, it is a critical parameter, especially for front-end receiver circuits, and plays an important role in audio systems and radio communications.
| Characteristics | Values |
|---|---|
| Other Names | Johnson-Nyquist noise, Johnson noise, Nyquist noise, resistor noise, white noise |
| Definition | Electrical noise present in equipment due to the vibration of charge carriers within a conductor |
| Cause | Thermal agitation of charge carriers (usually electrons) within an electrical conductor |
| Dependence on Applied Voltage | Independent |
| Dependence on Temperature | Directly proportional |
| Occurrence | Present in all electrical circuits and analog RF and baseband circuits |
| Impact | Can drown out weak signals and limit the sensitivity of electrical measuring instruments |
| Reduction Method | Lowering the temperature or resistance in electrical circuits |
| Fluctuation Distribution | Nearly Gaussian |
| Frequency Spectrum | Flat |
| Power Spectral Density | Nearly constant |
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What You'll Learn
Thermal noise is caused by the thermal agitation of charge carriers
Thermal noise, also known as Johnson-Nyquist noise, is the electrical noise present in equipment due to the vibration or thermal agitation of charge carriers within a conductor. It is directly proportional to the temperature of the conductor and is independent of the applied voltage. This means that the amount of thermal noise present increases with the temperature of the component.
The discovery of thermal noise is attributed to J. B. Johnson, who published a definitive experiment on noise in 1928 alongside Harry Nyquist's theoretical explanation. Johnson's work built upon the earlier research of Walter Schottky, who first postulated the existence of thermal noise and shot noise in 1918. Nyquist's 1928 paper, "Thermal Agitation of Electric Charge in Conductors", used concepts about potential energy and harmonic oscillators to explain Johnson's experimental results.
Thermal noise is present in all electrical circuits and sensitive electronic equipment, such as radio receivers. It can interfere with weak signals and impact the sensitivity of electrical measuring instruments. To mitigate this issue, some equipment is cooled to cryogenic temperatures to improve the signal-to-noise ratio. Additionally, thermal noise can be reduced by lowering the temperature or reducing the resistance in electrical circuits.
The power spectral density of thermal noise is nearly constant throughout the frequency spectrum, giving it a flat bandwidth. This characteristic is known as white noise, and it distinguishes thermal noise from other types of noise, such as shot noise, which is dependent on the application of voltage. The statistical distribution of thermal noise is almost Gaussian, and it can be treated as such for practical purposes.
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It is also known as Johnson-Nyquist noise
Thermal noise, also known as Johnson-Nyquist noise, is the voltage or current noise generated by the thermal agitation of charge carriers (usually electrons) inside an electrical conductor at equilibrium. This occurs regardless of any applied voltage and is present in all electrical circuits.
John B. Johnson, working in Bell Labs in 1927, discovered this noise in communication systems and described it in terms of frequencies. He shared his findings with Harry Nyquist, who was also working at Bell Labs. Nyquist then used the principles of thermodynamics and statistical mechanics to explain the results, which were published in 1928.
Johnson-Nyquist noise is independent of the resistor's value, and the temperature of the resistor alone should be used even if the resistor and capacitor are at different temperatures. The noise is not caused by the capacitor itself but by the thermodynamic fluctuations of the amount of charge on the capacitor. It is inherent in the RC circuit.
Johnson noise thermometry is used in precision measurements. For example, the NIST used Johnson noise thermometry to measure the Boltzmann constant with an uncertainty of less than 3ppm in 2017.
The Johnson-Nyquist noise is related to the resistive response in many generalized electrical cases as a consequence of the fluctuation-dissipation theorem. Nyquist's original paper provided the generalized noise for components with a partly reactive response, such as sources containing capacitors or inductors.
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It is present in all electrical circuits
Thermal noise, also known as Johnson-Nyquist noise, is the electrical noise present in equipment due to the vibration of charge carriers within a conductor. It is caused by the thermal agitation of these charge carriers, which are usually electrons, and occurs regardless of any applied voltage. In other words, thermal noise is present because all systems have a temperature above 0 Kelvin. As temperature increases, so does thermal agitation, resulting in higher noise levels.
Thermal noise is present in all electrical circuits and can be a significant issue in sensitive electronic equipment such as radio receivers, where it can drown out weak signals. It is a critical parameter in RF circuits, especially for front-end receiver circuits, and is an important consideration for audio systems where a good signal-to-noise ratio is crucial for optimal performance.
The impact of thermal noise can be mitigated by lowering the temperature or resistance in electrical circuits. Certain sensitive equipment, such as radio telescope receivers, are cooled to cryogenic temperatures to enhance their signal-to-noise ratio. Additionally, calculating thermal noise levels and understanding its value can help optimise system performance.
The spectrum of thermal noise is flat over a wide range of frequencies, giving it a white appearance. It is distinct from shot noise, which occurs when a voltage is applied and a macroscopic current starts to flow. Thermal noise is Gaussian distributed and has a unique frequency content and time-dependent behaviour. It is considered an intrinsic noise source, arising from the system's temperature rather than any external component.
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It can be modelled as a noiseless resistor in series with a voltage source
Thermal noise, also known as Johnson-Nyquist noise, is the electrical noise present in equipment due to the vibration of charge carriers within a conductor. It is caused by the thermal agitation of the charge carriers, typically electrons, inside an electrical conductor at equilibrium, regardless of any applied voltage. This noise is present in all electrical circuits and can interfere with sensitive electronic equipment, such as radio receivers, by drowning out weak signals.
The impact of thermal noise is significant in systems that require precise electronic and optical measurements, such as optical systems and communication systems. It is important to distinguish between extrinsic and intrinsic noise sources to address these issues effectively. Intrinsic noise sources like thermal noise are inherent to the system and arise due to the presence of temperature.
A resistor with thermal noise can be modelled in two ways:
- Thévenin Equivalent Circuit: In this model, the resistor with thermal noise is represented by a noiseless resistor in series with a Gaussian noise voltage source. The voltage fluctuations across the resistor terminals are a result of the random motion of electrons, with their kinetic energy depending on the temperature. The amount of thermal noise in a component is directly proportional to its temperature.
- Norton Equivalent Circuit: Here, the noisy resistor is modelled as a noiseless resistor in parallel with a Gaussian-distributed current source. This representation considers the current fluctuations caused by thermal noise.
By employing these models, engineers can gain valuable insights into the behaviour of thermal noise in circuits and work towards mitigating its effects. These models help in understanding the impact of thermal noise on the overall performance of electronic systems and facilitate the design of more robust and efficient circuits.
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It is important for audio systems to have a good signal-to-noise ratio
Thermal noise is defined as the electrical noise present in equipment due to the vibration of charge carriers within a conductor. It is also known as Johnson-Nyquist noise and is proportional to temperature. All electrical circuits exhibit thermal noise, and in sensitive electronic equipment, it can drown out weak signals.
For example, if you're trying to have a conversation with someone in a noisy environment, you may need to raise your voice to be heard over the background noise. Similarly, in an audio system, the signal (desired audio) needs to be stronger than the noise (unwanted sounds) for clear and accurate sound reproduction.
The signal-to-noise ratio is expressed in decibels (dB), and a higher number indicates better performance. For instance, an audio component with an SNR of 100 dB means the audio signal level is 100 dB higher than the noise level, resulting in clearer audio.
Some devices incorporate hardware or software elements specifically designed to improve the signal-to-noise ratio. Balanced circuits in home audio systems, for instance, eliminate noise from household electrical sources, ensuring only the desired audio is heard. Additionally, the best components are designed to maintain a low noise floor, which is the inherent noise present in the system even when no audio is playing.
In summary, a good signal-to-noise ratio in audio systems is crucial for minimizing distractions and ensuring a clear and immersive listening experience.
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Frequently asked questions
Thermal noise, also known as Johnson-Nyquist noise, is the electrical noise present in equipment due to the vibration of charge carriers within a conductor. It is caused by the thermal agitation of the charge carriers, which are usually electrons, and occurs regardless of any applied voltage. This type of noise is present in all electrical circuits and can interfere with sensitive equipment.
Thermal noise pollution is caused by the random movement of electrons within a conductor, which creates voltage and current fluctuations. These fluctuations are proportional to the temperature of the system and the resistance of the conductor. Any system with a temperature greater than 0 Kelvin will exhibit thermal noise.
Thermal noise can interfere with the performance of electronic equipment by drowning out weak signals. It is a major source of noise in electronic circuits, radio frequency systems, and communication systems. In RF circuits, it is a critical parameter for front-end receiver circuits and can limit the sensitivity of radios.
