Shot Noise : Circuit, Working, Vs Johnson Noise and Impulse Noise & Its Applications

The shot noise was first developed by the German physicist namely “Walter Schottky” who played a main role in the expansion of the electron & ion emission theory. While working on thermionic valves or vacuum tubes, he observed that even when all external noise sources had been removed two kinds of noise remained. One he determined was an outcome of the temperature which is known as thermal noise whereas the remaining one is shot noise. In electrical circuits, there are different types of noise sources like johnson/ thermal Noise, shot noise, 1/f noise, or Pink/ Flicker noise. This article discusses an overview of a shot noise – working with applications.

What is Shot Noise?

A type of electronic noise created from the discrete nature of electric charge is known as shot noise. In electronic circuits, this noise has random fluctuations in a DC current because actually current has a flow of electrons. This noise is noticeable mainly in semiconductor devices like Schottky barrier diodes, PN junctions, and tunnel junctions. Not like thermal noise, this noise mainly depends on the flow of current and it is more evident in PN tunneling junction devices.

Shot noise is significant with extremely small currents mainly when measuring on short time scales. This noise is particularly noticeable whenever current levels are not high. So this is mainly due to the statistical current flow.

Shot Noise Circuit

The shot noise experimental setup with a photo assembly circuit is shown below. This setup includes a variable-intensity light bulb & photodiode which are connected to a simple circuit. In the following circuit, the multimeter is used to measure the voltage supply across an RF resistor which is connected in series with the photo circuit.

A switch in the circuit chooses whether the photocurrent (or) the calibration signal can be given to the rest of the circuit. The op-amp which is on the right side is connected in parallel with the resistor causing the shot noise assembly box to have around tenfold gain.

The oscilloscope is used to digitally incorporate the resulting noise signal. A function generator is used in series with an attenuator to adjust the gain curve. Here, we began the Shot noise experiment with very careful calibration of the measurement chain through an attenuated sinusoidal signal using a function generator. The gain is recorded (g(f) = Vout(f)/Vin(f)).

During this experiment, we simply recorded the RMS voltage of the noise which is measured by the oscilloscope 20 times for 8 different voltages within the light photo circuit VF. After that, we broke the photo circuit & recorded the level of noise in the background.

In this circuit, the noise which is measured can be slightly changed depending on the integration time utilized by the oscilloscope, however, this is ranged on the order of 0.1% uncertainty & we can ignore it, as it is dominated through the uncertainty caused by random fluctuations within voltage.

Shot Noise Current Formula

Shot noise occurs when current flow throughout a PN junction. There are various junctions present on  integrated circuits. Barrier crossing is simply random & the DC current produced is the sum of various random elementary current signals. This noise is stable above all frequencies. The shot noise current formula is shown below.

In = √2qIΔf

Where,

‘q’ is the charge on an electron which is equivalent to 1.6 × 10-19 coulombs.

‘I’ is the flow of current throughout the junction.

‘Δf’ is the bandwidth in Hertz.

Difference B/W Shot Noise, Johnson Noise & Impulse Noise

The difference between shot noise, Johnson noise, and impulse noise are discussed below.

Advantages and Disadvantages

The advantages of shot noise include the following.

• The shot noise at high frequencies is the limiting noise for terrestrial detectors.
• This noise simply provides valuable information on basic physical processes beyond other experimental methods.
• Since the signal strength enhances more quickly, then the relative proportion of shot noise reduces & the S/N ratio increases.

The disadvantages of shot noise include the following.

• This noise is simply caused by the fluctuations within the number of detected photons at the photodiode.
• It needs a post-measurement data modification to compensate for the loss of signal because of the low-pass filter (LPF) formed through the tunnel junction.
• This is quantum-limited intensity noise. Various lasers are very close to shot noise, as a minimum for high-noise frequencies.

Applications

The applications of shot noise include the following.

• This noise is mainly visible in semiconductor devices like PN junctions, tunnel junctions, & Schottky barrier diodes.
• It is significant in fundamental physics, optical detection, electronics, telecommunications, etc.
• This type of noise is encountered in electronic & RF circuits as an effect of the granular current nature.
• This noise is very signification in a very low-power system.
• This noise is correlated to the quantized charge nature & the individual carrier injection throughout the pn-junction.
• This noise is simply distinguished from fluctuations of current in equilibrium which occur without any voltage applied & without any normal flow of current.
• Shot noise is the time-dependent fluctuations within electrical current that are caused by the electron charge’s discreteness.

Q). Why Shot Noise is Called White Noise?

A). This noise is frequently known as white noise because it has a consistent spectral density. The main examples of White noise are Shot noise & Thermal noise.

Q). What is the Noise Factor in Communication?

It is the measure of the S/N ratio degradation within a device. So, it is the ratio of the S/N Ratio at the i/p to the S/N ratio at the output.

Q). What is Shot Noise in Photodetector?

A). The shot noise within the photodetector in the detection of optical homodyne is attributed to either the zero point fluctuations of the quantized electromagnetic field, otherwise to the separate nature of the photon absorption procedure.

Q). How is Shot Noise Measured?

A). This noise is measured by using this like shot noise = 10 log(2hν/P) in dBc/Hz). The ‘c’ within dBc is relative to the signal, thus we multiply through the signal power ‘P’ to obtain the shot noise power within dBm/Hz.

Q). How do you reduce Shot Noise?

This noise can be reduced by

1. Increasing the signal strength: Increasing the amount of current in the system will reduce the relative contribution of shot noise.
2. Averaging the signal: Averaging multiple measurements of the same signal will reduce the shot noise, as the noise will be averaged out over time.
3. Implementing noise filters: Filters such as low-pass filters can be used to remove high-frequency noise components from the signal.
4. Reducing temperature: Increasing the temperature of the system will increase the amount of thermal noise, making shot noise relatively less significant.
5. Choosing the right detector: Using a detector with a larger active area or a higher electron collection efficiency can reduce the impact of shot noise.

Thus, this is an overview of shot noise and its applications. Usually, this noise happens whenever there is a voltage differential or potential barrier. Once the charge carriers like holes and electrons cross the barrier, then this noise can be generated. For instance, a transistor, a diode & a vacuum tube all will generate shot noise. Here is a question for you, what is noise?