Noise Johnson

The primary sources of noise in this region are classified as thermal or Johnson noise together with shot noise due to the particulate nature of photons and electrons. Additionally there is non-equilibrium or inverse frequency noise, often termed pink noise to distinguish it from the other two that have a uniform white noise frequency distribution. 

Thermal fluctuations are unavoidably present in all circuits and give rise to a 

This corresponds to a power 4 X 10 WHz at 290 K which produces an rms noise voltage (AkTBR) across any resistor combination of value R. The p — p fluetuation is 4-5 times greater than this value. Practical circuit devices such as amplifiers increase this apparent noise power by their power gain plus 

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Semiconductor devices ate affected by three kinds of noise. Thermal or Johnson noise is a consequence of the equihbtium between a resistance and its surrounding radiation field. It results in a mean-square noise voltage which is proportional to resistance and temperature. Shot noise, which is the principal noise component in most semiconductor devices, is caused by the random passage of individual electrons through a semiconductor junction. Thermal and shot noise ate both called white noise since their noise power is frequency-independent at low and intermediate frequencies. This is unlike flicker or ///noise which is most troublesome at lower frequencies because its noise power is approximately proportional to /// In MOSFETs there is a strong correlation between ///noise and the charging and discharging of surface states or traps. Nevertheless, the universal nature of ///noise in various materials and at phase transitions is not well understood. 

Many special-purpose electrical thermometers have been developed, either for use in practical temperature measurement, or as research devices for the study of temperature and temperature scales. Among the latter are thermometers which respond to thermal noise (Johnson noise) and thermometers based on the temperature dependence of the speed of sound. 

Let us remember that the bolometer noise consists of two contributions [82] the phonon noise caused by fluctuations in the transfer of thermal energy between the bolometer and the heat sink and the Johnson noise due to the thermistor resistance. 

As was mentioned before, noise is a term used to describe any random output signal that has no relationship with the incoming signal (the incoming light). In photomultipliers, noise can be classified, depending on its origin, into three types dark current, shot noise, and Johnson noise. The differences between these three classes are explained next ... 

COMPUTER ALGORITHMS SOFTWARE JOB PLOT Johnson noise,... 

Fig. 11.1. Two basic types of current ampliflers. (a) Feedback picoammeter. It consists of two components, an operational amplifier (op-amp) A, and a feedback resistor 1 fb- a typical value of the feedback resistor used in STM is 10 fl. The stray capacitance Cfb is an inevitable parasitic element in the circuit. In a careful design, Cfb 0.5 pF. The input capacitance Cm is also an inevitable parasitic element in the circuit. Those parasitic capacitors, the thermal noise of the feedback resistor, and the characteristics of the op-amp are the limiting factors to the performance of the picoammeter. (b) An electrometer used as a current amplifier (the shunt current amplifier). The voltage at the input resistance is amplified by the circuit, which consists of an op-amp and a pair of resistors R, and R2. The parasitic input capacitance Cm limits the frequency response, and the Johnson noise on Rm is the major source of noise. Also, the input resistance for this arrangement is large.

It is often called the Johnson noise, after its discoverer (see, for example, Ott, 1976). From Eq. (11.2), we find that the mis Johnson voltage noise in a resistor R is... 

In reducing Johnson noise, to incorporate all the amplification in one single stage is the best choice. However, a stray capacitance Cfb is always present. In this case, the relation between the input current and the output voltage is determined by the differential equation... 

Fig. 11.2. Broad-band current amplifiers, (a) By replacing the feedback resistor in Fig. 11.1 with a resistor network, the cutoff frequency of the amplifier can be greatly increased, but the Johnson noise is increased, (b) Broad-band current amplifier with a compensation capacitor. By introducing a condensation capacitor C2, the effect of Q can be reduced. Under the condition CiRi = C2R2, the frequency range is substantially expended. The Johnson noise is not affected.

Fermi-level DOS 115 Jellium model 92—97 failures 97 schematic 94 surface energy 96 surface potential 93 work function 96 Johnson noise 252 Kohn-Sham equations 113 Kronig-Penney model 99 Laplace transforms 261, 262, 377 and feedback circuits 262 definition 261 short table 377 Lateral resolution... 

The second contributor to Eq. (53) has to do with the type of noise assumed to be present. To keep things simple, we shall assume it to be additive and independent Gaussian but with generally position-dependent variance e . This describes, for example, Johnson noise and other noise types that enter into the output of the spectrometer. Thus, we have as the probability PN of noise... 

Thus, to maximize D in the region where Johnson noise dominates, it is desirable to maximize ... 

The bandwidth of the complete circuit is limited by the bandwidth of the operational amplifier and the parasitic capacitance of Rfb, while the main noise source is given by the Johnson noise density n rb of the resistor Rfb ... 

All signal detectors are required to detect the signal against a background of noise . Therefore, the signal-to-noise ratio must be optimized or, put another way, for maximum sensitivity the noise has to be minimized. The sensitivity of any detector is determined by the noise level in the amplified output signal. In the case of a pyroelectric detector and its associated circuitry, the principal sources of noise are Johnson noise, amplifier noise and thermal fluctuations. 

Johnson noise and thermal fluctuations are briefly discussed below, Johnson noise because it is usually the dominant noise and thermal fluctuations because they set a lower limit on the achievable noise level. 

Johnson noise arises because the random thermal motion of electrons in an isolated resistor produces random fluctuations in voltage between its ends, covering a broad frequency band. It can be shown that... 

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