Amplifier voltage noise

The coaxial cable is the major source of noise for yet another reason. At the input of the op-amp, there is always a small voltage noise, which is amplified by the op-amp and appears at the output end. To make a simplified analysis, let the input noise be represented by an ac source at the noninverting input end. The output voltage is (see Fig. 11.3)... 

Another way to reduce the fluorescence background, especially from plastic chips, is to modulate the velocity of a fluorescent analyte. The analyte velocity is modulated by periodic variation (in 7-20 Hz) of the separation voltage. Noise rejection is achieved using a lock-in amplifier because only the fluorescent signal but not the background from the chip substrate was modulated. With this method, a decrease in LOD by one order of magnitude has been obtained [685],... 

What really matters, of course, is not the current yield per se, but the signal-to-noise ratio at low concentrations, or detection limit, which depends on the ratio of the current yield (sensitivity) to current noise. Measuring equipment that employs suitable solid-state amplifiers contributes virtually nothing to the observed current noise (for a discussion of the influence of voltage noise, see ref. 35, however). 

Additionally, there is the inherent amplifier noise. It consists of two frequency-dependent components, the internal voltage noise source e and the voltage drop across the source resistance R, caused by an internal current noise generator i . The total input noise for the amplifier with a bandwidth of B=f2 -fi is calculated as the sum of its three independent components ... 

High signal-to-noise ratios thus require the use of very low noise amplifiers and the limitation of bandwidth. The current technology offers differential amplifiers with voltage noise of less than 10 nV/VHz and current noise less than 1 pA/VHz. Both parameters are frequency dependent and decrease approximately with the square root of frequency. The exact relationship depends on the technology of the amplifier input stage. Field effect transistor (FET) preamplifiers exhibit about 5 times the voltage noise density compared to bipolar transistors but a current noise density that is about 100 times smaller. 

In this work, we set the amplifier s noise power to be 1/lOth of the electrode noise power. This results in the small 4.8% increase in the total equivalent RMS noise voltage. From (15) and (28), we can express the relationship mentioned above... 

The major noise sources for a typical pyroelectric detector are the dielectric or Johnson noise, the amplifier current and voltage noise, and the thermal noise, caused by fluctuations in the power flow from the element to its heat sink. Each of these has an equivalent voltage generated at the amplifier input V y (given by equation (5.9)), and and Vj. respectively. These combine to give the total equivalent input noise according to the equation... 

Fig. 6.16 Predicted hydrophone noise spectra (I) sea-state zero (II) Johnson noise of dielectric loss (III), (IV) voltage noise of two existing pre-amplifiers. The material constants used were as follows = 44, tan d = 0.032 + 0.47 + 1.4 f if =20 Hz-10 kHz), 4 = 13.8 pC N gn = 0.035 V mN ...

Fig. 38. Voltage noise of resistors as a function of the amplifier bandwidth [214]...

CCD detector designers try to increase the signal-to-noise ratio of an amplifier in two ways (1) increase the responsivity, or (2) decrease the random current fluctuation between source and drain. The responsivity can be increased by decreasing the amplifier size. Decreasing the amplifier size decreases the capacitance of the MOSFET. The responsivity of a MOSFET obeys the capacitor equation which relates voltage, V, to the charge Q on capacitance C V = QIC. 

The radiation detector is located some distance from the readout. A shielded coaxial cable transmits the detector output to the amplifier. The output signal of the detector may be as low as 0.01 volts. A total gain of 1000 is needed to increase this signal to 10 volts, which is a usable output pulse voltage. There is always a pickup of noise in the long cable run this noise can amount to 0.001 volts. 

In the past, except for the low-temperature range, the uncertainties of noise thermometry were not comparable to those of the gas thermometry due to the non-ideal performance of detection electronics. Up to now, the most successful technique is the switched input digital correlator proposed by Brixy et al. in 1992 [89], In this method, the noise voltage is fed via two separate pairs of leads to two identical amplifiers whose output signals are multiplied together, squared and time averaged (see Fig. 9.10). 

This eliminates the amplifier and transmission line noise superimposed on the thermal noise, since the respective noise voltages are uncorrelated. 

The superconducting quantum interference device (SQUID) is formed from a superconducting loop containing at least one Josephson junction. Basically, a SQUID amplifier converts an input current to an output voltage with a transresistance of the order of 107 V/A. The input noise is of the order of 10-11 A/(Hz)1/2. The bandwidth of the SQUID amplifier can be up to 80kHz. The dynamic range in 1 Hz bandwidth can be 150dB. 

When constituted of metals, thermopiles exhibit a very low noise, in particular only thermal noise if the voltage amplifier used for signal amplification has a very high input impedance. 

The voltage drop across the platinum temperature sensor is small since the platinum resistor has a nominal resistance of only 75 Q. The fully-differential LNA amplifies the minute voltage drop in order to provide an useful feedback signal to the differential-analog proportional controller. A simplified schematic of the fully-differential low-noise amplifier is shown in Fig. 5.18. 

Thermal electric noise thermometry 1. Josephson junction point contact 2 Conventional amplifier 0.001-1 4-1400 Mean square voltage fluctuation Nyquist s law oc fegT Other sources of noise serious problem for T > 4 K... 

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