Logarithmic amplifier

Fig. 11.4. Logarithmic amplifier, (a) Schematic of a logarithmic amplifier. A diode is used as the feedback element in a current amplifier. The current-voltage characteristics are exponential. The output voltage is then proportional to the logarithm of the input current, (b) The transfer curve of a typical logarithmic amplifier, AD757N from Analog Devices. The reference current is internally set to be 10 p,A. It is accurate up to six decades.

The ultrasound system should have more independent channels and allow the transmitter pulse to be individually adjustable in width and amplitude, and an increased frequency range for the logarithmic amplifier was desired. The digitization should be improved both with respect to sampling rate and resolution. 

LOG Logarithmic amplifier with 60/100 dB dynamic range without gain setting 0.1 to 10 MHz (-3 dB)... 

The HILL-SCAN 3020LOG with a logarithmic amplifier provides A-seans with a single-shot dynamic range of 100 dB. 

Fig. 1 Scheme of the OCT system. SLD—superluminescent diode FBS—fiber-optic beam splitter M—mirror L—lenses S—sample PD—photo-diode F—band-pass filter A— logarithmic amplifier AD—amplitude detector ADC—analog-to-digital converter PC— computer. 

In preliminary evaluations a Nuclear Data ND-PSA Particle Sizing Amplifier (PSA) was interfaced between the Royco and the MCA. The PSA, which could serve as either a linear or logarithmic amplifier, was operated in the log mode to develop an output signal which would be more nearly proportional to the log of the particle diameter and spread the small-diameter particle count over a larger number of chemnels. The distribution displayed on... 

Sample solution is introduced into the solvent flow stream from a sample loop via a sample injection valve. The solvent flow pushes the sample solution through the analytical capillary vdiere the viscosity of the sample is detected. The AP signal from each pressure transducer is fed to a differential logarithmic amplifier as shown in the Figure. The viscometer output is the In signal of the sample solution. A pump is used to... 

To make the entire eleetronic response linear with respect to tunneling gap s, a logarithmic amplifier is attached at the output of the current amplifier. A logarithmic amplifier can be made from a feedback amplifier, by replacing the feedback resistor with a diode, as shown in Fig. 11.4. The current-voltage characteristics of a good-quality, forward-biased silicon diode follow an exponential law over more than five orders of magnitude ... 

For every decade of input, the output changes about 120 mV. In commercial logarithmic amplifiers, a subsequent linear amplification stage further amplifies the output voltage to a preset value. A typical input-output characteristic of a commercial logarithmic amplifier is shown in Fig. 11.4. 

It is possible to use a logarithmic amplifier as the first-stage current amplifier, which may become the only stage before being compared with the set point of the tunneling current. The disadvantages are The leakage current... 

A schematic of the feedback system of an STM is shown in Fig. 11.5. The tunneling current, after the current amplifier and the logarithmic amplifier, is... 

The minus sign indicates that the further away the tip is from the surface, the smaller the tunneling current. After the current is amplified by the current amplifier and the logarithmic amplifier, the signal becomes... 

Fig. 11.5. A schematic of the feedback loop in an STM. The tunneling current, after the current amplifier and the logarithmic amplifier, is compared with a predetermined voltage, which represents the current setpoint. The error signal is processed by the feedback electronics, which typically contains an amplifier and an integration circuit. The output of the feedback electronics is applied to the z piezo, to keep the error between the actual tunneling current and the reference current very small. The voltage applied to the z piezo is recorded as the topographic image.

Local density of states (continued) definition 119 s-wave-tip model, and 29 Sommerfeld metal 93 STM corrugation, and 142 total charge density, and 120 Local modification of sample wavefunctions 195 Local-density approximation 114 Logarithmic amplifier 257 Louse 269... 

The photomultiplier output is proportional to the transmission of the flame, yet concentration is proportional to absorbance. A logarithmic readout is preferred as being linear in absorbance This can be achieved by using an appropriate logarithmic amplifier, or when normalizing the output from the integrating condenser. 

Fig. 1. Analysis of fluospheres (Dako Ltd.) using logarithmic (open peaks) and linear (filled peaks) amplification. Logarithmically amplified peaks are identified by upper-case letters, and the corresponding bead peak amplified on a linear scale by lower-case letters. The linear scale discriminates between bright signals (for example, d and e) better than the log scale (D and E). but the range of fluorescence covered by the log scale is much greater than the linear scale.

A second consideration with logarithmic amplifiers is that because of the complex way in which they operate (3), quantitative data often require reconversion to a linear scale before further calculations can be done. This process requires information about the exact number of channels/decade, and any deviation from true logarithmic behavior (4). Such deviations are sometimes found, particularly in the lowest decade. 

The detector response of an IR analyzer is not linearly related to concentration. It follows the Beer-Lambert law, and a logarithmic amplifier provides an acceptable linear output. The Beer-Lambert law relates the amount of light absorbed to the sample s concentration and path length. 

All proportional detectors are designed to provide a response that is as close to linear as possible in order to facilitate calculation procedures in quantitative analysis. In many detectors the output from the detector sensor is not linearly elated to the solute concentration, in which case appropriate signal modifying circuits must be used to furnish a linear output. For example the output from a light adsorption sensor will be exponential and consequently it must be used with a logarithmic amplifier to produce an output that is linearly related to solute concentration. 

Photodiodes are used as detectors in many automated systems either as individual components or in multiples as an array. Photomultiplier tubes are required in many immunoassay systems to provide adequate sensitivity and fast detector response times for fluorescent and chemiluminescent measurements. Several approaches have been used for the electrooptical integration and packaging in different analyzers. A logarithmic amplifier or microprocessor and/or computer software converts transmittance to absorbance. Low-cost analog-to-digital converters with conversion times... 

Each operating channel will consist of a gamma compensated ion chamber and a seven-decade or more logarithmic amplifier. 

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