The Amplifier

There are many types of commercial preamplifiers, two of which are shown in Fig. 1.8. In most cases, the HV is fed to the detector through the preamplifier. 

The main amplification unit is the amplifier. It increases the signal by as many as 1000 times or more. Modern commercial amplifiers produce a maximum signal of 10 V, regardless of the input and the amplification. For example, consider a preamplifier that gives at its output three pulses with heights 50 mV, 100 mV, and 150 mV. Assume that the amplifier is set to 100. At the output of the unit, the three pulses will be 

Note that the third value should be 15 V, but since the amplifier produces a maximum signal of 10 V, the three different input pulses will show, erroneously, as two different pulses at the output. If only the number of particles is measured, there is no error introduced—but if the energy of the particles is measured, then the error is very serious. In the example given above, if gammas of three different energies produce the pulses at the output of the preamplifier, the pulses at the output of the amplifier will be attributed erroneously to gammas of two different energies. To avoid such an error, an observer should follow this rule  

Before any measurement of particle energy, make certain that the highest pulse of the 

In addition to signal amplification, an equally important function of the amplifier is to convert the signal at the output of the preamplifier into a form 

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After amplification both signals change their initial phases due to the delay r of the amplifier unblank (r = 0.1 - 0.5 ms), phase shift in it and wave propagation in passive vibrator s elements. All the mentioned phase changes are proportional to the frequency. The most contribution of them has unblank delay z. Thus frequency variations changes the initial phases) f/, and j(/c) of both signals and their difference A - Vi ... 

Figure Bl.10.2. Schematic diagram of a counting experiment. The detector intercepts signals from the source. The output of the detector is amplified by a preamplifier and then shaped and amplified friitlier by an amplifier. The discriminator has variable lower and upper level tliresholds. If a signal from the amplifier exceeds tlie lower tlireshold while remaming below the upper tlireshold, a pulse is produced that can be registered by a preprogrammed counter. The contents of the counter can be periodically transferred to an online storage device for fiirther processing and analysis. The pulse shapes produced by each of the devices are shown schematically above tlieni.

The amplified signal is passed to a double-balanced mixer configured as a phase-sensitive detector where the two inputs are the NMR signal (cOq) and the frequency of the synthesizer (03. gf) with the output proportional to cos(coq - co gj.)t + 0) + cos((coq + + 9). The sum frequency is much larger than the total bandwidth of the... 

The amplified signal can be recorded directly by analog means but, more likely, the signal is digitized and processed by a computer (data system). 

Direct photography of drops in done with the use of fiber optic probes using either direct or reflected light. StiU or video pictures can be obtained for detailed analysis. The light transmittance method uses three components a light source to provide a uniform collimated beam, a sensitive light detector, and an electronic circuit to measure the amplified output of the detector. The ratio of incident light intensity to transmitted intensity is related to interfacial area per unit volume. 

The noise is expressed as noise density in units of V/(Hz), or integrated over a frequency range and given as volts rms. Typically, photoconductors are characterized by a g-r noise plateau from 10 to 10 Hz. Photovoltaic detectors exhibit similar behavior, but the 1/f knee may be less than 100 Hz and the high frequency noise roU off is deterrnined by the p—n junction impedance—capacitance product or the amplifier (AMP) circuit when operated in a transimpedance mode. Bolometers exhibit an additional noise, associated with thermal conductance. 

Where stray currents are involved, several measurements have to be taken that are continually changing with time, simultaneously with each other. A double recorder is most suitable for this. Linear recorders with direct indication of the measurements cannot be used for potential measurements because the torque of the mechanism is too small to overcome the friction of the pen on the paper. Amplified recorders or potentiometer recorders are used to record potentials. In amplified recorders, as in amplified voltmeters, the measured signal is converted into a load-independent impressed current and transmitted to the measuring mechanism, which consists of a torque motor with a preamplifier. The amplifier results in an... 

Figure 4-6 is an amplitude-speed eurve showing the loeation of the running speed to the eritieal speed, and the amplitude inerease near the eritieal speed. When the rotor amplifieation faetor, as measured at the vibration probe, is greater than or equal to 2.5, that frequeney is ealled eritieal and the eorresponding shaft rotational frequeney is ealled a eritieal speed. For the purposes of this standard, a eritieally damped system is one in whieh the amplifieation faetor is less than 2.5. 

That eompletes its set-up. Tater, the eompensation around the amplifier needs to be done to set the de gain and the bandwidth performanee. 

First one assumes that the final closed loop compensation network will have a continuous -20dB/decade slope. To achieve a 15 kHz cross-over frequency, the amplifier must add gain to the input signal and push-up the gain curve of the Bode plot. 

For the amplifier pulse to be recognized in the ADC, it must exceed the lower level set by a discriminator, which is used to prevent noise pulses from jamming the converter. Once the pulse is accepted it is used to charge a capacitor that is discharged through a constant current source attached to an address clock typically... 

Electronic instrumentation is available for the measurement of D.C. and A.C. voltage, current and power as well as impedance. Such instruments usually have higher sensitivities, operating frequencies and input impedance than is normally found in the electromechanical instrumentation described above. However, they may need to incorporate amplifiers and they invariably need power to operate the final display. Hence, an independent power source is needed. Both mains and battery units are available. The accuracy of measurement is very dependent on the amplifier, and bandwidth and adequate gain are important qualities. 

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