Preamplifiers amplifier gain

FIG. 8.18. Voltage sensitive (a), charge sensitive resistive feedback (b), and pulsed optical feedback (c) preamplifiers. A is amplifier gain. 

Wandering peaks 1. Amplifier gain drift 2. Other faulty electronics 3. Detector-to-preampUfier connection loose 1. Repair/replace amplifier 2. Check cables/connectors for intermittent connection of signals or ground 1. T gently on detects to induce shift. Send for repair 2. Clean preamplifier contacts if possible... 

The pulses received from preamplifier have wide variation in energies of the particles. The gain in the amplifier is of the order of 8000 such that a 1 mV signal is amplified to approximately 8 V while still maintaining the proportionality of the energy delivered by the particle (more often y-Ray) to the detectors. The amplified pulse is then delivered to pulse height analyzer. 

The receiver system consists of a preamplifier (Radiation Devices model BBA-1P, bandwidth 3-500 MHz at 15 dB gain and 3 dB noise), a 10 MHz amplifier (F+H Instruments mixer-amplifier, model 39 A/M), and a phase detector (F+H Instruments phase detector 1-30 MHz). 

A current-sensitive preamplifier is used to transform fast current pulses produced by a photomultiplier into a voltage pulse. The current-sensitive preamplifier is an amplifying instrument. The sensitivity (or gain) of such a unit is expressed as h ut/ in> > mV/mA with typical values of the order of 500 mV/mA. The risetime of the pulse is 1 ns. 

Because of the high speed of the biased amplifier and the ADC in a TCSPC board, the photon pulses delivered to the ADC need not be broader than 50 to 100 ns. Therefore an extremely high preamplifier gain is not required, and AC coupling can be avoided. The setup can therefore be used up to a count rate of several 10 pulses per second. Examples for pulse height distributions recorded this way are shown in Fig. 6.13, page 227. 

The eommonly used MCPs and PMTs deliver single-electron pulses of 20 to 50 mV when operated at maximum gain. Although these pulses can be detected by the input diseriminators of most TCSPC modules, a preamplifier is recommended for several reasons. The most obvious one is that a good preamplifier, if it is con-neeted elose to the detector output, improves the noise immunity of the system. Moreover, with the amplifier the CFD ean be operated at a higher discriminator threshold, which improves the timing and the threshold stability. 

The main signal detection system was a commercial EG G phase-coherent detector that consisted of a broadband preamplifier of adjustable 30-60 dB gain that transmitted both 1 kHz and 2 kHz signals from the MMW detector, but was selectively tuned to 2 kHz. The outputs from this were split, with one half input to a home-made 1 kHz phase detector. This generated the piezoelectric actuator control signal that scanned the cavity. The other half went to the EG G precision low-noise 2 kHz phase sensitive amplifier that passed the spectral to the computer for processing and display (Figure 2.1). 

A typical AE detection system commercially available is illustrates in Fig. 2.8. AE waves are detected by AE sensor, which converts dynamic motions at the surface of a material into electrical signals. Because AE signals are weak, they are normally amplified by two amplifiers of a preamplifier and a main amplifier. The signal-to-noise ratio of equipments shall be low, and the amplifiers often provide more than 1000 times gain. Lately, it is set to normally 100 times or so. As discussed above, the bandpass filter is successfully employed to eliminate the noises. In engineering materials, the band width from several kHz to several 100 kHz or 1 MHz is recommended in the measurement. 

If fast signals with medium strength need to be recorded, then photodiodes are used in connection with so-called transimpedance amplifiers. In this case, the photodiode is driven in the photocurrent mode, i.e. it is driven in short-cut mode with Ud being nearly equal to zero. This mode requires that any preamplifier to be used is in transimpedance mode, i.e. its input impedance can be matched to that of the photodiode. This mode, also termed current-to-voltage converter, allows one to achieve low input impedance even for high gain. 

Figure 5.95 Oscillator and preamplifier of a laser-pumped dye laser with beam expander and grating. The same dye cell serves as gain medium for oscillator and amplifier [courtesy of Lambda Physik, Gottingen]...

The signal from the preamplifier of an X-ray spectrometer is the input to a linear fast-response amplifier whose gain can be varied by a factor up to 10,000. Voltage pulses as large as 10 V result. 

Linear pulses carry information in their size, that is, the pulse height or the pulse area. The output pulses from the preamplifier and amplifier are of this type. The linearity of the linear pulses is clearly crucial and much of the expense of the system comes down to maintaining this linearity. As an example, if we wish to measure an energy of say 2000 keV to a precision of 0.2 keV or better we must be sure that the gain of the system will be constant to better than 0.01 % over the period of a count. (Compare this with the integral linearity specification of typical spectroscopy amplifiers of better than 0.025 % .)... 

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