The Preamplifier

The primary purpose of the preamplifier is to provide an optimized coupling between the output of the detector and the rest of the counting system. The preamplifier is also necessary to minimize any sources of noise that may change the signal. 

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After the preamplifier, the beam is expanded to 2 mm, collimated and imaged onto a 1 mm aperture, producing a flat-top intensity profile. A 3-element telescope relays the aperture plane to the amplifier with a collimated 0.5-mm diameter. The telescope contains a spatial filter pinhole. The nominal power levels are 3 mW into the preamp, 500 mW out of the preamp and 200 mW out of the aperture. A 6° angle of incidence bounce beam geometry is utilized in the amplifier cell. The "bounce" foofprinf overlaps with the 4 pump beam fibers, arranged in 2 time sefs of 13 kHz. The pump fibers have f 50-60% fransmission. The amplifier brings the power up to < 20 W at 26 kHz. 

Figure 2.2.11 shows a typical block diagram of the MRI transceiver used for the compact MRI system. The waveform generator can be replaced by the DA converter on the DSP board described in the previous section. Because the typical input/ output power level for the transmitter and from the preamplifier is about 1 mW, a commercially available transmitter and preamplifier are directly connected. 

The preamplifier amplifies the voltage pulse. Further amplification is obtained by sending the signal through an amplifier circuit (typically about 10 volts maximum). The pulse size is then determined by a single channel analyzer. Figure 10 shows the operation of a single channel analyzer. 

The preamplifier/amplifier amplifies the voltage pulse to a usable size. 

Radiation detector output signals are usually weak and require amplification before they can be used. In radiation detection circuits, the nature of the input pulse and discriminator determines the characteristics that the preamplifier and amplifier must have. Two stages of amplification are used in most detection circuits to increase the signal-to-noise ratio. 

Fig. 12 (a) Current-distance retraction traces recorded with a goid STM tip for f mM 1,9-nonanedithiol in 1,3,5-trimethyibenzene on Au(lll)-(1 x 1), at bias = 0.10 V. The setpoint current before disabling the feedback was chosen at i0 = 0.1 nA. The retraction rate was 4 nm s-1. (b) Same conditions as in (a), except that the preamplifier iimit was chosen at 10 nA. The dotted lines represent characteristic regions of the low, mid, and high conductances... 

Figure 2 Readout pattern of a two-dimensional CCD. The bottom row corresponds to the shift register. Its output is connected to the preamplifier input. (Reproduced by permission, from the Book of Photon Tools, Oriel, Stafford, CT.)...

An electronic noise component is also generated by the transfer of charges and by the preamplifier. For each readout process, one readout noise is generated. This readout noise is not very sensitive to temperature but increases with reading-out speed. Readout noise for a HCCD is about 10 electrons RMS or less. 

Another important feature regarding the whole receiver chain (including the probe and the preamplifier) is the total dead time which must be kept as short as possible in order to allow measurements of NMRD profiles of solids with extremely fast decaying FIDs. 

Fig. 12.4. Single-tube STM. The tube piezo scanner is adhered inside a sturdy metal cylinder, which sits on three screws on the base plate. The two front screws make the coarse approaching. The rear screw makes fine approaching by using the two front screws as the pivot axis. The rear screw is actuated by a stepping motor for automatic approaching. The preamplifier (not shown) is mounted directly on top of the metal cylinder to eliminate the microphone effect of the coaxial cable between the tip and the input of the preamplifier. The entire unit is rigid enough that a mediocre vibration isolation device can provide atomic resolution. (After Hansma et al., 1988.)...

An external charge-sensitive preamplifier (type LE 14551) also designed by A. Bradley (3, 4), completes the preamplifier loop. 

A special zero suppressor was connected to the output of the preamplifier so that the transmission from 80 to 100% of the incident energy could be recorded. The relationship of concentration to signal output was found to be essentially linear in this range. An example of the recorder output on a 10 inch chart is shown in Figure 2. 

Logic modules are used to monitor the counting rate of single detectors and the relative times at which radiation is detected. A fast signal derived from the detector itself, the preamplifier, or from a timing-filter amplifier is sent to a discriminator. 

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