Resistance Feedback

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

In preliminary tests performed on the X-ray camera a subarray of 5x5 strips was used to obtain a 25 pixel detector each strip coupled to a hybrid charge sensitive preamplifier (CSP), model CS 507, produced by Clear Pulse (Tokyo) in a modified version with external input FET and a resistive feedback loop CSP is characterised by an equivalent noise (r.m.s.) of about 1 keV. Pulses from CSP... 

Figure 4.21 A simplified representation of the Si(Li) detector cryogenic mount for a resistive feedback system. The length of the cryostat is foreshortened in the figure cryostat length is typically on the order of 70 cm. The section above the mounting flange is the end cap and may vary from a 2.5- to a 6-cm outside diameter. (Reprinted by courtesy ofEG G ORTEC.)...

TWo general classes of preamplifiers are in common use with Si(Li) detectors in x-ray fluorescence spectrometers (a) the continuous feedback preamplifier and (b) the pulsed feedback preamplifier. Resistive feedback and the dynamic charge restoration feedback method [21-24] both fall under the continuous feedback category. The pulsed feedback system ordinarily encountered in x-ray spectrometry is the pulsed optical feedback [25-29]. 

Figure 4.26 The resistive feedback preamplifier. (Reprinted by courtesy of EG G ORTEC.)...

The dynamic charge restoration feedback differs from the resistive feedback in that a more complicated, active feedback network is substituted for the simple resistor feedback. This results in a lower preamplifier noise at long shaping time constants, but maintains an energy rate capability at short shaping time constants similar to the resistive feedback preamplifier. Pulse shapes at the output of the dynamic charge restoration preamplifier are similar to the pulses from the resistive feedback preamplifier. In both types of continuous feedback there is no deadtime or deadtime loss associated with the preamplifier. 

Figure 4.27 shows the pulsed optical feedback preamplifier of the Landis and Goulding design [28]. It is identical to the resistive feedback preamplifier except that the feedback resistor has been removed. This change permits lower preamplifier noise at long shaping time constants. In place of the feedback resistor an optical reset circuit has been incorporated. 

Ikuta, K., M. Tsukamoto, and S. Hirose, Shape Memory Alloy Servo Actuator System with Electric Resistance Feedback and Application for Active Endoscope, in Computer-Integrated Surgery, R. H. Taylor et al. (eds.), 1996, MIT Press, Cambridge, Mass, pp. 277-282. 

In this subsection, we will discuss noise sources that arise from the nanopore itself, Erec, and the head stage during nanopore sensing. For the noise analysis, we use the simplified electrical models shown in Figure 29.3. Here, and model the resistance and capacitance of the nanopore, respectively. These vary with nanopore diameter, thickness, and material. Erec is modeled by the series resistance and a double-layer capacitance Cj, where R is much smaller than Rj. Note that in this chapter, we pay attention to the resistive-feedback transimpedance amplifier (TIA) due to its simple hardware structure and robust reliability, rather than a capacitive-feedback TIA, which requires a disruptive periodic reset [12,15]. Readers who are interested in the capacitive-feedback TIA can refer to the paper of Kim et al. [17]. 

To analyze an op-amp circuit with resistive feedback, we also need noise models for resistors. Two models for a noisy resistor are shown in Fig. 7.100. The thermal noise (Johnson noise) of the resistor is modeled by a voltage noise source with spectral density as shown in Fig. 7.100(b). The power density of this source is e = 4fcTJ V /Hz where fi is in ohms, k is Boltzmann s constant = 1.38 x 10" J/K, and T is temperature in degrees Kelvin. Thus, thermal noise is white. At 25 C, the noise spectral density is approximately Cr Ay/R nV/ Hz where K is expressed in kilohms. Figure 7.100(c) shows an equivalent resistor noise model that includes a current noise source having power density i = AkT/R A /Hz. 

Figure 4.6 Schematic diagram of a resistive feedback charge coupled preamplifier...

Figure 4.7 Shape of the output pulse from a resistive feedback preamplifier (a) definition of rise time and fall time (b) actual rising edge shapes derived from a 45 % detector...

Figure 4.8 Pile-up at the output of a resistive feedback preamplifier...

Figure 4.9 Block diagram of Canberra 2002 resistive feedback preamplifier indicating the function of the various panel sockets and indicators...

The second limitation of the resistive feedback preamplifier is that the feedback resistor has an intrinsic noise associated with it Johnson noise) and this can be a significant problem with pulses of very small size. In order to minimize this source of noise the value of is chosen to be high. This, in turn, means that the decay time of the output pulse is long, which exacerbates the pile-up problem. In principle, it would be possible to reduce the time constant by reducing Cf but doing that would affect the linearity of the preamplifier. There is, however, scope for reducing the value of Rf, in order to trade off resolution for count rate performance and this will be referred to in Chapter 14, Section 14.3.2. 

The very sharply peaked pulses emanating from the traditional resistive feedback preamplifier are not suitable for... 

Figure 4.12 Piled-up resistive feedback preamplifier output and the desired converted pulses...

The reader should remember that throughput will also be constrained by the amplifier and that, at very high count rates, a resistive feedback preamplifier may lock up. Throughput of complete systems is discussed in Chapter 14. 

The second factor in Equation (6.6), the thermal noise term for the feedback resistor, can be eliminated altogether by removing it, i.e. by using an autoreset mechanism. Thus, both transistor-reset (TRP) and pulsed-optical-reset (POR) preamplifiers can give better resolution than the resistive feedback preamplifier. (Transistor reset preamplifiers were discussed in Chapter 4, Section 4.3.2.) Note that parallel noise is independent of the detector capacitance. 

The detector is installed and ready to power-up. 1 assume initially that we are dealing with the more common RF or resistive feedback preamplifier and that the electronic system is NIM based. The instructions below are very detailed, intended for when installing a new detector, checking out a detector after repair, or for those wishing to understand more about their detector. On a routine basis, most people would not be as pernickety — switch on and wind it up being more likely ... 

Source activity too high resistive feedback preamplifier blocked... 

Resistive-feedback Energy rate Dynamic range — 4 X lO to 7 X 10 ... 

The mechanisms of the resistive feedback (RF) preamplifiers and transistor reset preamplifiers (TRPs) were discussed in Chapter 4, Section 4.3. Some of their properties are compared in Table 14.2, and some of the differences between them are explored in the following sections. 

Table 14.3 Effect of changing the feedback resistor in resistive-feedback preamplifiers...

Figure 14.3 Throughput curves for two complete spectrometry systems using a transistor reset preamplifier (continuous line) and a resistive feedback preamplifier (dashed line), with each feeding the same high count rate amplifier and ADC (reproduced by permission of Canberra Nuclear)...

At high count rate, for resistive feedback preamplifiers, the correction can be more difficult to achieve than at lower rates. The automatic PZ button on some ampUfiers is a welcome user friendly feature but, at high count rate where settings are more critical, the user may prefer the comfort of visual feedback whUe performing the correction manually. The operations of Chapter 10,... 

Transistor reset preamplifiers do not shnt down at high rates as do resistive feedback preamplifiers. They also show better resolution at high rates. However, the resetting process introduces extra dead time, particularly due to overloading of the amplifier. 

RESISTIVE FEEDBACK PREAMPLIFER. The conventional preamplifier in which the input voltage step is reset by a feedback resistor. For high count rate systems the Transistor Reset... 

TRANSISTOR RESET PREAMPLIFIER A type of preamplifier particularly suited to high count rates. The alternative to the resistive feedback preamplifier. See Chapter 4. 

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