Preamplifiers feedback

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... 

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

The output from a voltage sensitive or resistor feedback preamplifier is a tail pulse with a rather long decay time. Hence, some pulse pile-up is unavoidable, except at very low count rates. Pile-up will cause the average level of this signal to increase with pulse rate, which may approach the limit of linear operation of the preamplifier. 

An imaging high-pressure detector can be envisioned from an array of vertically cylindrical ionization chambers, with spatial resolution set by each tube diameter. It may further be possible to segment the collection anode, to derive an azimuthal co-ordinate within each detector and to use signal risetime to get a radial co-ordinate. The precision of such techniques, and the low-energy performance of such detectors is critically dependent upon the preamplifier noise. It may be possible to achieve around 50 electrons rms with modern (optical feedback, or no feedback) amplifiers resulting in an energy resolution of a few percent at 100 keV. 

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... 

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. 

Er is the equivalent energy of the reset, and Tr is the reset and recovery time associated with each reset. For a situation where E = 30 keV, I = 10" counts/s, Er = 3000 keV, and Tr = 10" s, the percent reset deadtime is 10%. In the pulsed optical feedback preamplifier the maximum energy rate that can be handled is determined by the resetting deadtime. The limiting energy rate is reached when... 

The Kandiah pulsed optical feedback design [25-27] differs from the Landis and Goulding design in that the preamplifier is reset after each x-ray photon is processed. In this case a short deadtime is added to each processed pulse. 

The increase in the peak width as well as the amplitude and slope of the skew term over time is a natural consequence of detector aging, and can be tracked by the periodic calibration of these parameters. The appearance of a long left skew and a signiftcant, irremovable right skew can be signs of the damage to the semiconductor crystal, which can temporarily be fixed by adjusting the pole/zero in the RC-feedback preamplifier. 

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...

---