| The preamplifier quickly terminates capacitance. It works as
an impedance matcher, presenting a high impedance to minimise loading,
while providing a low impedance output to drive succeeding components.
The preamplifier effectively integrates the detector charge pulse, but
because of the long decay time of the pulse at other than low count rates,
successive pulses pile up on top of one another. At higher count rates the
preamplifier will 'lock up' and no pulses will output at all.
One solution is to use an automatic reset. This allows the DC level to
build up stepwise, relying on the amplifier to measure the step height
until the limit of linearity is approached. At that point the DC level is
returned rapidly to zero and the stepwise iteration process continues. |
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