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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.

[ Preamplifier applet here ]


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1999, 2000 The University of Liverpool, Department of Physics

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