Amptek CoolFET A250CF Podręcznik - Strona 2
Przeglądaj online lub pobierz pdf Podręcznik dla Wzmacniacz Amptek CoolFET A250CF. Amptek CoolFET A250CF 7 stron. Charge sensitive preamplifier
Drukuj stronę1 Warnings
Under normal operation, a high voltage bias is applied
across the detector that is used with the Amptek A250CF
preamplifier.
The user must exercise care when the
detector is biased. Moreover, the bias circuitry has a very
long time constant and so this circuitry, including the
A250CF's input node, can remain at a high voltage for a
very long time.
If the user does not exercise suitable
caution, this voltage can possibly cause personal injury.
More likely, however, is that this voltage could damage or
destroy either the detector or the A250CF and void
warranty if proper care and operation are not exercised as
discussed in this manual.
The input circuit will be destroyed if the detector input
capacitor is shorted while the detector bias components are
charged. These capacitors retain charge for a long period of
time. A short circuit, which can result from connecting a
detector, cable, or capacitor, will cause the applied bias
voltage (stored on the input capacitor) to be coupled
directly to the input FET, causing catastrophic damage to
the the CoolFET hybrid and also the A250 hybrid. To
avoid the possibility of such damage, please observe the
following precautions:
1. Completely discharge the detector bias circuit by turning
off the bias supply before connecting a cable, capacitor,
or any other device to the Detector Input connection on
the front panel of the preamplifier.
2. To discharge the circuit if a variable HV supply is used,
turn the voltage control to zero and leave it there for are
least 30 seconds. The bias circuitry will discharge itself
through the output of the bias supply.
connect a low impedance, ideally a short circuit, across
the Detector Bias SHV connector on the rear panel of the
preamplifier for at least 30 seconds. Do not short the
Detector Input connector on the preamplifier.
The preamplifier includes a protection circuit that is
enabled when the unit ships from the factory, which
protects the input FET from destruction due to large
transients under abnormal operating conditions.
important to note that this circuit protects the FET and not
the detector.
If the detector breaks down due to over
voltage, the protection circuit prevents damage to the
preamplifier/FET. The protection circuit does add to the
electronic noise of the preamplifier, so if the lowest noise is
required, this circuit can be removed, as described later in
this manual.
However, this leaves the FET/preamp
susceptible to damage by a variety of transients. The "half-
life" of an unprotected FET is much shorter than that of a
protected FET, so there is a trade-off between the very
lowest noise and risk. Damage caused during unprotected
operation is not covered under warranty unless the
precautions outlined here are followed.
Amptek A250CF CoolFET User Manual –Rev. A5
Charge Sensitive Preamplifier
A250CF CoolFET
2 Overview
2.1
Description
The A250CF is a charge sensitive preamplifier. The input
to the preamplifier is the signal from a radiation detector, a
current pulse of short duration. The total energy deposited
is proportional to the total charge generated.
The primary output of A250CF (the energy output) is a
voltage step proportional to the input charge, the time
integral of the current pulse, with a gain of 176 mV/MeV
(Si). This step has a fast rise (the rise time of the A250CF is
2.5 nsec for 0 capacitance) and a slow decay to baseline
(500 msec).
shaping
measurements. The timing output has a much faster decay,
1 msec, and is used as the input to a timing circuit.
The A250CF consists of several main function blocks. The
core of the preamplifier is the charge amplifier itself, which
consists of the CoolFET hybrid, the A250 amplifier, and the
feedback components. This circuit produces the voltage
output for a current input, and determines the output noise
and rise time.
The series noise of a preamplifier is at its minimum when
the input capacitance is comparable to the FET capacitance.
The A250CF includes a jumper that permit the user to select
1 of 3 FETs, to match capacitance. Jumpers are used to
connect the gates to the input, to connect the commensurate
drains, and to select the proper drain resistor, to set the drain
current.
There are additional circuit elements in the A250CF,
including (1) connections to detector bias, (2) optional input
Otherwise,
protection circuitry (enabled when shipped from the
factory), (3) a test input, (4) an amplifier which buffers the
energy output and provides for polarity and offset
adjustments, (5) an amplifier which buffers the timing
output and provides for polarity adjustment, and (6) power
supply circuitry.
It is
This energy output is generally sent to a
amplifier
and
is
used
for
spectroscopic
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