2
ORTEC 142IH PREAMPLIFIER
1. DESCRIPTION
The ORTEC 142IH charge-sensitive
Preamplifier is
designed as a minimum-cost general-purpose unit.
It is intended for use in systems where the detector,
rather that the preamplifier, is the major noise
source in the system or in systems where the
ultimate in performance is not required and the cost
of the system must be minimized. It can be used
with semiconductor detectors, proportional
counters, scintillation counters, ionization
chambers, etc.
A bias circuit is included to accept the operating
voltage required by the detector and to furnish this
bias out to the detector through the signal input
cable. The bias input circuit in the 142IH
Preamplifier includes a 100-M
S
load resistor, R4,
and any detector leakage current will have to pass
through this resistance as well as through R1,
1.5 M
S
, in series. A considerable voltage drop will
be expected across this 100-M
S
resistor for a
detector with high leakage, and resistor R3 (shipped
as an accessory) can be installed in parallel with
R4; R3 has a value of only 10 M
S
.
An input protection circuit is built into the
preamplifier. It protects the input FET from any
large transient voltages that would otherwise
damage the transistor. This is discussed in the
Notice inside the front cover. The protection circuit
also provides a damping resistance on the input so
that relatively long cable lengths can be used
between the detector and the preamplifier without
disrupting the system stability.
A Test Pulse connector with built-in charge
terminator is provided for use with a pulse
generator such as the ORTEC 419, 448, or 480 to
simulate the signal from the detector. This allows a
check of the system performance while an
experiment is in progress.
The 142IH will accommodate up to ±3000 V for the
detector. The output pulse polarity is the same as
the applied bias polarity. The 142IH Preamplifier
output can be connected to a shaping main
amplifier such as the ORTEC 451, 485, or 572 for
energy spectroscopy or to a timing filter amplifier
such as the ORTEC 474 for time spectroscopy.
Output connectors are provided so that both types
of analysis can be operated simultaneously.
If it is necessary to open the case for any reason,
observe the following instructions carefully to
prevent serious injury to yourself and/or damage to
the instrument.
Observe the steps that are included in the
Notice at the front of the manual to discharge
the high voltage and prevent shock; the voltage
levels that can be used are lethal and the
capacitors are very high quality so they retain a
charge much longer than normally expected.
Do not touch the high-megohm resistors, R4
and R7, with your bare fingers; the presence of
skin oil can reduce the resistance of the
component and alter operating characteristics.
See Section 4 for instructions that involve the
protection circuit.
Summary of Contents for 142IH
Page 6: ...vi...
