1
ORTEC MODEL 142A, 142B, and 142C PREAMPLIFIERS
1. DESCRIPTION
The ORTEC 142A, 142B, and 142C Preamplifiers
are charge-sensitive units that are designed for use
with room-temperature-operated silicon surface-
barrier detectors. They are designed to give the
ultimate in both energy and timing resolution, with
no compromise through either output circuit. The
142A Preamplifier is designed to operate over a
detector input capacitance range from 0 to 100 pF;
the 142B is for detector input capacitance of 100 to
400 pF; and the 142C from a capacitance of 400 to
2000 pF. The 142A and 142B can operate with
higher capacitance, but with performance
characteristics that are slightly degraded.
The 142A Preamplifier has a low noise intercept
and a moderate slope. The 142B and 142C have a
moderate noise intercept and a low slope and are
preferred for high-capacitance detectors and high-
energy measurements. The wide bandwidth of the
142B is compatible with detector capacitances
greater than 100 pF but less than 400 pF and
should not be used for capacitances less than 100
pF. The 142C should not be used with detectors
less than 400 pF capacitance.
All three models in the 142 Series feature a
transformer- coupled differentiated timing output
that is directly compatible with most timing
applications.
The energy range expected in typical applications is
from 0 to 200 MeV. Two simultaneous outputs are
provided; the output marked E is for energy
measurements and the output marked T is for
timing applications. Either or both outputs may be
used as desired, since their circuits are isolated
from each other. For best results, however, the T
output should be terminated in 50
S
when not in
use.
A bias circuit is included to accept the operating
voltage required by the surface-barrier detector.
The bias input circuit in the preamplifier includes a
100-M
S
load resistor, and any detector leakage
current will have to pass through this high
resistance. A considerable voltage drop will be
expected across this load resistor for a high-
leakage detector, and a smaller value of resistance
can then be substituted. A 10-M
S
resistor is
furnished as an accessory to the preamplifier to be
soldered in parallel with the 100-M
S
load resistor
when it is required (Section 4.2).
An input protection circuit is built into the
preamplifier circuits to protect the input FET from
any large transient voltages that would otherwise
damage the transistor. This is discussed in the
Notice on page vi.
An internal rise time compensation adjustment is
accessible through a hole in the case of the unit.
See Section 4.5 for adjustment information.
Under normal conditions, the case of the
preamplifier should not be opened. However, it is
necessary to remove the cover for the addition of
the 10-M
S
) resistor in the bias circuit or for
removing or reconnecting the input protection
circuit. Whenever the case is opened for any
purpose, observe the following instructions
carefully:
1. Do not touch the high-value resistors, R4 and R7,
with your fingers; the presence of skin oil can
reduce the resistance of the component.
2. Observe the steps that are included in the Notice
on page vi to discharge the high voltage to prevent
shock; the capacitors in this preamplifier are very
high quality and retain a charge much longer than is
normally expected.
See the information in Section 4 for instructions that
cover the action to be taken in the bias or protection
circuit.
