8
Fig. 4.4. 142AH Typical Rise Time.
4.6. INPUT PROTECTION
A provision is built into the preamplifier to protect
the input FET stage from damage when high-
voltage transients are applied to its input. These
transients can result from any one or more of
many causes, including detector breakdown,
moisture condensation on the input connector,
short circuits or uncharged capacitance
connected across the input while bias is being
applied through the preamplifier, or disconnection
of a bias voltage without first reducing it gradually
to zero.
The protection circuit is installed in the
preamplifier when the unit is shipped from the
factory. Although it offers protection to the FET, it
also causes some degradation of the noise
performance of the preamplifier, which increases
as detector capacity increases.
With the protection circuit in, the collector lead of
Q11 is attached to the center tap of the two series
input resistors to the FET, R39 and R5. Transistor
Q11 is connected as a diode, with both the base
and the emitter tied through R8 to ground. This
prevents the voltage in the input circuit from
increasing beyond the safe limit for the FET input.
To take the protection circuit out, simply remove
the collector lead of Q11 from its circuit
connection and install a wire jumper across R39
and R5.
Inherent to all high-speed charge-sensitive
preamplifiers is the problem of keeping the total
system stable while interconnecting the detector
and preamplifier with long cables. To help ease
this problem and permit more flexibility for
installations, the protection circuit is designed to
serve not only as a protection for the FET input
but also to terminate the input cable in a nominal
100
. So it is recommended that the protection
circuit be left in the preamplifier circuits whenever
possible.
In order to take full advantage of the rise time
capabilities of the 142AH for timing experiments
(typical rise times of 4 to 10 ns for detector
capacities of 1 to 100 pF), the total cable length
should be kept as short as possible even though it
is terminated. Due to vagaries in the scattering
chamber system - ground loops, stray
inductances, etc. - and since the maximum cable
length is a factor in the input capacitance to the
preamplifier, it is not possible to give absolute
numbers. Generally, two feet is a typical
maximum length to obtain fastest rise times for
low-detector capacities. Of course, the system
can be compensated for cable lengths of up to 5
or 6 feet but slower rise times will be obtained.
The screwdriver adjustment inside the case can
be used for this type of system. The rise times
under these conditions will be in the range of 10
to 25 ns for capacitances from 1 to 100 pF.
Typical results are shown in Fig. 4.4.
