the
end, which
is
fed
into
the
preamplifier
is
very small. Also, each
time
the
cable length
is
changed, a new voltage sensitivity
factor
must be calculated,
since
the
voltage divider has new
component
values.
If,
however, ·an amplifier which gives an
output
voltage proportional
to
input
charge
is
used, it can be seen
from
Fig. 1.1
a)
that
the
cable capacitance
would have
no
effect
on
the
sensitivity.
This,
then,
is
the
difference between a charge amplifier and a voltage
amplifier.
A voltage amplifier
is
sensitive
to
variations
in
input
voltage and
so
is
sensitive
to
cable capacitance. A charge amplifier
is
sensitive
to
variations
in
input
charge, and since
the
capacitance across which the charge
appears does
not
alter
the
charge,
the
charge amplifier
output
is
independent
of
the
cable
tapacitance.
1.3. PRINCIPLE OF THE CHARGE AMPLIFIER
Basically, a charge amplifier consists
of
a high gain operational amplifier
with a feedback capacitor, Ct, as
in
the
equivalent circuit
of
Fig.1.2. It can
be shown
that
the
voltage
output
of
this circuit
is
given
by
OA
Fig.1.2. Equivalent
circuit
of
charge amplifier
7
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