Datron 1062 - Page 28

20
FIG. 3.9
PREAMPLIFIER GAIN
CIRCUITS
3. 2.
2.2
Preamplifier (430328
sheet
1)
The
preamplifier
is designed
to
present
an
input
impedance
of
greater
than
10,000MS2
for
signals up
to
±
20
volts. It
is also
bootstrapped
(tracking
of both
ground
lines
and
supply
voltages
with
input
signal)
being
essential
for
correct
operation
of
input
bias
compensation,
temper-
ature
compensation
and
common
mode
rejection.
Q12
is a
well
matched
monolithic
NPN
transistor
pair
exhibiting
minimal
voltage
drift and
low
noise
charact-
eristics,
the
output
being
buffered
by M31.
To
compensate
for
the
current
gain
drift
of Q12
(approx.
-1%/°C),
the
change
in
the
base-emitter
voltage
of
one
half of
Q12
is
sensed
by
IVI30.
The
drift
compensation
is
linearised
to
1%/°C
by
thermistor
R218.
Thus
the input
bias
current is
kept
constant
with
temperature.
3.2.2.3
D.C.
Bootstrap
(430328
sheet
2)
Bootstrapped
supplies are
generated
which track
the
input
signal
directly
(BS),
track
the
input
signal with
a
positive
offset of -H2
V(-hBS) and
track
the input
signal
with a
negative
offset
of
—
12(— BS).
M32
is the high
inpedance
buffer which tracks
the
inverting input
of the
preamplifier.
The
offset of M32
is
adjusted so
that its
input is
within
lOO/tV of the input
of
the
preamplifier. M32
thus
functions as
the low
impedance
rail (BS)
following
the input
signal.
Selection of DC(M20-3)
enables
the
capacitive
inverter
driven
from IVI33 to
provide
an
unregulated
-i-42V(TL4) and -42V
(TL5) supply
from the±15V
supply.
The positive
bootstrap
supply
(h-BS)
is
generated as
a
current
source
comprising Q26 and
the
shunt
regulator,
Q27,
referenced
to
D50. When
the output
voltage of the
regulator
is approximately
1.2 volts
above
D50
cathode,
Q27 conducts current into
R175. Since
the current in R175
is controlled to be
constant by
Q30,
referenced to D50,
the current
flowing through R174
is reduced. Hence the
supply current,
"mirrored” in R173,
is
reduced and
the
output voltage
controlled.
The
negative bootstrap
supply (—
BS)
is generated
in
a
similar manner.
Thus bootstrapped supplies
of
approx-
imately
±12
volts are produced, tracking the input signal
exactly.
3.2.2.4
Filtering (430328
sheet
1
)
Selection
of filter causes
an
active
filter
to
be
switched
in
by
relay, RL2,
(via Q32).
The
filter gives an
attenuation
of
-34
dB
at
50Hz.
The
essential components
of
the
filter
are
shown
in Fig.
3.10.
Vi
J