Datron 1061 - Page 28

I
20
3.2.2.2
Preamplifier
(430328
sheet
1)
The
preamplifier
is
designed to
present an
input
impedance
of
greater
than 10,000Ml) for signals
up
to
+
20
volts.
lt
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.
O12
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
dri-ft
of
012
(åpprox.
-l%/ocl,
the
change
in
the
basecmitter
voltage of
one
half
of
O12
is
sensed
by
M30. The drift
compensation
is
linearised
to
l%loC
Oy
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
+12V(+BS)
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 wíthin
100ltV
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
f rom
M33 to
provide
an
unregulated
+42V(TL4) and
-42V(TL5)
supplv
from the
+15V
supply'
The
positive bootstrap
supply
(+BS)
is
generated
as
a
current
source
comprising
026 and
the
shunt regulator,
O27,
referenced
to
D50.
When
the
output
voltage
of
the
resulator
is
approxímately
1.2
volts
above
D50 cathode,
O27
conducts
current
into
R'175.
Since
the current
in Rt75
is controlled
to
be
constant
by
O30,
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
senerated
in
a similar
manner.
Thus
bootstrapped
supplies
of approx-
imately
!12
volts
are
produced, trackíng
the
input
signal
exactly.
g.2.2.4
Filtering
(430328
sheet
1)
Selection
of
filter
causes
an
active
filter
to be
switched
in by
relay,
RL2,
(via
O32).
The
filter
gives
an
attenuation
of
-g+
dó
at
50Hz'
The
essential
components
of the
filter
are shown
in
Fig.
3.10.
lnput
IOOmV
100k
o8
1k
IOOmV
RANGE
lnput
,IV
100k
o8
10k
1V
RANGE
lnput
10v
100k
21k6
o7
'lok
1OV
RANGE
9M9
lnput
100v
o8
100k
10k
1OOV
RANGE
9M9
lnput
1000v
100k
21k6
o7
10k
IOOOV
RANGE
FIG.
3.9
PREAMPLIFIER
GAIN
CIRCUITS
30k6
o6
21k6
010