Datron 1061 Calibration And Servicing Handbook 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