in series
with
A2R4, The meter and A2R4 are also placed
in
parallel with the series-pass
transistor A2Q2 with the
POWER switch,
thus providing a check of BTl under an
actual load condition.
3-20. THE REFERENCE
SUPPLY
3-21. GENERAL.
Reference voltages of
1.
1,
11,
110,
and 1100
volts dc, upon
which the stability of the
instru-
ment
relies, and
a
3 kHz
output
signal
are produced by
the Reference
Supply. The
circuitry
of
the Reference
Supply consists of
two regulator stages and a
dc-dc con-
verter. A block diagram
of these
circuits is illustrated
in
Figure
3-2.
3-22. BLOCK DIAGRAM
DESCRIPTION.
The dc to dc
converter
regulator provides a
pre-regulated voltage
to the zener
current
regulator
and a
regulated voltage
to the dc
to
dc
converter. The zener
current regulator
provides
a
closely
regulated dc current for
the refer-
ence
zener diodes of CR3 and CR4
from which
the
1,
1
and 11
volt dc reference
voltages are derived.
These
same
zener diodes also
provide
a
stable
reference
voltage for
the dc to dc
converter comparison
ampli-
fier.
The
dc
to dc converter
produces the 110 and
1100
volt dc reference
voltages and a 3
kHz drive signal.
The reference divider
junction provides a
sample of
the 110 and 1100
volt
dc
reference
voltage to the dc
to
dc
converter
comparison amplifier
which controls
the
input dc
voltage to the dc to dc
converter. Conse-
quently, a
stable 110 or 1100
volt dc reference
voltage
related
to the zener reference
voltage is obtained.
The
3
kHz drive signal
is used to produce
operating voltages
for
null
detector,
ac
converter, and optional
recorder
output
circuits.
3-23.
CIRCUIT DESCRIPTION. The unregulated dc
voltage
from
the
power supply or
the
dc
output voltage
of the battery pack is applied to terminal 3 of the Ref-
erence Amplifier Assembly. This voltage is used
to
provide
an
input dc voltage to the dc to dc
converter
regulator circuitry. Initial turn-on
of
the dc to dc
converter regulator circuitry
is
accomplished with the
circuitry
comprised of A2R1, CRl, CR2,
CRSand
Q4.
The
voltage developed at the junction of R1 and
CR5 forward
biases CRl and transistor
Q4.
Conduction
of
Q4 is
initially
tlirough the low impedance
path of CR2,
thus providing
a
dc input voltage to the zener
current
regulator. The resulting zener reference voltage biases
the
comparison amplifier
circuitry
of
Q3, Q5,
and
Q6
into conduction.
As
the
dc
to
dc
converter regulator
output approaches
the
nominal +18.
5 volt dc level, di-
odes CRl and CR2 are
reversed biased, disconnecting
the turn-on circuitry from
the regulator circuitry.
Transistor
Q4
then functions as
a
buffer
amplifier be-
tween the
series-pass
driver
Ql
and
the comparison
amplifier output from
Q5.
3-24. The zener current
regulator circuitry
is com-
posed of transistors
Q7
through
Q9,
zener
diodes CR3
and CR4 and associated resistors.
Transistor
Q7
is
the
series-pass element
while resistor
R13
provides
starting current to
initially turn-on
the reference
ele-
ments of
CR3 and CR4. The
resistive network
of R17
through
R19 form a divider
across the output
of the
zener current
regulator and
provides
a
sample
of its
output
voltage to the
base of
Q9. Resistor
R19 provides
adjustment
of the output voltage
of the
zener current
regulator.
Transistors
Q8
and
Q9
coxnprise
a differ-
entially
connected
comparison amplifier
with the zener
voltage
of
CR3 and CR4 providing
a
reference at
the
base of
Q8.
This
reference voltage
has a
very low
temperature
coefficient
from which the
instrument
3-3
893A
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