341
A
343A
to
the high
voltage power supply.
This allows the
auxiliary power
supplies time to
stabilize and reach
equillibrium
before high voltage is
applied. Secondly, the
control circuit acts to
trip the instrument to
STANDBY
when
the output terminals
are shorted at voltage settings
of 300 or
above
on
the
1000
volt range. The control
circuit
consists of an
AND gate, which maintains the high voltage
relay in an
energized condition as long as both the delay
and trip
inputs are present.
3-11. The high
voltage power
supply is a
conventional
full-wave voltage doubler.
Output voltage is determined
by
the range
switch and most significant decade control
switch, which select one of
six voltage taps at the power
transformer secondary.
3-12.
CIRCUIT
ANALYSIS
3-13.
+15 VOLT
REFERENCE AND
AUXILIARY
SUPPLIES
3-14. The
+15 volt reference
supply is a
conventional
series regulated
power
supply,
employing a
reference
amplifier and an
error amplifier
to
provide
a
stable,
well
regulated output.
Filtered,
unregulated dc
voltage for the
+15 volt supply
is provided by CR22,
CR25,
and
C23.
Darlington
amplifier
Q15,
Q23
together with
zener diodes
CR19
and
CR20
form a
simple preregulator.
The pre-
regulator improves
line regulation
and reduces
noise due
to
power line
transients. Reference
amplifier Q14
is a
single device
containing
matched zener
reference and
amplifier
elements, thereby
providing a
voltage
reference
that
is extremely
stable with
respect to
temperature
variations.
Output voltage
variations are
sensed at the
input of
differential
amplifier
Q15,
Q17.
The
amplified
output
of the differential
amplifier
is
applied to the
input
of the Darlington
amplifier pair
Q20,
Q19,
which
varies
the conduction of Q19
to maintain a
constant
output
voltage. Frequency
response for
the overall servo loop
is
controlled by R14 and C20.
3-15. Filtered
dc
voltage for
the -15 volt auxiliary
supply
is
provided
by CR23,
CR24, and
C22.
The
voltage
reference
for this
supply is
the +15 volt
reference supply
output.
The error amplifier consists of
differential, ampli-
fier
Q16,
Q18.
Series control of
the output is provided by
Darlington amplifier
Q22,
Q21.
Transistor
Q24
and
associated components
comprise a constant
current source
for
Q16,
thus achieving a high
overall loop gain. Capacitor
C21
and
resistor R6
control the
frequency for this
servo
loop.
3-16. CHOPPER-STABILIZED
AMPLIFIER
3-17.
The
chopper amplifier consists of an input modu-
lator, a high gain
amplifier,
a
synchronous demodulator,
and an
output filter network. Input signals are restricted to
less than 30 Hz by a
low-pass filter consisting of R172,
R171,and C47. The resulting
baseband signal, dc
to
30 Hz,
is applied through R170 to
the
drain of MOS FET chopper
Q39. Q39
is switched off and on at a 215 Hz rate by a
squarewave signal applied to its gate, thus modulating the
input signal and producing a squarewave output signal
having
an
amplitude proportional to the amplitude of the
baseband signal. The resulting signal is coupled to the in-
put of JFET amplifier Q38. Use of the
JFET
as an
input
amplifier provides high
input impedance
and
assures
low
noise operation. From
Q39,
the signal passes through IC1,
an operational amplifier
having
a gain of approximately
420
and a
frequency response of 20 Hz to 10 kHz. The
output of IC1 is inverted in
Q37
and appears with approxi-
mately equal amplitude at both collector and
emitter of
Q37.
The collector signal is synchronously demodulated
by
shunt switch
Q36.
The resulting voltage is filtered
by
R129
and
C31,
leaving only the amplified dc and low
frequency signals. The
Q37
emitter signal is applied
through
a
filter network to C31 to assist
in reducing
chopper ripple and to cancel any base band signal that is
present. The purpose of CR57 is to assist in stabilization
and quick recovery of the amplifier
by
providing a quick
charge path for
C36.
Diode
CR56
clamps the voltage at
the emitter of
Q36 to a
safe value.
3-18.
The
source of the 215 Hz chopper
drive signal
is
multivibrator
Q2,
Q3
and driver Ql. The
signal at the
collector of Ql is coupled to
the drain of
Q39
to compen-
sate for the dc error introduced by the
gate-to-drain
capacitance
of Q39 and
is adjusted by R162.
3-19.
The
preamplifier consists of two differential impli-
fiers and an
output emitter follower. The signal passes
through the input FET amplifier
Q35,
through a
second
differential pair
Q33, Q34,
and out the
emitter
of Q32
to
the series regulator. The purpose of
Cl
and
C2
on
the ref-
erence
board
assembly
is to
reduce noise, principally 60 Hz
ripple, by increasing
the
ac
feedback at the undesired
frequency.
3-20. SERIES
REGULATOR
3-21
.
The series
regulator is composed of a
preregulator,
a
control transistor, and a
fixed
current limiter. The pre-
regulator consists
of four Darlington amplifiers in series
Q42
through
Q49.
Resistors R58,
R60, R61, and R64
divide the high
voltage
equally
among the four amplifiers
3-3
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