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3330B
signal
applied
to
the
gate
of Ql.
The
collector signal
of
Q5
is
applied
to the
drain
of
Ql
to
compensate
for
spikes
coupled
between
the
gate
and
drain.
Variable
resistor
R34
provides
adjustment of
the
compensation
signal.
An
output
signal at
the collector
of
Q7
is
applied
to
the
base
of
Q4, which
synchronously
demodulates
the
Chopper
Amplifier output.
3-52.
Differential
Amplifier
(A4A3)
3-53.
The
Differential
Amplifier provides
a
control
signal
to the
Series
Pass
Driver. Part
of
the current
limit circuitry
is
also
located
in
this
assembly.
The
differential
amplifier
consists
of
Q2
through
Qll, and
the current
limit circuitry
consists
of
Qlj
Q12,
CRIO
through
CR12,
and
R7
through
RIO.
A
restraining circuit
of
Q13,
and
Q14, and
associated
circuitry,
prevents the
chopper
Amplifier
from
saturating
in
the
standby
mode.
3-54.
DIFFERENTIAL
AMPLIFIER.
Input
control
signals
from
the
A4A10
through
A4AI6
Ladder
summing
junction
are
applied to terminal
5
for amplification
by
the
differential
amplifier
of
Q2
through
QIO. High
frequency
input
signals are
coupled through
Cl
to
one
input
of
the
differential
amplifier.
Low
frequency and dc
control
signals are
applied
through
R21
to
terminal
6,
where
they
are
amplified
by
the
A4A2
Chopper
Amplifier.
Over-
voltage protection for the
differential
amplifier
and Chop-
per Amplifier inputs
is
provided
by
CR2
through
CR9
and
associated
components.
The
amplified control
signals
from
the
Chopper
Amplifier
are
applied
to
terminal
3
as
the other input
of
the
differential amplifier.
The
resulting
output
at
the collector
of
Q9
is
applied
to the
emitter
follower
Q
i 1
,
which
provides
a
low impedance
drive
signal
required
by
the
A4A4
Series
Pass Driver
at
terminal
1.
Transistor
Q8
is
a
current source
for
one
stage
of
the
amplifier
and
provides
common
mode
rejection
at
the
input
to
the
amplifier.
3-55
.
CURRENT
LIMIT CIRCUITRY.
The
current
limit
circuitry
consisting
of
Ql, Q12,
CRIO
through
CR12,
and
R7
tlirough
RIO
provide
a
means
of bypassing
Ladder
cur-
rent
during
a
current
limit
condition.
Tranj>istor
Ql
receives
base
and
emitter operating voltages
from
the
A4A7
Current Limiter and
is
normally
cut-off.
However,
if
the
output
current
of
the instrument
exceeds
a
preset value
established
by
the
CURRENT
LIMIT
control,
Ql
is
driven
into
conduction
by
a
corresponding drop
in
emitter
vol-
tage
and
turns
on
transistor
diode
Q12.
Conduction
of
these
transistors
shunts
the
Ladder
current
through
the
range
resistor
R9,
thus
limiting the control signal
applied
to the
A4A4
Series
Pass Driver
and
the
output current of
the instrument.
3-56.
RESTRAINING
CIRCUIT. The
circuitry
of
Q13
and
Q14
ensures that the Differential Amplifier
is
pro-
vided
with
a
dynamic
load during
a
standby
condition
to
keep
the
output of
the amplifier
relatively
constant.
This
also
maintains
fast
response
during turn-on and
range
changes.
The
emitter voltage of
Q14
is
derived
from
the
emitter
of
Qll,
while the base voltage
of
Q14
is
derived
from
the voltage divider
of
R33
and R34. The
collector
of
Q14, whose conduction depends
primarily
on
its
emit-
ter
voltage, supplies
base
drive
to
Q13.
The
output of
Q13
drives
diode-connected
Q12,
controlling the current
appearing
in
the
summing
junction.
In
this
manner,
the
loop
is
completed and
the
output of
the
Differential
amplifier
will
remain
nearly
constant,
thus preventing
the
Chopper from
going
into
saturation.
3-57.
Series Pass
Driver
(A4A4)
3-58.
The
Series Pass
Driver applies amplified control
signals
to the
A4A5
Series
Pass assembly.
Additional
circuits
found on
the
assembly
are
the overcurrent/voltage
trip circuit
and
the front panel
meter
multiplier
resistors.
A
three-stage amplifier
composed
of
Q5
through
Q7
forms
the
series
pass
driver.
The
overcurrent/voltage
trip
circuitry
consists
of
Ql
througli
Q4
and Q8. The
meter
multipliers
are
R1
through.
R6.
3-59.
SERIES PASS
DRIVER.
The
series
pass
driver
of
Q5
through
Q7
amplifies the
input
regulation control
signal
at
terminal
1
sufficiently
to drive the
A4A5
series-pass
transistor
Q8.
The common-base
configuration of
Q7
provides
isolation
of
the input
circuitry
and
the required
voltage gain
of
the control
signal.
The
common-collector
stages
of
Q5
and
Q6
provide
sufficient
current gain
of
the
drive
signal
applied
out
terminal
4.
3-60.
OVERCURRENT/VOLTAGE
TRIP.
The
over-
current/voltage
trip circuitry,
through
relay
A5A2K.2,
re-
moves
ac
power
from
the high voltage transformer
when-
ever
a
catastropliic
overcurrent or
voltage
trip
occurs.
Transistor
Q3
is
a
current source
for the solenoids
of
this
relay
and
produces
a
-30
volt
dc
operating voltage
at
terminal
10.
Transistors
Ql
and
Q2
activate
the voltage
trip
feature.
Transistors
Q4
and
Q2
activate the over-
current
trip
feature.
Resistors
R22
and
R29
are
the
sensing
resistor
for the
overcurrent
trip
feature.
Transistor
Q8
is
the
driver
amplifier for the
OPR
indicator
lamp.
3-61.
The
voltage
trip
feature
is
activated
when
the
output of
the
instrument exceeds
the
level
established
by
the
VOLTAGE
TRIP
controls, or
when
triggered
by
the
rate limiter.
(Paragraph
3-74). Transistor
Ql
receives
3-7
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