Table of Contents
Advertisement
341
343A
Amplifier
output
is
controlled
by
the
range
and
beta
string
resistors,
which
control the input
signal
and
the negative
feedback.
3-5.
The
signal
source
is
the
+15
volt
reference supply.
A
reference amplifier within the supply assures high
sta-
bility
with
respect to
time
and
temperature.
An
output
current
of 10 microamps, 100 microamps,
or
1
milliamp
for
the
10
volt,
100
volts,
and 1000
volt
ranges respectively
is
selected
by
the range control
switch.
The
resulting
volt-
age
at
the
summing
junction S
is
applied to the input of the
chopper-stabilized amplifier.
The
function of
this
amplifier
is
to
control
conduction of
the
series
regulator so that the
resulting
voltage
at
S approaches
zero.
As shown
on
the
sig-
nal
flow
line,
signal
current
from
the
series
regulator
passes
through
the
rectifier
and
filter
and through
the
volt-
age
control
network
into the
summing
junction, thereby
completing
the servo loop.
3-6.
The
chopper-stabilized
amplifier
consists
of
a
chopper
amplifier
and
a
preamplifier.
The
chopper
ampli-
fier
amplifies
all
frequencies
from
dc
to
approximately
30
Hz;
the preamplifier
is
used
principally for
frequencies
from 30
to
approximately 500 kHz.
3-7.
The
controlled current
limiter
together
with
the
chopper-
stabilized amplifier
and
series
regulator
comprise
a
second
servo loop,
which
operates
under abnormal
load-
ing
conditions,
such
as
short
circuited
output,
to
ensure
linear
operation of the
amplifier.
The
controlled current
limiter
continuously monitors output
current
and compares
it
to the setting
of
the current
limit control.
When
the
vol-
tage across
current sensing
resistor
R70
exceeds
the
refer-
ence
voltage
set
by
the current
limit
control, the
limiter
generates
a
current,
which
subtracts
from
the
+15
volt
reference
current.
Thus,
the
output
is
limited or
clamped
to the preset value.
3-8.
The
function
of
the negative voltage
limiter
is
to
ensure
linear
operation of the shopper-stabilized
amplifier
during
periods,
such
as
downranging,
when
the
series
regu-
lator
is
cut off
and
the
main
servo
loop
is
opened.
The
negative voltage
limiter
is
activated
when
the amplifier
output attempts
to
go
below
-2.5
volts.
The
limiter
then
generates
a
current
into the
summing
junction,
which
balances
the
input
reference
and
beta
string currents,
thereby
limiting
the
negative
output of
the
amplifier.
The
action
of
the limiter
thus
assures
fast
recovery
and
settling
time
for
the amplifier.
3-9.
The
primary
function
of
the sink
supply
and
crowbar
circuitry
is
to
quickly
discharge
the
output
capacitor
and
the
high
voltage
filter
capacitors
when
downranging
the
instrument.
These
circuits also
provide
protection
for
the
instrument during
abnormal
load con-
ditions,
such
as
short circuited
output.
The
sink
supply
serves
two
purposes:
It
discharges the
output
capacitor,
and
it
protects the
series
regulator
transistors
by
tempor-
arily
clamping
the voltage across
them
to
a safe value.
The
crowbar
circuit
protects the
series
regulator
transistors
by
discharging the high voltage
filter
capacitors.
It
also
generates
a
signal
for
the control
circuit,
which, under
certain
conditions,
is
used
to
trip
the
instrument
to
standby.
3-10.
The
control
circuit
provides
a
fixed delay
of
3 to
5
seconds, following
initial
turn-on, before
ac
is
connected
3-2
Advertisement
Table of Contents
