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SECTION III
CIRCUIT DISCUSSION
The basic
element
of the Model 610 is a highly
accurate,
stable
dc volt-
meter with
a full
SC
"E
e sensitivity
of 10 millivolts
and an input
imped-
ance greater
than Id-
ohms shunted by 30 micro-microfarads.
Amperes
and ohms are measured by the use of resistance
standards.
The various
connections
necessary
for
amperes and ohms measurements will
be discussed
following
the detailed
description
of the amplifier.
A.
VOLTMETER
Refer to DR 11861 D at the rear
of the manual.
The amplifier
proper
consists
of Vl through
V5,-A.
Vg-B is
a Cathode follower
which drives
--O
INPUT
0
the amplifier
at the same in-
stantaneous
potential
as the
-
input
signal.
In other
words
the neutral
or ground termi-
nal of the amplifier
is not
grounded to the chassis
but
is attached
either
directly
I
or through
divider
Fi165
)
through
Rl71 to the output
cathode follower.
Fig.
61.0-l
OUTPUT QROUND
shows this
dia@'amatically.
The amplifier
A is driven
by
FIGURL
610-I
the cathode follower.
If
it
is desired
to have unity
gain,
the amplifier
ie connected
directly
to the cathode.
To increase
the voltage
gain,
a fraction
of the output
voltage
rather
than all
of it
is fed back.
'
The purpose
of this
arrangement
is to allow
the input
to accept
relatively
large
input
voltages
without
use of input
dividers,
which are neither
stable
nor accurate
at high impedance.
Consequently,
the Model 610 will
accept
100 volts
without
the use of input
dividers,
preserving
the high input
impe-
dance and accuracy
of the amplifier.
Accessory
probes are available
for
ex-
tending
the voltage
range at reduced
input
resistance
and accuracy.
Since the amplifier
proper
is driven
by the cathode follower,
the plus and
minus 220 volt
supplies
for the cathode follower
are referred
to input
pound
while
the t225 volt
and the -85 and -150 volt
supplies
for the amp-
lifer
are referred
to amplifier
ground which is "floating".
In subsequent
discussion,
reference
will
be made to the amplifier
ground as "floating
ground"
and to cathode follower
ground as "output
ground".
The amplifier
input
consists
of two balanced
5886 electrometer
tubes.
The
filaments
are operated
in parallel
through
a dropping
resistor
from the
regulated
B-plus
supply.
The control
grid
of Vl,
the active
electrometer
tube,
is protected
by Rl23,
a one megobm resistor,
bypassed for
high fre-
quencies
by ClOl.
The control
grid
of V2, the "dummy" tube,
is returned
III
- 1
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