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NOLLOHS
SECTION
IV
CIRCUIT
DESCRIPTION
4.1
BLOCK DIAGRAM DISCUSSION
Refer
to
the
Block and Wiring diagram
on
page VII-2_
Note
that
the
Model
113
is
composed,
essentially,
of
two
operational
amplifiers in
tandem,
with
associated coupling,
interstage LP/HP filter circuits, overload recovery circuit,
and
battery
charging and
test
circuits_
4.1A INPUT COUPLING CIRCUIT
Individual
input
coupling switches select
the
inputs direct-
Iy,
through
dc
blocking capacitors,
or
select ground, and
apply
the
selected inputs
to
the
corresponding
field-effect
input
transistors
of
ALI
n
the
ac positions,
the
100
megohm bleeders, R4 and R6, keep charge
from
building
up
on
the
capacitors, which
would
otherwise render
the
amplifier inoperative by
offsetting
the
input
FET's
to
cutoff_
In
the
dc
positions
the
signals go directly
to
the
FET's,
so
that
no bleeder
is
needed_ (When
in
a
dc
position,
if
the
signal source
cannot
bleed
off
charge,
then
the
operator
should
provide an external bleeder.)
The
ground
positions are
convenient
to
use
when
setting
the
zero
control,
and
when
using
the
ampl ifier single-ended
the
coupling switch
for
the
unused
input
should
be set
to
the
ground
position.
t 4.1B GROUND-LOOP SUPPRESSION
A typical amplifier
hook-up
consists
of
a signal source
or
sources, amplifier, and
readout
device such as an oscillo-
scope_ This
type
of
hook-up
usually has
two
ground
connections
between
the
signal source and amplifier:
that
which
accompanies
the
power
source
and
mounting
rack,
and
that
which accompanies
the
signal
between
·the source
and amplifier; see Figure IV-l.
The
two
ground
paths
together
form
a loop
that
is
exposed
to
varying environ-
mental electric and magnetic fields. These fields,
cutting
the
loop,
generate
emf's
and
currents
in
it. Various
other
sources
of
current
in
the
loop may exist,
sLlch
as
current
due
to
battery
action
at
the
connectors,
etc. A sign ificant
R
(SIGNAL
GROUND)
CHASSIS GROUND emf CHASSIS GROUND
(DISTRIBUTED
AROUND LOOP)
F~ureIV-l_
GROUN~LOOPCURRENTPATH
IV-l
part
of
the
ground-loop
voltage (V
I)
is
developed
across
the
signal
ground
path,
especially
i~
the
chassis
ground
is
very
good
(it usually
is
and
should
be). This
portion
of
the
ground-loop
voltage
is
amplified along with
the
desired
signal, and,
at
very low signal levels, can cause a
great
deal
of
trouble_
The
Model
113
has provisions
for
eliminating
this
ground-loop
voltage
from
the
signal being amplified_
For
doing this,
the
signal
ground
is
not
connected
directly
to
chassis ground
but
to
a
10
ohm
resistor (R7)
connected
betWeen
the
insulated
outer
shells
of
the
input
BNC's and
chassis
ground.
Since
10
ohms
is
very much
greater
than
the
signal
ground
and chassis
ground
resistances, virtually all
of
the
ground-loop
voltage develops across
the
resistor.
The
first amplifier stage amplifies,
either
differentially
or
single-ended with respect
to
signal
ground,
and
does
not,
therefore,
amplify these
ground-loop
voltages.
The
output
signal
is
referenced
to
chassis
ground,
and
is
free
of
ground-loop
components_
4.1C GAIN DETERMINATION
The
gain switch selects
either
xl0
or
xl00
gain
of
the
first
amplifier,
either
xO.l,
xO.2, xO.5,
or
xl
attenuation
of
an
interstage
attenuator,
and
either
xl
0
or
xl
00
gain
of
the
second amplifier, as required
to
yield
the
gain
marked
at
each position
of
the
switch.
For
the
first
four
switch positions,
the
gain
of
Al
is
xl0.
For
the
last six gain positions,
the
D
section
of
the
gain
switch,
Sl,
connects
pins
10
and
12
of
the
PC
board,
thereby
changing
the
gain
of
A 1
to
xl
00.
How
the
gain
of
A 1
is
determi ned
is
explained
in
the
schematic
discussion
for A
1.
The
interstage
attenuator
comprises
R12
through
-R15.
The-:
A section
of
the
gain switch selects
the
required
attenuation
level.
The
series
attenuator
output
resistors, R
16
through
R 19, adjust
the
attenuator
output
impedance
so
that
the
attenuator
presents
the
same
impedance
to
the
rolloff
circuit
for
all switch positions.
For
all
gain-switch positions,
the
500
ohm-4.5
kilohm
feedback
attenuator
at
the
inverting
input
of
A2
makes
A2
operate
with a gain
of
at
least 10. I n
the
last
three
positions
another
10: 1
attenuator,
comprising R9 and R 10,
is
switched
in
by
the
F section
of
Sl
to
increase
the
gain
to
xl00.
The
gain vern ier, R 11,
is
also
in
th
is
feedback
circuit_
4.10
ROLLOFF FILTER CIRCUIT
The high
frequency
and low
frequency
rolloff circuits are
simple RC filters
connected
in
tandem.
The
HF
and
LF
roll
off
switches select
different
capacitors for each rolloff
frequency
of
the
corresponding
filter.
The
H F rolloff filter
is
composed
of
the
series resistance
presented
by
the
gain
attenuator
circuit
(15.9 kilohms for
all
gain switch posi-
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