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Model
3484A
I
NPUT
ATTENUATOR
VARIABLE
FI LTER
3482A·
8
•
3031
9
9ROTECTtON
L[Al<AGE
COMPEN-
SAT
ION
+30V
-30V
+30V
FILTER
AMP
-
30V
Section IV
3460A/8
E1N
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FEEDBACK
I
ATTENUATOR
I
SENSEi
L __ _
Figure 4-2. Simplified
Diagram,
DC
Amplifier.
4-5.
DC
AMPLIFIER.
4-6.
Figure
4-2
is a simplified
diagram
of the
DC
Amplifier. This
is a non-inverting amplifier
having
a
differential
input stage
and
a
push-pull
output stage.
The dual
field effect
transistors used
in the input stage
are employed as source-followers to
provide input im·
pedance
greater
than
1ol
0 ohms on the three
lower
ranges.
4-7.
The
"bootstrap"
diode
string, in conjunction with
constant
current
sources
11
and 13,
is
designed
to
im·
prove the common
mode rejection
characteristics
of
the
amplifier.
Any
common
mode voltage
appearing at the
gates of
ICIA
and
IClB
will
be
coupled
to
the
emitters
of
Q2A
and
Q2B.
This
common emitter connection
is
"bootstrapped" to the common
drain
connection of
IClA
and lClB.
Since
constant
current
sources
11
and
I3
maintain the
proper bias
currents for common
mode
voltages
up
to
±
1
S
V,
this
prevents
the common
mode
voltage from appearing as a
voltage
change at the ampli·
fier output.
4
-8.
The dual
field
effect transistor
is
a special part
containing a
heater which
maintains a
high
temperature
within
the
unit.
Consequently,
the
performance of this
part is affected
very little by
external temperature
changes.
The Leakage Compensation
circuit prevents
leakage
current in
the
field effect
transistor,
lCl
A,
from
causing an offset
voltage
at t he input.
4-9.
When
the instrument is on the 100
mV
range,
the
Overload
Protection
circuit
limits
the
voltage
at the gate
of
IClA
to about ±
1.8
V.
On
all
other ranges,
the
limit is about
±
18
V. This circuit,
in conjunction
with
the resistance
and
zener
diodes
in the input
circuit,
prevents an excessive
input
voltage
from damaging the
input transistor.
Two
zener diodes at the
amplifier
out-
put
limit
the output voJtage to
about
±
18
V.
4-10.
VARIABLE FIL TEA AND FIL
TEA
AMPLIFIER.
The Filter
Amplifier,
used
in
conjunction
with
RC
filter
components, permits a
high
degree
of
filtering with a
relatively fast response time. Filter switching
is
accom·
plished by reed relays
controlled
by circuits
on
the
Range
and
Function Assembly
,
A3. The Filter
Amplifier
is
a unity gain non-inverting amplifier, having its output
capacitively coupled to the input filter circuit.
4-11. MANUAL
AND REMOTE FIL TEA
SELECTION.
When
the
FILTER
OUT
position
of
the
front panel
switch
is
selected, a reed
relay
on the Attenuator As-
sembly bypasses the
RC filter components,
and the
Filter Amplifier
output is
disconnected. When FILTER
A
is
selected,
the
Filter
Amplifier
output and
certain
RC
filter components are connected, and
the bypass
relay
is
opened. In the
FILTER B
position
, additional
filter
capacitance
is
added.
When t
he
FILTER
switch
is
set to
the
REM
position,
either
Filter
A
or
B
may be
selected
remotely
.
If
neither A
or
B is
selected,
the
Filter Out
condition
results.
The
Filter Selection circuits
also affect the amount of delay
present in the Auto·
ranging and
External Trigger Delay
circuits.
4.3
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