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HP 3400A - Detailed Circuit Description (Part 3)

HP 3400A
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Theory
of
Operation
Model
3400A
4-25.
Driver
amplifier
Q403
further
amplifies
the
ac
signal
and
the
output
at
Q403
collector
is
fed
to
the
base
circuit
emitter
follower
Q404.
The
feedback
path
from
the
collector
of
Q403
to
the
base
of
Q402
through
C405
(10
MHz
ADJ)
prevents
spurious
oscillations
at
high
input
frequencies.
A
dc
feedback
loop
exists
from
the
emitter
circuit
of
Q403,
to
the
base
of
Q401
through
R425.
This
feedback
stabilizes
the
Q401
bias
voltage.
Emitter
follower
Q404
acts
as
a
driver
for the
output
amplifier
consisting
of
Q405
and
Q406;
a
complimentary
pair
operating
as
a
push-pull
amplifier.
The
video
amplifier
output
is
taken
from
the
collectors
of
the
output
amplifiers
and
applied
to
thermocouples
TC401.
A
gain
stabilizing
feedback
is
developed
in
the
emitter
circuits
of
the
output
amplifiers.
This
negative
feedback
is
applied
to
the
emitter
of
input
amplifier
Q401
and
establishes
the
overall
gain
of
the
video
amplifier.
4-26.
Trimmer
capacitor
C405
is
adjusted
at
10
MHz
for
frequency
response
of
the
video
amplifier.
Diodes
CR402
and
CR406
are
protection
diodes
which
prevent
voltage
surges
from
damaging
transistors
in
the
video
amplifier.
CR401,
CR407,
and
CR408
are
temperature
compensating
diodes
to
maintain
the
zero
signal
balance
condition
in
the
out-
put
amplifier
over
the
operating
temperature
range.
CR403,
a
breakdown
diode,
establishes
the
operating
potentials
for
the
output
amplifier.
4-27.
Chopper
Op
Amp
Assembly
AG
And
Thermocouple
Pair
(Part
Of
A4)
4-28.
The
chopper
op
amp
and
thermocouple
pair
form
a
servo
loop
which
functions
to
position
the
direct
reading
meter
M1
to
the
rms
value
of
the
ac
input
signal.
The
chopper
op
amp
and
thermocouple
pair
schematic
diagram
is
illustrated
in
Figure
6-3.
4-29.
The
video
amplifier
output
signal
is
applied
to
the
heater
of
thermocouple
TC401.
This
ac
voltage
causes
a
dc
voltage
to
be
generated
in
the
resistive
portion
of
TC401
which
is
proportional
to
the
heating
effect
(rms
value)
of
the
ac
input.
The
dc
voltage
is
applied
to
the
chopper
op
amp.
4-30.
The
chopper
op
amp
assembly
consists
of
a
chopper
op
amp
(U601),
filter,
oscillator,
and
output
amp
(U602).
The
chopper
op
amp
is
a
high
gain
low
drift
op
amp
and
is
used
to
modulate
the
applied
de
voltage,
amplify
the
voltage,
and
then
demodulate
the
voltage
back
to
de
(the
output
voltage
of
the
amplifier).
The
output
of
the
amplifier
is
the
ac
stabilizing
and
summing
feedback
for
thermocouple
TC402,
and
is
also
applied
to
the
filter.
The
ac
stabilizing
feedback
(using
R614,
R615,
and
C605)
is
used
to
prevent
the
circuitry
from
oscillating.
4-31.
The
oscillator
consisting
of
Q601,
Q602,
and
associated
circuitry,
is
used
to
reduce
the
noise
of
the
chopper
op
amp.
It
operates
at
a
frequency
of
approximately
2KHz.
4-32.
The
output
from
the
chopper
op
amp
(summing
feedback)
is
applied
to
the
heating
element
of
thermocouple
TC402.
The
dc
voltage
developed
in
the
resistive
portion
of
TC402
is
effectively
subtracted
from
the
voltage
developed
by
TC401.
The
input
signal
to
the
chop-
per
op
amp
then
becomes
the
difference
in
the
dc
outputs
of
the
two
thermocouples.
When
the
difference
between
the
two
thermocouples
becomes
zero,
the
de
output
from
the
chop-
per
op
amp
will
be
equal
to
the
ac
from
the
video
amplifier.
4-33.
The
filter,
consisting
of
R617,
R618,
R619,
C607
and
C608,
is
used
to
further
reduce
the
noise
on
the
output
of
the
chopper
op
amp.
This
is
to
make
the
meter
reading
quiet.

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