Second Attenuator Assembly; Video Amplifier Assembly; Photochopper Assembly, Chopper Amplifier Assembly, Thermocouple Pair - HP 3400A Operating And Service Manual

Section IV
Model 3400A
voltage
on V201 thereby
establishing the operating
point of this stage.
CR202
and
R211
across
the
base-
emitter junction
of
Q201
protects Q201
in
the
event
of
a failure
in
the
+75
volt
power
supply.
Regulated dc
is
supplied
to
V201
filaments
to
avoid inducing
ac
hum
in
the signal path.
This
also
prevents
the
gain
of
V201
changing with
line
voltage variations.
4-19.
SECOND
ATTENUATOR ASSEMBLY
A3.
4-20.
The second
attenuator is
a
resistive divider
which attenuates the ac
input signal
while maintaining
a low
impedance
output
for the
following amplification
stages. See
second attenuator
assembly schematic
diagram illustrated in
Figure 6-1.
4-21.
The ac input signal
is
applied
to
a
precision
resistance voltage divider consisting of
R302 through
R312.
These resistors are
arranged
to give six
ranges
of
attenuation at 10
dB per
range.
The
six
ranges
of
the
second attenuator
combined with the
two
ranges
of
the input
attenuator
make
up
the 12
ranges
of
attenuation
(0.
001
to
300V),
Trimmer
capacitor
C303
(10
MHz
0.
3V
ADJ)
provides an adjustment for
frequency
response
at the
higher frequencies.
4-22.
VIDEO
AMPLIFIER ASSEMBLY
A4.
4-23.
The video amplifier functions
to
provide con-
stant
gain to the ac signal
being
measured
over
the
entire
frequency range
of
Model 3400A. See video
amplifier
assembly
schematic diagram
illustrated
on
Figure 6-2.
4-24.
The ac input signal
from the
second
attenuator
is
coupled through
C402
to
the base of input
amplifier
Q401. Q401, a class
A
amplifier,
amplifies
and
in-
verts the signal
which
is
then direct
coupled to the
base
of
bootstrap amplifier Q402.
The
output,
taken
from Q402 emitter
is
applied
to
the
base
of
Q403 and
fed
back
to the
top
of
R406 as a bootstrap feedback.
This
positive
ac feedback increases the effective ac
resistance
of
R406
allowing a greater portion
of
the
signal to
be
felt at
the
base
of
Q402. In this
manner,
the effective
ac gain
of
Q401
is
increased for the
mid-
band frequencies
without disturbingthe
static
ope
rating-
voltages
of
Q401.
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 oscil-
lations
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
ampli-
fiers.
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 output amplifier over
the
operating
temperature range.
CR403, a
break-
down
diode,
establishes the operating potentials
for
the output amplifier.
4-27.
PHOTOCHOPPER ASSEMBLY
A5,
CHOPPER
AMPLIFIER ASSEMBLY
A6,
AND THERMO-
COUPLE PAIR (PART
OF
A4).
4-28.
The
modulator /demodulator, chopper ampli-
fier,
and thermocouple
pair
form
a
servo
loop
which
functions
to
position the direct
reading
meter
Ml
to
the
rms
value of the
ac
input signal.
See modulator/
demodulator, chopper amplifier,
and thermocouple
pair
schematic diagram
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
photocell V501.
4-30. Photocells
V501 and
V502
in
conjunction with
neon lamps DS501 and DS502 form a modulator
cir-
cuit.
The
neon lamps
are lighted alternately
between
90
and 100 Hz.
Each lamp illuminates one
of
the
photocells.
DS501
illuminates
V501;DS502 illuminates
V502.
When
a photocell
is
illuminated
it
has a low
resistance
compared
to
its
resistance
when
dark.
Therefore,
when V501
is
illuminated, the output
of
thermocouple
TC401
is
applied to the input
of
the
chopper
amplifier through V501.
When
V502
is
illumi-
nated, a
ground
signal
is
applied
to the
chopper
amp-
lifier.
The
alternate illumination of
V501 and V502
modulates
the dc input
at
a frequency between 90
and
100 Hz.
The modulator output
is
a square
wave whose
amplitude
is
proportional to the dc input level.
4-31.
The chopper amplifier, consisting of
Q601 through
Q603,
is
a high gain
amplifier
which amplifies the
square
wave developed by the
modulator.
Power
supply voltage variations
are reduced by diodes
CR601
thru
CR603.
The amplified output
is
taken
from
the
collector of
Q603 and applied
to
the
demodulator
through emitter follower Q604.
4-32.
The
demodulator
comprises two
photocells,
V503
and V504, which operate
in
conjunction with
DS501 and DS502; the
same neon lamps usedto
illumi-
nate the photocells
in
the modulator. Photocells
V503
and
V504 are
illuminated by
DS501 and DS502, res-
pectively.
4-33.
The
demodulation process
is
the
reverse
of
the
modulation process discussed
in
Paragraph 4-30.
The
output
of
the
demodulator
is
a
dc
level
which
is
proportional
to the
demodulator
input.
The magnitude
and phase
of
the input
square wave
determines
the
magnitude and
polarity
of
the dc output
level.
This
dc
output level
is
applied to
two
emitter follower
output
stages.
i
%
4-2