HP 3400A Service Manual

Model
3400A
Theory
of
Operation
4-15.
Impedance
Converter
Assembly
A2
4-16.
The
impedance
converter
assembly
uses
a
FET
follower
circuit
to
match
the
high
out-
put
impedance
of
the
input
attenuator
to
the
low
input
impedance
of
the
second
attenuator.
The
follower
circuitry
has
unity
gain
with
no
phase
inversion
between
the
input
and
output
signals.
The
circuitry
is
illustrated
in
Figure
6-1.
417.
The
ac
input
signal
to
the
impedance
converter
is
resistor/capacitor
coupled
via
R202
and
C201
to
the
gate
of
FET
Q201.
The
output
of
the
converter
is
developed
across
a
Q202
which
acts
like
a
variable
load
resistor
for
the
FET.
4-18.
The
bootstrap
circuitry,
consisting
of
C202
and
R205,
increases
the
input
impedance
of
the
converter
circuitry
and
the
compensating
feedback
circuitry,
consisting
of
C204,
decreases
the
output
impedance
of
the
converter.
The
C202
feedback
circuitry
is
also
used
to
flatten
the
converter’s
frequency
response
at
high
frequencies.
The
bias
voltage
for
Q202
is
developed
by
resistors
R209,
R210,
R211,
and
CR206.
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
at-
tenuator
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
(10MHz
0.3V
ADJ)
provides
an
adjustment
for
frequency
response
at
the
higher
frequen-
cies.
4-22. Video Amplifier Assembly A4
4-23.
The
video
amplifier
functions
to
provide
constant
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
inverts
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
feed-
back.
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
midband
frequencies
without
disturbing
the
static
operating
voltages
of
Q401.
4-3

HP 3400A Specifications

General

Brand	HP
Model	3400A
Category	Measuring Instruments
Language	English