HP 5254C - VIDEO AMPLIFIER A5

Model
5254C
Section
IV
INPUT
frequency
and
selected
mixing
frequency).
The
difference
frequency
signal in
particular is
coupled
to
the input
of
the
differential
amplifier
(A5Q1
and
A5Q2)
through
coupling resistors
(A3R3
and
A3R4)
as
a
dif-
ferential
signal.
4-13. VIDEO
AMPLIFIER
A5.
(See
Figure
4-3)
4-14.
The two
signals
from mixer
A3
are
applied to
the bases of
differential
amplifier
A5Q1 and
A5Q2.
A5Q1
and A5Q2
output
is applied to
the second
stage
of differential
amplification
(A5Q3
and
A5Q4).
A5Q3
collector output goes
to ground
through
A5C4 and
A5C5,
and
A5Q4 collector
output
is
applied to
the
base of A5Q5.
A5Q5 and
A5Q6
form a
feedback
amp-
lifier
with a constant
gain of
10.
A5Q7 and
A5Q8
limit
the signal and
provide
a high impedance
a the
collector
of
A5Q8.
This high
impedance is
shunted with
100
ohms
(A5R24) to provide
the input
impedance
required
by
the
low-pass
filter.
The
low-pass filter,
a three
-
section
Tschebycheff
filter,
provides a
53-MHz
cut-
off
frequency
for the
video
amplifier.
A5Q9
andA5Q10
form
a shunt
feedback
amplifier
providing a
low im-
pedance
input
at the base
of
A5Q9.
This,
in
series
with
91
-ohm
resistor
A5R25
provides
100
ohms out-
put
impedance
required by
the low
-pass filter.
A5Q11
is a
buffer
amplifier
with a
resistive
divider
output
providing
the
auxiliary
output
to Jl.
A5Q11 output
also
goes to
trigger
circuit
A5Q13,
A5Q14,
and
A5Q15.
Tunnel
diode
A5CR3 shapes
the
signal
into a
rectangu-
lar
pulse
with
very
fast rise
and
fall times.
This
sig-
nal causes
the
trigger
transistors
A5Q14 and
A5Q15
to
switch
quickly.
With
AUX
OUT jack Jl
terminated
in
50 ohms,
output to
the counter
is
twice as
large as
the
auxiliary
output.
There is
sufficient
isolation
in
the
divider
network
to
prevent
the
signal
to the
coun-
ter
from being
affected
by
termination
at
the
auxiliary
output.
4-3