Video Amplifier A5 - HP 5254C Operating And Service Manual

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