Akai VS-2EGN

11-3-3 AGC CIRCUIT
VIDEO IN
1) Two AGCs are provided: one is for controlling RF
amplifier gain of the tuner and the other is for con-
trolling VIF amplifier gain. The former is RF AGC, and
the latter is IF AGC.
This IC (TA7607AP) uses a peak value AGC circuit.
2) In the peak value AGC, gain is controlled by volt-
age proportional to the peak value of SYNC signal.
3).Fig. 7-113(D) shows the block diagram of the AGC
circuit. (Bold line)
VIF
gain
is
changed
by
controlling
the
collector
cur-
rent
of
the
VIF
transistor
by
the
IF
AGC
voltage,
and
RF
gain
is
changed
by
controlling
the
second
gate
of
the
tuner
RF
amp
FET
by
the
RF
AGC
voltage.
The
RF
AGC
is
a
delay
type
AGC
designed
so
that
it
does
not
operate
when
the
input
is
below
a
certain
level.
4)
The
basic
operation
of
the
peak
value
AGC
cir-
cuit
will
be
described
with
reference
to
Fig.
7-163.
(This
AGC
is
within
the
IC.)
Since
the
peak
value
AGC
is
easily
affected
by
noise,
a
noise
canceller
TR1
is
provided.
A
video
signal
is
fed
to
the
base
of
TR1.
When
noise
of
a
large
negative
ampli-
tude
is
present
here,
the
forward
bias
of
TRI
is
greatly
reduced,
and
a
noise
of
a
large
positive
amplitude
ap-
pears
at
the
collector.
Since
this
is
superimposed
on
the
video
signal
via
R2,
the
noise
is
below
the
SYNC
signal
level.
(Fig.
7-164)
5)
Noise
cancelled
video
signal
of
negative
polarity
is
fed
to
the
base
of
TR2,
and
a
sync
signal
of
positive
polarity
is
picked
up
at
the
collector.
The
signal
is
de-
tected
at
D3,
passes
through
the
integrating
circuit
(R8,
C3),
and
is
averaged.
Then,
an
AGC
control
voltage
is
obtained
via
TR3.
In
Fig.
7-113(D),
this
integrating
circuit
is
represented
by
RI
and
C17
(IF
AGC
filter).
Fig. 7-163 An example of AGC circuit
(a) INPUT SIGNAL
(TRI BASE)
(b)
TRI
COLLECTOR
(c) OUTPUT SIGNAL
((a)=(b))
Fig. 7-164 Noise canceller I/O waveform
6)
TR4
is
a
delay
type
RF
AGC
circuit.
When
the
IF
AGC
voltage
is
lower
than
the
emitter
voltage,
the
tran-
sistor
does
not
operate.
The
voltage
is
divided
by
R10
and
R12,
and
the
RF
AGC
voltage
becomes
constant.
When
the
IF
AGC
voltage
becomes
higher
than
the
emit-
ter
voltage
as
the
input
level
rises,
a
collector
current
flows,
the
RF
AGC
voltage
falls,
and
the
RF
amp
gain
is
lowered
accordingly.
7)
RO
and
R10
of
Fig.
7-113(D)
correspond
to
R10
and
R12
of
Fig.
7-172.
There
is
shown
in
Fig.
7-113(D)
a
filter
(C7,
C35),
which
removes
the
ripple
from
the
AGC
voltage.
(RF
AGC
filter)
The reason why VR4 and VRS are adjusted for each
channel
band
in
Fig.
7-113(D)
is
that
the
tuner
RF
amp-
lifier differs from VHF to UHF.
———
Service
Manill
VS-2EGN
95