GE 4PE350A3 Instructions Manual page 32

EBI-6351
COLOR CAMERA
Luminance Video Amplifier Board,
PL -7674514
Refer to the Waveforms, Figs. 18 and 19,
the Printed Circuit Diagram, Fig. 20, and
the Schematic Diagram, Fig. 21.
Transistor Q1 is a video amplifier with
an approximate gain of 9.3. The frequency
response of this stage is +1 db to 8 mHz
and 3 db down at 10 mHz. Capacitor C2 is
used to adjust the frequency response of the
amplifier. Q2 is an emitter follower which
provides a low impedance for the clamped
amplifier, Q4. Transistor Q3 is used as a
squasher/clamp. This differs from a regular
clamp in that
it
operates for most of the
horizontal blanking interval. When used in
conjunction with a series resistance such as
R11, it reduces the horizontal interval inter-
ference by a large factor. The remainder
of the amplifier is devoted to aperture cor-
rection.
It
is preferable from a signal-to-noise
standpoint for an aperture correction cir-
cuit (as used in this Camera) to be ampli-
tude -dependent, that is to limit the aperture
correction so that only the white parts of
the picture are corrected. This is desirable
because the noise level is inherently raised
when aperture correction is accomplished,
and since noise is more apparent in the gray
and black regions of the picture than in the
light or white regions,
it follows that the
noise in the dark regions should not be in-
creased.
In
the
nonaperture-corrected mode of
operation, the video is applied through de-
lay
line L8 to
L13, etc, to the base of
Q9 and then to the feedback output stages,
Q12 and Q13. The output of Q6 is ap-
plied to the clipper, CR2 and CR3. This
clipper is so arranged that only the white
part
of
the
picture
is
passed through
and the
gray and black portion is clip-
ped away. The clipping point is adjust-
able by varying R12. The remaining por-
tion of the signal
is
applied through R25
to the base of the phase splitter, Q7. To
illustrate how the remainder of the cir-
cuit operates, assume that the video con-
sists
of a
single pulse as
shown in
Sketch A (1). This video has rounded cor-
ners because
of
the finite
spot size of
18
(2)
--
Q7- e
(3)
Q7- C
(4)
Q9
c5)
Q II
( 6)
QI0
(7)
QII
(7Q10
Q9
Sketch A