Circuit Description
Figure 5
is a block diagram of one of the video amplifiers
along with the contrast and brightness controls The con-
trast control is a dc-operated attenuator which varies the ac
gain of all three amplifiers simultaneously while not introduc-
ing any signal distortions or tracking errors The brightness
control function requires a ‘‘sample and hold’’ circuit (black
level clamp) which holds the dc bias of the video amplifiers
and CRT cathodes constant during the black level reference
portion of the video waveform The clamp comparator
when gated on during this reference period will charge or
discharge the clamp capacitor until the plus input of the
clamp comparator matches that of the minus input voltage
which was set by the brightness control
Figure 6
is a simplified schematic of one of the three video
amplifiers along with the recommended external compo-
nents The IC pin numbers are circled with all external com-
ponents shown outside of the dashed line The video input
is applied to pin 6 via the 10
mF
coupling capacitor DC bias
to the video input is through the 10 kX resistor which is
connected to the 2 4V reference at pin 11 The low frequen-
cy roll-off of the amplifier is set by these two components
Transistor Q1 buffers the video signal to the base of Q2
The Q2 collector current is then directed to the V
CC
1 sup-
ply directly or through the 1k load resistor depending upon
the differential DC voltage at the bases of Q3 and Q4 The
Q3 and Q4 differential base voltage is determined by the
contrast control circuit which is described below RF decou-
pling capacitors are required at pins 2 and 3 to insure high
frequency isolation between the three video amplifiers
which share these common connections The black level dc
voltage at the collector of Q4 is maintained by Q5 and Q6
which are part of the black level clamp circuit also described
below The video signal appearing at the collector of Q4 is
then buffered by Q7 and level shifted down by Z1 and Q8 to
the base of Q9 which will then provide additional system
gain
TL H 9178 – 5
FIGURE 5 Block Diagram of LM1203 Video Amplifier with Contrast and Black Level Control
7
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