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 24V 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

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

TLH9178 – 5

FIGURE 5 Block Diagram of LM1203 Video Amplifier with Contrast and Black Level Control

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