Circuit Description

Figure 5 is a block diagram of the LM1201 along with the

contrast and brightness controls The contrast control is a

DC operated attenuator which varies the AC gain of the

amplifier without introducing any signal distortions or DC

output shift The brightness control function requires a

‘‘sample and hold’’ circuit (black level clamp) which holds

the DC bias of the video amplifier and CRT cathodes con-

stant 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 non-inverting input of the clamp compara-

tor matches that of the inverting input voltage which was set

by the brightness control

Figure 6 is a simplified schematic of the LM1201 video am-

plifier along with the recommended external components

The IC pin numbers are circled with all external components

shown outside of the dashed line The video input is applied

to pin 16 via the 10 mF coupling capacitor DC bias to the

video input is through the 10 kX resistor which is connected

to the 26V reference at pin 3 The low frequency 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

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 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 col-

lector 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

TLH10006 – 5

FIGURE 5 Block Diagram of LM201 Video Amplifier with Contrast and Black Level Control

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