The ‘‘Drive’’ pin will allow the user to trim the Q9 gain of

each amplifier to correct for differences in the CRT and high

voltage cathode driver gain stages

A small capacitor

(33 pF) at this pin will extend the high frequency gain of the

video amplifier by compensating for some of the internal

high frequency roll off To use this capacitor and still provide

variable gain adjustment the 51X and series 100X pot

should be used with the red and green drive pins The 91X

resistor used with the blue drive pin will set the system gain

to approximately 62 and allow adjustment of the red and

green gains to 62 plus or minus 25% The video signal at

the collector of Q9 is buffered and level shifted down by

Q10 and Q11 to the base of the output emitter follower Q12

Between the emitter of Q12 and the video output pin is a

40X resistor which was included to prevent spurious oscilla-

tions when driving capacitive loads An external emitter re-

sistor must be added between the video output pin and

ground The value of this resistor should not be less than

390X or package power limitations may be exceeded when

worst case (high supply max supply current max temp) cal-

culations are made If negative going pulse slewing is a

efficient method of emitter pull down would be to connect a

suitable resistor to a negative supply voltage This has the

effect of a current source pull down when the minus supply

10 mA The system gain will also increase slightly because

less signal will be lost across the internal 40X resistor Pre-

cautions must be taken to prevent the video output pin from

going below ground because IC substrate currents may

cause erratic operation The collector currents from the vid-

eo output transistors are returned to the power supply at

current at 12V The IC power dissipation contribution of

load

In applications that require video amplifier shut down be-

cause of fault conditions detected by monitor protection cir-

cuits pin 11 and the wiper arms of the contrast and bright-

ness controls can be grounded without harming the IC This

assumes some series resistance between the top of the

Figure 7 shows the internal construction of the pin 11 24V

reference circuit which is used to provide temperature and

supply voltage tracking compensation for the video amplifier

inputs The value of the external DC biasing resistors should

not be larger than 10 kX because minor differences in input

bias currents to the individual video amplifiers may cause

offsets in gain

TLH9178 – 7

FIGURE 7 LM1203 Video Input Voltage Reference and Contrast Control Circuits

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