The ‘‘Drive’’ pin will allow the user to set the maximum gain

of the amplifier based on the range of input video signal

levels and the CRT stage gain if it is fixed or limited When

using three LM1201 devices for high resolution RGB appli-

cations the ‘‘Drive’’ pin allows the user to trim the gain of

each channel to correct for differences in the three CRT

cathodes A small capacitor (12 pF) in shunt with a 51X

drive resistor at this pin will extend the high frequency gain

of the video amplifier by compensating for some of the inter-

nal high frequency roll off The 51X resistor will set the sys-

tem gain to approximately 8 or 18 dB 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

50X resistor which is 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

330X otherwise package power limitations may be exceed-

ed when worst case (high supply max supply current max

temp) calculations are made If negative going pulse slewing

more efficient method of emitter pull down would be to con-

nect a suitable resistor to a negative supply voltage This

has the effect of a current source pull down when the minus

mately 10 mA The system gain will also increase slightly

because less signal will be lost across the internal 50X re-

sistor Precautions must be taken to prevent the video

output pin from going below ground since IC substrate cur-

rents may cause erratic operation The collector current

from the video output transistor is returned to the power

emitter pull down load

In normal operation the minimum black level voltage that

can be set at the video output pin is approximately 2V at

maximum contrast setting In applications that require a low-

er black level voltage a resistor (approximately 16 kX) can

be added from pin 3 to ground This has the effect of raising

the DC voltage at the collector of Q4 which will extend the

range of the black level clamp by allowing Q5 to remain

active In applications that require video amplifier shutdown

due to fault conditions detected by monitor protection cir-

cuits pin 3 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 3 26V

reference circuit which is used to provide temperature and

supply voltage tracking compensation for the video amplifier

input The value of the external DC biasing resistors should

not be larger than 10 kX when using more than one

LM1201 (eg in RGB systems) because minor differences in

input bias currents on the individual video amplifiers may

cause offsets in gain

TLH10006 – 7

FIGURE 7 LM1201 Video Input Voltage Reference and Contrast Control Circuits

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