Circuit Description
(Continued)
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 6 2 and allow adjustment of the red and
green gains to 6 2 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
problem because of high capacitive loads (
l
10 pF) a more
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
voltage is
b
12V and the emitter current is approximately
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
V
CC
2 pin 23 When making power dissipation calculations
note that the data sheet specifies only the V
CC
1 supply
current at 12V The IC power dissipation contribution of
V
CC
2 is dependent upon the video output emitter pull down
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
control pots and V
CC
Figure 7
shows the internal construction of the pin 11 2 4V
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
TL H 9178 – 7
FIGURE 7 LM1203 Video Input Voltage Reference and Contrast Control Circuits
9
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