LM1205 National Semiconductor, LM1205 Datasheet - Page 17

• Manufacturer Part Number: LM1205
• Description: 130 MHz/85 MHz RGB Video Amplifier System with Blanking
• Manufacturer: National Semiconductor

Figure 13 is a simplified schematic of the Blank Gate circuit
Circuit Description
comparators of the three video channels Q103 is added to
help drive the base of Q86 in the clamp comparator in-
creasing the accuracy of the current mirror Q101 drives
R79 and R80 This sets the current thru Q102 thus setting
the current thru Q86 of the clamp comparator
BLANK GATE CIRCUIT
With the exception of the simple output stage and the spot
killer circuit this circuit is almost identical to the clamp gate
circuit The only difference is that the output stage is driven
from the opposite side of the differential pair Thus Q111 is
connected as a diode instead of Q109 With the input at pin
13 at a low level Q108 is turned on also turning on Q29 the
output transistor Q29 is part of the blanking circuit in the
output stage shown in Figure 14 When Q29 is turned on the
output is clamped to a blanking level that is ‘‘blacker than
black’’ allowing blanking to be done on the cathodes of the
CRT
The spot killer circuit is used to force the outputs of the
LM1205 LM1207 into blanking when the V
10 6V Forcing the outputs to a blacker-than-black level will
FIGURE 12 Simplified Schematic of LM1205 LM1207 Clamp Gate Circuit
(Continued)
CC
drops below
17
drive the cathode driver stage well above the black level
cutting off the beam current in the CRT This prevents the
bright spot from occurring when the monitor is turned off
preserving the phosphor of the CRT The CRT will also have
its beam current cut off during the time the monitor is first
turned on This is not a critical period for the CRT since the
filaments have not warmed up to generate a current flow
The comparator along with R89 R90 and Q115 all form the
spot killer circuit Q115 acts the same as Q106 When Q115
has a high signal at its base it is turned off and the outputs
of the LM1205 LM1207 are in the normal operating mode
A low signal at the base of Q115 turns on this transistor
blanking the outputs of the LM1205 LM1207 Q115 is driv-
en by the output of the comparator The inverting input of
the comparator is connected to an internal 1 2V reference
The non-inverting side is connected to a resistor divider net-
work R89 and R90 When V
verting input is above the 1 2V reference therefore the out-
put of the comparator is high This high output turns off
Q115 Once the V
output goes low turning on Q115 which forces the outputs
into the blanking mode
CC
drops below 10 6V the comparator’s
CC
is above 10 6V the non-in-
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