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LM1212 データシート(PDF) 7 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
部品番号 LM1212
部品情報  230 MHz Video Amplifier System with OSD Blanking
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メーカー  NSC [National Semiconductor (TI)]
ホームページ  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM1212 データシート(HTML) 7 Page - National Semiconductor (TI)

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Circuit Description (Continued)
VIDEO AMPLIFIER SECTION (Output Stage)
A simplified schematic of LM1212’s video amplifier output
stage is shown in
Figure 4 The output stage is the second
gain stage Ideally the gain of the second gain stage would
be AV2 ebR21R18 eb16 Because of the output
stage’s low open loop gain the gain is approximately
AV2 eb10 Thus the maximum gain of the video amplifier
is AV e AV1 c AV2 e 20 Transistors Q23 and Q24 provide
a push-pull drive to the load The output voltage can swing
from 02V to 10V
Q82 is the OSD blanking transistor Its base is connected to
pin 20 via R34 A high TTL signal at pin 20 turns on Q82
This forces a current flow-through R20 and R31 These two
resistors form a voltage divider from the emitter of Q82 If
the input at pin 20 ia a minimum TTL high of 20V then the
juction of R20 and R31 will have a voltage of 06V This
voltage is applied to the bases of Q22 and Q24 06V is just
enough voltage to drive transistors Q22 and Q24 into satu-
ration forcing the output voltage at pin 17 to drop below
100 mV The OSD video signal is applied during this period
The emitter-base junction of Q82 can withstand a maximum
reverse bias voltage of 5V thus pin 17 must not be driven
above 5V during OSD blanking
CONTRAST CONTROL SECTION
A simplified schematic of LM1212’s contrast control section
is shown in
Figure 5 A 0V to 4V DC voltage is applied at the
contrast input (pin 8) Transistors Q29 Q30 and Q34 buffer
and level shift the contrast voltage to the base of Q36 The
voltage at the emitter of Q36 equals the contrast voltage
(Vcont) and the current through Q36’s collector is given by
IC36 e Vcont R28
Transistor Q36’s collector current is used to unbalance the
current through the differential pair comprised of Q38 and
Q40 Q40’s base is internally biased at 53V The 53V refer-
ence is also connected to pin 1 and to the base of Q10 (see
Figure 3 ) The base of Q38 (pin 3) is externally connected
to pin 2 through a 10X resistor (see
Figures 2 and 3 ) With
Vcont e 2V the differential pair (Q38 Q40) is balanced and
the voltage at pins 1 and 2 is 53V Under this condition
Q8’s collector current is equally split between Q9 and Q10
(see
Figure 3 ) and the amplifier’s gain is half the maximum
gain If contrast voltage at pin 8 is greater than 2V then
Q36’s collector current increases thus pulling Q38’s collec-
tor node lower and consequently moving Q38’s base below
53V With pin 2 at a lower voltage than pin 1 current
through Q10 (see
Figure 3 ) increases and the amplifier’s
gain increases With Vcont e 4V the amplifier’s gain is max-
imum
If the contrast voltage at pin 8 is less than 2V then Q36’s
collector current decreases and Q38’s base is pulled above
53V With pin 2 voltage greater than pin 1 voltage less
current flows through Q10 (see
Figure 3 ) consequently the
amplifier’s gain decreases With Vcont e 0V the amplifier’s
gain is minimum (ie maximum attenuation)
TLH12354 – 10
FIGURE 4 Simplified Schematic of LM1212 Video Amplifier Output Stage
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