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LM1209 Datasheet

  • LM1209

  • National Semiconductor [130 MHz/100 MHz RGB Video Amplifier...

  • NSC

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Circuit Description
(Continued)
transistors Note that the bias voltage passes through three
diode drops (Q5 Q6 and Q7) before setting the voltage
across R9 Q2 Q3 and Q4 also provide three diode drops
to the bias voltage at the base of Q5 temperature compen-
sating for the diode drops of Q5 Q6 and Q7 This insures
that the bias voltage across R9 remains very constant over
temperature providing an accurate bias current for the dif-
ferential transistor pair Q8 and Q9 thus assuring proper op-
eration of the contrast control
Q6 serves as a buffer to the input video signal Its emitter
drives the base of Q7 Thus the video signal modulates the
current flowing through R9 which in turn modulates the cur-
rents through the differential pair formed by Q8 and Q9 The
current flow through Q8 and Q9 is controlled by a DC volt-
age from the Contrast Control circuit This DC voltage is
common to all three channels Increasing the voltage to the
base of Q9 with respect to the base of Q8 increases the
current flow through Q9 A higher current flow through Q9
increases the video gain (contrast) of the LM1208 LM1209
Q10 and Q11 also form a differential pair at the collector of
Q9 The operation of this differential pair is similar to Q8 and
Q9 The DC control voltage is from the Drive Control cir-
cuits Each channel has its own drive control circuit In-
creasing the voltage to the base of Q11 increases the video
gain (drive) of the LM1208 LM1209 At a 4V control voltage
the gain of the LM1208 LM1209 is at maximum with all the
current flowing through Q11 If all the current was flowing
through Q10 the video signal attenuation would be over
40 dB the maximum attenuation Q12 through Q17 are part
of the final section shown in
Figure 8
DC restoration is done
at this stage The clamp comparator
(Figure 11)
drives the
clamp cap at pin 8 to a voltage that sets the correct black
level of the video signal This cap is also connected to the
base of Q17 Q17 and Q16 are one half of the darlington
differential pair The clamp cap voltage establishes the cur-
rent flow through R16 Q15 and R15 With the bases of
Q14 and Q15 held to the same voltage the current through
Q15 is mirrored into Q14 and the other half of the differen-
tial pair Q12 and Q13 By this current mirror the voltage at
the collector of Q14 is set to the correct DC value for the
video signal by controlling the voltage drop across R13
completing the DC restoration
TL H 11884 鈥?13
FIGURE 8 Simplified Schematic of LM1208 LM1209 Video Amplifier Input Stage
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