LM1205 National Semiconductor, LM1205 Datasheet - Page 12

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

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Circuit Description
and R4 is used to compensate for beta variations of the
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 LM1205 LM1207
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-
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FIGURE 8 Simplified Schematic of LM1205 LM1207 Video Amplifier Input Stage
(Continued)
12
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 LM1205 LM1207 R10 and R11 are of
the same value but R10 is common to both Q10 and Q11 If
all the current is flowing through Q10 the video amplitude
would only be half of the maximum gain (all current flowing
through Q11) This gives the drive control a total gain ad-
justment range of 6 dB Since the drive control is only used
to balance the color of each channel a small adjustment
range is desirable 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 current 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 differential pair
Q12 and Q13 By this current mirror the voltage at the col-
lector of Q14 is set to the correct DC value for the video
signal by controlling the voltage drop across R13 complet-
ing the DC restoration
TL H 11881 – 16