LM1207AN National Semiconductor, LM1207AN Datasheet - Page 13

• Manufacturer Part Number: LM1207AN
• Manufacturer: National Semiconductor

Specifications of LM1207AN

• Power Supply Requirement: Single
• Dual Supply Voltage (typ): Not RequiredV
• Dual Supply Voltage (min): Not RequiredV
• Dual Supply Voltage (max): Not RequiredV
• Mounting: Through Hole
• Lead Free Status / RoHS Status: Not Compliant

Circuit Description
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 compensating for the diode drops
of Q5, Q6, and Q7. This insures that the bias voltage across
R9 remains very constant over temperature, providing an ac-
curate bias current for the differential transistor pair Q8 and
Q9, thus assuring proper operation 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 voltage
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 LM1205A/LM1207A. 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 circuits. Each
FIGURE 8. Simplified Schematic of LM1205A/LM1207A Video Amplifier Input Stage
(Continued)
13
channel has its own drive control circuit. Increasing the volt-
age to the base of Q11 increases the video gain (drive) of the
LM1205A/LM1207A. 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 adjustment 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 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.
DS012866-16
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