LM1207AN National Semiconductor, LM1207AN Datasheet - Page 16

• 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

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Circuit Description
CLAMP COMPARATOR CIRCUIT
Figure 11 is a simplified schematic of the clamp comparator
circuit. Q85 and its input transistors, Q81 and Q82 are one
half of the differential pair. The base of Q81 is connected to
pin 19 via R62. This is the positive input to the comparator.
Q88 and its input transistors, Q90 and Q91 are the other half
of the differential pair. The base of Q92 is connected to the
junction of R19 and R20 in Figure 14 via R73. This is the
negative input to the comparator. R73 is included only to
match the input characteristics of the positive input, which
requires the 100 resistor. The negative comparator input is
the feedback from the output stage as briefly described in
the block diagram and covered in more detail in the output
stage circuit description. Q86 is the current source for the dif-
ferential pair. It is turned on and off by the output of the clamp
gate circuit ( Figure 12 ). Q102 of the clamp gate circuit has a
current flow of about 225 µA when it is turned on. This cur-
rent is mirrored into Q86. Assume that the inputs to the com-
parator are equal, making the differential pair balanced. ln
this condition Q85 and Q88 each have a current flow of
113 µA. Looking at the Q85 side of the circuit, Q84 will also
have 113 µA of current flow. Q80 is set up as a current mirror
to Q84, but its emitter resistor is one fourth the emitter resis-
tance of Q84. Thus the current flow for Q80 is four times the
current flow thru Q84, or 450 µA. Q83 has been added to
help drive the base of Q80, increasing the accuracy of the
current mirror. The collector of Q80 directly charges the ca-
pacitor as a current source of 450 µA. R65 is added to dis-
charge the charge stored in the bases of Q80 and Q84. This
is necessary to quickly turn off the current charge of the
clamp capacitor as the comparator section is turned off.
Q87, Q89, and Q90 work in exactly the same way. However,
the collector of Q91 drives another current mirror with the
450 µA. This current flows thru Q78. Q77 is a current mirror
with Q78, thus 450 µA also flows thru Q77. Q79 has been
added to help drive the base of Q77, again adding to the ac-
curacy of the current mirror. Since Q77 is on the ground side
(Continued)
16
of the circuit it discharges the clamp capacitor with 450 µA.
ln this balanced condition the charge and discharge current
are equal, thus the voltage across the clamp capacitor re-
mains unchanged.
Going back to the input stages, note that both inputs, Q81
and Q92, are driven by a 50 µA current source. This keeps
both transistors turned on even when the differential pair,
Q85 and Q88, is turned off. Q82 and Q90 are added to help
drive the bases of Q85 and Q88 respectively. R64 and R72
are added to help discharge the charge stored in the bases
of Q85 and Q88 as these two transistors are turned off.
Since the input stage remains active the differential pair is
quickly turned off. The comparator can also be more quickly
turned on with the input stages remaining active. R67 is used
to assure that the potential difference across the differential
pair is minimal during turnoff. Without R67 there could be a
little extra charge or discharge of the clamp capacitor during
turnoff, creating an error in the black level of the video signal.
Now assume that the input to pin 19 is slightly higher than
the reference voltage to the negative input of the compara-
tor. The voltage at the base of Q85 is now higher than the
base of Q88. This creates an increased current flow thru
Q85 and an equal decrease of current flow thru Q88. This
current change is multiplied by four in the increase of current
flow thru Q80. Likewise the current flow thru Q77 and Q91 is
decreased by four times the current change in Q88. ln the
extreme case the current flow thru Q80 can increase to
900 µA and there would be no current flow thru Q77. Q80
does charge the clamp capacitor, thus the voltage across the
capacitor will increase. The above is all reversed when the
input to Q92 rises above the input level of Q81. If the base of
Q86, the current source to the differential pair, is forced close
to ground, then there is no current flow thru Q86 and the dif-
ferential pair, Q85 and Q88. With the current flow thru the dif-
ferential pair set the zero, all the current mirrors would also
have no current flow. Thus the voltage on the camp capacitor
would remain constant, the desired result during active
video.