LM1205 National Semiconductor, LM1205 Datasheet - Page 15

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

Figure 11 is a simplified schematic of the clamp comparator
Circuit Description
CLAMP COMPARATOR CIRCUIT
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
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
differential pair It is turned on and off by the output of the
clamp gate circuit (Figure 12) Q102 of the clamp gate cir-
cuit has a current flow of about 225 A when it is turned on
This current is mirrored into Q86 Assume that the inputs to
the comparator are equal making the differential pair bal-
anced ln this condition Q85 and Q88 each have a current
flow of 113 A Looking at the Q85 side of the circuit Q84
will also have 113
current mirror to Q84 but its emitter resistor is one fourth
the emitter resistance of Q84 Thus the current flow for Q80
is four times the current flow thru Q84 or 450 A 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 capacitor as a current source of 450 A R65 is
added to discharge 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 A This current flows thru Q78 Q77 is a
current mirror with Q78 thus 450
Q79 has been added to help drive the base of Q77 again
adding to the accuracy of the current mirror Since Q77 is on
the ground side of the circuit it discharges the clamp
resistor The negative comparator input is
A of current flow Q80 is set up as a
(Continued)
A also flows thru Q77
15
capacitor with 450 A ln this balanced condition the charge
and discharge current are equal thus the voltage across the
clamp capacitor remains unchanged
Going back to the input stages note that both inputs Q81
and Q92 are driven by a 50 A 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 dif-
ferential pair is minimal during turnoff Without R67 there
could be a little extra charge or discharge of the clamp ca-
pacitor 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 comparator 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 A 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 differential pair Q85 and Q88
With the current flow thru the differential 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
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