LM1207AN National Semiconductor, LM1207AN Datasheet - Page 17

• 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
CLAMP GATE CIRCUIT
Figure 12 is a simplified schematic of the Clamp Gate circuit.
A voltage reference is setup by Z3 and by Q104 and Q105
connected as diodes, generating a 7V base drive to Q94,
Q99 and Q101. Q94 is used to bias the input stage. This
stage is designed to accept TTL levels at pin 14. Q95 and
Q97 form a differential pair. The base of Q97 is set to 2.1V by
Q99 driving the voltage divider formed by R77 and R78. In a
balanced condition the base of Q95 is also at 2.1V. Q96 is
connected as a diode and the current flow thru it is mirrored
into Q98. Also the input to pin 14 would be one diode drop
below 2.1V, or around 1.4V. R74 is added to the input for cur-
rent limiting during any possible voltage surge at pin 14. With
no resistors at the emitters of Q96 and Q98 this circuit will
quickly switch. Below 1.4V (1.2V typical) Q95 is turned on
and Q97 is turned off. Above 1.4V (1.6V typical) Q97 is
turned on and Q95 is turned off. With Q97 turned on Q100 is
also turned on. This pulls the current thru R79 to ground,
turning off Q102 and Q103. Remember Q102 is a current
mirror to Q86 in the clamp comparator. With Q102 turned off,
the clamp comparator is also turned off. When the input sig-
nal goes below 1.2V, Q97 and Q100 will be turned off. This
FIGURE 11. Simplified Schematic of LM1205A/LM1207A Clamp Comparator Circuit
(Continued)
17
allows Q102 to turn on, turning on the clamp comparators of
the three video channels. Q103 is added to help drive the
base of Q86 in the clamp comparator, increasing the accu-
racy of the current mirror. Q101 drives R79 and R80. This
sets the current thru Q102, thus setting the current thru Q86
of the clamp comparator.
DS012866-19
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