LM1207AN National Semiconductor, LM1207AN Datasheet - Page 19

• 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
VIDEO AMPLIFIER OUTPUT STAGE WITH BLANK
CIRCUIT
Figure 14 is a simplified schematic of the Video Amplifier
Output Stage including the blanking circuit. Q18 serves as a
buffer between the DC restoration stage shown in Figure 8
and the output stage. A current source is used to fix the cur-
rent flow thru Q18 keeping it well within its operating range.
The emitter of Q18 drives the bases of Q19 and Q24 with the
current thru Q24 being twice that of Q19. Q19, along with
Q20 thru Q23 duplicate the actual output stage going to pin
20. Q19 inverts the video signal (note that the video signal
was inverted at Q7 in Figure 8 ). With two internal inversions
of the video signal in the LM1205A/LM1207A, the output is
non-inverted. The collector of Q19 gives a gain of −10 to the
video signal and drives the base of Q20. Q21 through Q23
are all connected as diodes with the emitter of Q23 driving
R19 and R20. The junction of R19 and R20 is connected to
the base of Q92 via R73 (shown in Figure 11 ), this being the
feedback to the negative input of the clamp comparator. This
stage is independent of the actual output stage at pin 20, but
is where the feedback is done for DC restoration. Therefore
it is possible to blank the actual output stage below the black
level without affecting the DC restoration feedback loop. Q24
is the equivalent part of Q19 in the actual output stage. It
also inverts the video signal with a gain of −10 and drives the
base of Q30. Q30 thru Q32 each give a diode drop to the
level of the video signal, similar to being connected as di-
odes. Being connected as emitter-followers these transistors
also give current gain to the signal. Q33 comes close to also
giving a diode drop to the signal, the voltage drop across
R27 being insignificant. R27 has been added to give some
FIGURE 13. Simplified Schematic of LM1205A/LM1207A Blank Gate Circuit
(Continued)
19
isolation between Q33 and the internal circuits of the
LM1205A/LM1207A, adding to the stability of the device.
Q33 also has R29 in its emitter for isolation from capacitive
loads and current limiting from any possible voltage surges.
R28 is at the collector of Q33 is also for current limiting from
voltage surges and minimizing crosstalk between the three
channels through the V
feedback section the output at pin 20 should have a load of
390 . To minimize power consumption the feedback section
uses resistor values 10 times larger than those at pin 20. The
current source at the emitter of Q33 provides for the capabil-
ity to set the black level as low as 0.5V.
The video signal does go thru a number of diode drops at the
output stage. One may be concerned that the tracking over
temperature could be a problem. The feedback section has
been designed to temperature track the output stage. The
feedback for DC restoration eliminates the temperature coef-
ficients of the diode junctions. The remaining section to be
covered is the blanking section. This section comprises of
Q25 thru Q29. Q26 thru Q28 are connected as diodes. Q25
provides current gain to this stage to adequately pull down
the base of Q30 during blanking and also adding another di-
ode potential. During blanking the base of Q30 will be four di-
ode drops above ground, plus the saturation voltage of Q29.
There are also four diode drops from the base of Q30 to the
output, pin 20. Therefore during blanking pin 20 will be less
than 100 mV above ground, enabling the designer to blank
at the cathode of the CRT. R23 is added to quickly turn off
Q25 by discharging its base when the blanking signal is re-
moved.
CC
line. To match the loading of the
DS012866-21
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