LM1205 National Semiconductor, LM1205 Datasheet - Page 18

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

http
Figure 14 is a simplified schematic of the Video Amplifier
Circuit Description
VIDEO AMPLIFIER OUTPUT STAGE WITH BLANK
CIRCUIT
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
current 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 inter-
nal inversions of the video signal in the LM1205 LM1207
the output is non-inverted The collector of Q19 gives a gain
of
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 out-
put 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
Q32 each give a diode drop to the level of the video signal
similar to being connected as diodes Being connected as
www national com
b
10 to the video signal and drives the base of Q20 Q21
b
10 and drives the base of Q30 Q30 thru
FIGURE 13 Simplified Schematic of LM1205 LM1207 Blank Gate Circuit
(Continued)
18
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 isolation between Q33
and the internal circuits of the LM1205 LM1207 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
To match the loading of the feedback section the output at
pin 20 should have a load of 390
sumption 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 capability 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
coefficients 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 an-
other diode potential During blanking the base of Q30 will
be four diode drops above ground plus the saturation volt-
To minimize power con-
TL H 11881 – 21
CC
line