LM1205 datasheet(18/24 Pages) NSC

18 / 24 page

Circuit Description (Continued)

TLH11881 – 21

FIGURE 13 Simplified Schematic of LM1205LM1207 Blank Gate Circuit

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

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 LM1205LM1207

the output is non-inverted The collector of Q19 gives a gain

of b10 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 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 b10 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 diodes 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 isolation between Q33

and the internal circuits of the LM1205LM1207 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 VCC line

To match the loading of the feedback section the output at

pin 20 should have a load of 390X To minimize power con-

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 05V

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-

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