LM1209 datasheet(11/24 Pages) NSC

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Functional Description (Continued)

TLH11884 11

FIGURE 6 Block Diagram of LM1208LM1209 Video Amplifier

TLH11884 12

FIGURE 7 Timing Diagram

Circuit Description

VIDEO AMPLIFIER INPUT STAGE

Figure 8 is a simplified schematic of one of the three video

amplifiers input stage along with the recommended external

components The IC pin numbers are circled and all external

components are shown outside the dashed line The video

input is applied to pin 6 via a 10 mF coupling capacitor and a

30X resistor The resistor is added to limit the current

through the input pin should an applied voltage surge rise

above VCC or drop below ground The performance of the

LM1208LM1209 is not degraded by the 30X resistor How-

ever if EMI is a concern this resistor can be increased to

well over 100X where the rise and fall times will start to

become longer DC bias to the input pin is provided by Q5

and its associated input circuitry Z1 is a 56V zener that

generates the input bias voltage Q1 is a buffer to the zener

reference voltage with 50V generated at its emitter Q3 and

Q4 are connected as diodes Q2 is close to being a diode in

this circuit This configuration will give about 20V at the

collector of Q2 R2 and R3 are a voltage divider setting the

base of Q5 to about 35V This sets the emitter of Q5 to

about 28V the bias voltage of the video input This bias

voltage is necessary to assure that the entire video signal

stays within the active operating region of the LM1208

LM1209 The bias voltage goes through R6 a 20k resistor

to the video input at pin 6 R4 and R6 are of the same value

and R4 is used to compensate for beta variations of the

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