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LM1208N データシート(PDF) 11 Page - National Semiconductor (TI) |
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LM1208N データシート(HTML) 11 Page - National Semiconductor (TI) |
11 / 23 page Functional Description (Continued) 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 µF coupling capacitor and a 30 Ω resistor. The resistor is added to limit the current through the input pin should an applied voltage surge rise above V CC or drop below ground. The performance of the LM1208 is not degraded by the 30 Ω resistor. However, if EMI is a concern this resistor can be increased to well over 100 Ω 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 5.6V zener that generates the input bias voltage. Q1 is a buffer to the zener reference voltage with 5.0V 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 2.0V at the collector of Q2. R2 and R3 are a voltage divider, setting the base of Q5 to about 3.5V. This sets the emitter of Q5 to about 2.8V, 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. 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 transistors. Note that the bias voltage passes through three diode drops (Q5, Q6, and Q7) before setting the voltage across R9. Q2, Q3, and Q4 also provide three diode drops to the bias voltage at the base of Q5, DS100083-11 FIGURE 6. Block Diagram of LM1208 Video Amplifier DS100083-12 FIGURE 7. Timing Diagram www.national.com 11 |
同様の部品番号 - LM1208N |
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同様の説明 - LM1208N |
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