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LM1202 データシート(PDF) 7 Page - National Semiconductor (TI) |
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LM1202 データシート(HTML) 7 Page - National Semiconductor (TI) |
7 / 18 page Circuit Description (Continued) VIDEO AMPLIFIER SECTION (Output Stage) A simplified schematic of LM1202’s video amplifier output stage is shown in Figure 4 The output stage is the second gain stage Ideally the gain of the second gain stage would be AV2 ebR21R18 eb16 Because of the output stage’s low open loop gain the gain is approximately AV2 eb10 Thus the maximum gain of the video amplifier is AV e AV1 c AV2 e 20 Transistors Q23 and Q24 provide a push-pull drive to the load The output voltage can swing from 02V to 10V CONTRAST CONTROL SECTION A simplified schematic of LM1202’s contrast control section is shown in Figure 5 A 0V to 4V DC voltage is applied at the contrast input (pin 8) Transistors Q29 Q30 and Q34 buffer and level shift the contrast voltage to the base of Q36 The voltage at the emitter of Q36 equals the contrast voltage (Vcont) and the current through Q36’s collector is given by IC36 e Vcont R28 Transistor Q36’s collector current is used to unbalance the current through the differential pair comprised of Q38 and Q40 Q40’s base is internally biased at 53V and made available at pin 20 Pin 20 is externally connected to pin 1 through a 100X resistor (see Figures 2 and 3 ) The base of Q38 (pin 3) is externally connected to pin 2 through a 100X resistor (see Figures 2 and 3 ) With Vcont e 2V the differ- ential pair (Q38 Q40) is balanced and the voltage at pins 1 and 2 is 53V Under this condition Q8’s collector current is equally split between Q9 and Q10 (see Figure 3 ) and the amplifier’s gain is half the maximum gain If contrast voltage at pin 8 is greater than 2V then Q36’s collector current increases thus pulling Q38’s collector node lower and consequently moving Q38’s base below 53V With pin 2 at a lower voltage than pin 1 current through Q10 (see Figure 3 ) increases and the amplifier’s gain increases With Vcont e 4V the amplifier’s gain is maximum If the contrast voltage at pin 8 is less than 2V then Q36’s collector current decreases and Q38’s base is pulled above 53V With pin 2 voltage greater than pin 1 voltage less current flows through Q10 (see Figure 3 ) consequently the amplifier’s gain decreases With Vcont e 0V the amplifier’s gain is minimum (ie maximum attenuation) TLH11440 – 5 FIGURE 4 Simplified Schematic of LM1202 Video Amplifier Output Stage http www nationalcom 7 |
同様の部品番号 - LM1202 |
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同様の説明 - LM1202 |
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