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LM1202M データシート(PDF) 5 Page - National Semiconductor (TI) |
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LM1202M データシート(HTML) 5 Page - National Semiconductor (TI) |
5 / 18 page Circuit Description Figure 2 shows a block diagram of the LM1202 video ampli- fier along with contrast and brightness (black level) control Contrast control is a DC-operated attenuator which varies the AC gain of the amplifier Signal attenuation (contrast) is achieved by varying the base drive to a differential pair and thereby unbalancing the current through the differential pair As shown in Figure 2 pin 20 provides a 53V bias voltage for the positive input of the attenuator (pin 1) Pin 3 provides a control voltage for the negative input (pin 2) of the attenua- tor The voltage at pin 3 varies as the voltage at the contrast control input (pin 8) varies thus providing signal attenuation The gain is maximum (0 dB attenuation) if the voltage at pin 8 is 4V and is minimum (maximum attenuation) if the voltage at pin 8 is 0V The 0V to 4V DC-operated drive control at pin 9 providesa6dB gain adjustment range This feature is necessary for RGB applications where independent gain ad- justment of each channel is required The brightness or black level clamping requires a ‘‘sample and hold’’ circuit which holds the DC bias of the video ampli- fier constant during the black level reference portion of the video waveform Black level clamping often referred to as DC restoration is accomplished by applying a back porch clamp signal to the clamp gate input pin (pin 14) The clamp comparator is enabled when the clamp signal goes low dur- ing the black level reference period (see Figure 2 ) When the clamp comparator is enabled the clamp capacitor con- nected to pin 12 is either charged or discharged until the voltage at the minus input of the comparator matches the voltage set at the plus input of the comparator During the video portion of the signal the clamp comparator is disabled and the clamp capacitor holds the proper DC bias In a DC coupled cathode drive application picture brightness func- tion can be achieved by varying the voltage at the compara- tor’s plus input Note that the back porch clamp pulse width (tW in Figure 2 ) must be greater than 100 ns for proper operation VIDEO AMPLIFIER SECTION (Input Stage) A simplified schematic of LM1202’s video amplifier input stage is shown in Figure 3 The 54V zener diode Q1 Q6 and R2 bias the base of Q7 at 26V The AC coupled video signal applied to pin 6 is referenced to the 26V bias voltage Transistor Q7 buffers the video signal VIN and Q8 converts the voltage to current The AC collector current through Q8 is IC8 e VIN R9 Under maximum gain condition transistors Q9 and Q11 are off and all of IC8 flows through the load resistors R10 and R11 The maximum signal gain at the base of Q13 is AV1 eb(R10 a R11)R9 eb2 Signal attenuation is achieved by varying the base drive to the dif- ferential pairs Q9 Q10 and Q11 Q12 thereby unbalancing the collector currents through the transistor pairs Base of Q10 is biased at 53V by externally connecting pin 1 to pin 20 through a 100X resistor Pin 2 is connected to pin 3 through a 100X resistor Adjusting the contrast voltage at TLH11440 – 3 FIGURE 2 Block Diagram of the LM1202 Video Amplifier with Contrast and Brightness (Black Level) Control http www nationalcom 5 |
同様の部品番号 - LM1202M |
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同様の説明 - LM1202M |
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