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VIDEO_AMPLIFIER_WITH_SYNC_STRIPPER_AND_DC_RESTORER

2016-09-11 21:14  
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VIDEO_AMPLIFIER_WITH_SYNC_STRIPPER_AND_DC_RESTORER
This circuit transmits 220-MHz (-3-dB bandwidth) video signals while stripping off the sync pulse and performing dc restoration. It is configured for a typical video cable-driver application driving a double-terminated 75-Ω load. In other words, the HFA1103 (IC3) is configured for a gain of +2 to ensure unity gain overall. The HFA1103 video op amp is specially designed to perform sync stripping. Its open-emitter NPN output forms an emitter-follower with the load resistor, and passes the active video signal while virtually eliminating the negative sync pulse. Residual sync, defined as the remainder of the original -300-mV sync pulse, referenced to ground, is only 8 mV at the cable output of the HFA1103. Because the HFA1103 contains no active pull-down, output linearity degrades as the signal approaches 0 V. To deal with this, a 6.8-kΩ pull-up resistor (R8) and a 75-Ω pull-down resistor (R10) on the output ensure a fixed positive offset voltage, in this case, +50 rnV. This offset was arbitrarily chosen as a good compromise between linearity near the dc level and minimum residual sync. Increasing R8 decreases residual sync at the expense of linearity. Conversely, lowering R8, decreases linearity error, but increases residual sync. This circuit achieves dc restoration by using a CA5260 dual op amp (IC1a, IC1b) coupled with a sample-and-hold circuit, based on a 74HC4053 switch (IC2). Vin ,consisting of the input video signal and a dc offset (Vdc) ,is connected to the noninverting input of