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This is another example where we like to amplify a video signal and we would like to maintain the negative going sync pulses present in a composite video waveform at the input. Choosing a rail-to-rail input/output device to do the job of video amplification can simplify the task. No diode string would be needed in this case to shift the output voltage to get close to zero volt out as you can see. The resistor string on the input consisting of R1, R2, and R4 will shift the -.3 volts to .7 volts input signal up in order to comply with the input common volt voltage range of the device chosen given the fact that the composite video signal would have sync tips that actually go below ground to -.3 volts. As you can see the slight loss of signal amplitude through the said resistor string, which is about 10%, is made up in the closed loop gain being raised by the same percentage, i.e., the closed loop gain in this case is 2.2 volts per volt total. Thus, the overall signal gain is still maintained at two. Note that the input voltage range, which is determined by the input resistor string, is interdependent with the closed loop gain setting set by R8 and R7 and the two need to be set together. If the incoming signal was pure video with no sync signal included and still using the LMH6645 which has the rail-to-rail input/output Op Amp, the 75 ohm termination, R4, could be tied directly to the Op Amp positive input and the closed loop gain could have been set to two exactly with an overall simplified resultant circuit. You don’t need to input shift circuit as you see here.
