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When used in an AC coupled video load configuration as shown, the amplifier used would need to both source and sink current into the load. Let’s say the amplifier is biased such that the output is at 2.5 volts with no swing on the input. The output capacitor would then charge up to this voltage. Each time the input signal excursions take the output positive beyond the 2.5 volt there is current flow through the capacitor. The same thing happens in the negative going excursions where the amplifier would have to sink the current flowing through the capacitor. Due to duty cycle variations, the excursions from mid-point could be as high as the actual video signal amplitude or about two volts in each direction of the mid supply bias point. In this case, the limitation would, of course, be the weaker of the amplifier characteristics and both need to be examined for output current. Looking at the plots shown, it can be seen that the ISINK, or the sinking characteristics of this particular device, is the weaker of the two by a small amount. This is the plot shown on the right hand side. For example, at one volt from the supply, the device can source 30 milliamps and sink only less than that amount. With an AC coupled video signal of two volt peak-to-peak and the amplifier biased to mid supply or 2.5 volts in this case, with worst case duty cycle the output could be at .5 volts from V-. Sinking characteristics, again, the right-hand side plot, show that there would be more than 20 milliamps available. As we had seen earlier, standard video would need about 14 milliamps; that’s for one 150 ohm load. Even though the AC coupled arrangement forced us to consider a larger output swing range, the LMH6645 output still had enough drive for a standard video load of 150 ohm loads. At low enough values of RF and RG, one must also consider the current flow through these components as they could tend to limit output current drive even further.
