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High Speed Amplifiers for Video Applications Pt 3 Slide 13

The simple peaking circuit mentioned previously consisting of one R and one C is a crude way to compensate for high-frequency cable losses. Shown here is a more elaborate scheme which would do a much better job at cable equalization. The amount of compensation required for a given cable increases with the cable length. The circuit shown consists of two shunt RC paths that cross to gain setting resistor RG. Intuitively, each RC path does the following, it boosts the gain at a frequency where the capacitance value equals the resistance in parallel with that capacitor, it stops being effective as a booster when the impendence of the shunt RC path is dominated by the resistance in that path. The statements above translate to getting a pole and a zero due to each RC path. Together, the two RC paths will contribute to two poles and two zeroes that will shape the closed loop frequency response. By proper selection of these component values, the designer can tailor the gain curve as a function of frequency if the cable length is fixed. In the example shown, the components calculated will equalize a 50 meter coax, in this case cable, up to about 100 megahertz. Longer lengths and or higher frequencies of interest would require more poles and zeroes for proper equalization. The plot on the top left-hand side shows the overall frequency response with the components selected.

PTM Published on: 2011-11-21