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High Speed Amplifiers for Video Applications Part 2 Slide 6

Being a CFA, the LMH6723 will need a specific value for the feedback resistor. This value is specified in the data sheet as a good compromise between loop stability and excessive frequency roll-off. For the LMH6723, this value is around 470 ohms, as you can see from the graph, for a closed loop gain of five. Let’s discuss this plot a little bit more. This is a fairly typical plot found in the data sheet or otherwise implied for most CFAs. As the required gain is increased from the minimum of one, you can see that the recommended RF is lowered. At higher gains, speed tends to suffer due to the second order parasitic effects that are inherent to the CFA. Lowering RF is intended to counteract these slowing effects and is meant to preserve speed. If this RF reduction with gain were to be continued, pretty soon the other gain setting resistor, known as RG, would then become too small compared to the resistance it sees through the inverting node of the CFA. That is why, beyond a gain of five, you see that RF no longer drops but rather starts to creep back up again.

PTM Published on: 2011-11-21