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One of the key features of the LNP architecture is the ability to employ active-feedback termination to achieve superior noise performance. Active-feedback termination achieves a lower noise figure than conventional shunt termination, es- sentially because no signal current is wasted in the termina- tion resistor itself. Another way to understand this is as follows: Consider first that the input source, at the far end of the signal cable, has a cable-matching source resistance of Rs. Using conventional shunt termination at the LNP input, a second terminating resistor of value Rs is connected to ground. Therefore, the signal loss is 6dB due to the voltage divider action of the series and shunt Rs resistors. The effective source resistance has been reduced by the same factor of 2, but the noise contribution has been reduced by only the only a 3dB reduction. Therefore, the net theoreti- cal SNR degradation is 3dB, assuming a noise-free amplifier input. (In practice, the amplifier noise contribution will de- grade both the unterminated and the terminated noise fig- ures, somewhat reducing the distinction between them.) See Figure 5 for an amplifier using active feedback. This diagram appears very similar to a traditional inverting ampli- fier. However, the analysis is somewhat different because the gain A in this case is not a very large open-loop op amp gain; rather it is the relatively low and controlled gain of the LNP itself. Thus, the impedance at the inverting amplifier terminal will be reduced by a finite amount, as given in the familiar relationship of Equation (3):
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