AN3169| Application Note
Maxim/Dallas > App Notes > WIRELESS, RF, AND CABLE Keywords: MAX2655, MAX2656, lna, optimizing noise figures, low-noise amplifier
Apr 06, 2004
APPLICATION NOTE 3169
Low-Noise Amplifier (LNA) Matching Techniques for Optimizing Noise Figures
An RF amplifier is an active network that increases the amplitude of weak signals, thereby allowing further processing by the receiver. Receiver amplification is distributed between RF and IF stages throughout the system, and an ideal amplifier increases the desired signal amplitude without adding distortion or noise. Unfortunately, amplifiers are known to add noise and distortion to the desired signal. In a receiver chain, the first amplifier after the antenna contributes most to the system noise figure. Adding gain in front of a noisy network reduces the noise contribution from that network.
Amplifier Noise Figure
To analyze the effect of circuit noise, one can model the noisy circuit as a noiseless circuit plus external noise sources. For a noisy, two-port network with internal noise sources (Figure 1a), the effects of those sources are represented by the external noise-voltage sources Vn1 and Vn2, placed in series with the input and output terminals, respectively (Figure 1b). Those sources must produce the same noise voltage at the circuit terminals as the internal noise sources. The values of Vn1 and Vn2 are calculated in equations 1 and 2. Representing the noise-free, two-port network in Figure 1b by its Z parameters:
and:
Equations 1 and 2 show that the Vn1 and Vn2 values can be determined from open-circuit measurements in the noisy two-port network. It follows from these equations that when the input and output terminals are open (I1 = I2 = 0) (equations 3 and 4):
and:
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