No Noise Is Good Noise: Noise Matching, Noise Canceling, and Maybe a Bit of Both for Wide-Band LNAs

Researchers have described the concept of noise matching since at least the 1950s, with studies demonstrating the interrelationships among the noise factor of a low-noise amplifier (LNA), the LNA's noise parameters, and the signal-source impedance Z
_s
. Noise matching is accomplished when an LNA is driven by a signal source, the impedance (or admittance) of which is designed-perhaps using a matching network-to equal the LNA's optimum signal-source impedance for minimum noise, or Zopt. The complex Z
_opt
= G
_opt
+ jX
_opt
represents two of the four noise parameters that completely characterize the noise behavior of a linear two-port device, such as an LNA. The other two noise parameters are the minimum noise factor, F
_min
, and the Lange invariant N, which is, arguably, a more fundamental parameter than the often-used equivalent noise resistance, R
_n
.