The noise figure of a system is defined as
If the system have n stages, it can shown that
where , is the noise figure and the power gain of of the nth stage. Clearly, the first stage, front-end amplifier, should have low noise and high gain in order to achieved a low noise system.
The noise figure of a two-port network active device is given by
where is the minimum noise figure of the two-port active device when its source terminated at . is the equivalent noise resistance and is the system impedance to be taken as 50 . Hence, the noise performance of the active device is completely described by , and which can be found in data sheets or measurement.