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Noise figure

The noise figure of a system is defined as
If the system have n stages, it can shown that
where text_wrap_inline3311, text_wrap_inline3313 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 text_wrap_inline3317 is the minimum noise figure of the two-port active device when its source terminated at text_wrap_inline3319. text_wrap_inline3321 is the equivalent noise resistance and text_wrap_inline2827 is the system impedance to be taken as 50 text_wrap_inline2807. Hence, the noise performance of the active device is completely described by text_wrap_inline3317, text_wrap_inline3321 and text_wrap_inline3319 which can be found in data sheets or measurement.