# Noise figure in PSPICE

There is an example in OrCad PSPICE which calculates noise figure for an RF amplifier.

I know the following formula for noise figure:

$$\text{NF}_\text{dB}=10\log\frac{P_{no}}{G_aP_{ni}}$$

But in this example the software uses another formula:

10*Log10(V(inoise)*V(inoise)/8.28e-19)


Where V(inoise) is the equivalent input voltage noise.

I don't understand why this formula should be true and where does 8.28e-19 come from? Is it a general formula that can be used in every simulation?

Here is the result

and the circuit

The formula also reduces to 20 log$_{10}(\frac{V_{NOISE}}{9.1e-10}$) i.e. it compares actual RMS noise against 1Hz-limited voltage noise from a 50 ohm resistor.