# Finding the Noise Figure for Circuit – Example

**Find the Noise Figure for the given circuit**

This circuit consists of source V_{s} and two resistors R_{s} and R_{p}. R_{s} is the source resistance, and R_{p} is the parallel resistance. The noise effect at the output of the circuit can be calculated as: PSD/gain total multiplied by one by 4kTRs due to thermal noise, so we get the NF. This formula was calculated in the __previous example section.__

If we have different kinds of noise in the circuit, we have to transfer them to the output to see its effect. Combing them, we reach the PSD of output noise.

Let’s find the noise sources in this given circuit.

The first noise source is R_{s}, making V_{s} = 0 to find the noise effect.

The effect of R_{s} would be:

Now, the effect of R_{p} would be:

Simplifying the formula for NF we get:

Therefore, for circuits like this with resistance from source Rs and parallel resistance Rp where is the parallel resistance Rp>> Rs then Rp has no noise contribution in the system. As it has no effect on the noise figure, and NF would be equal to 1, the noise effect is only from the source.

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Tag:NF, Noise Figure