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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