Conclusion

Knowing the noise factor of a quadrupole is not enough to describe its noise behavior, especially if it's an active quadrupole. It is therefore necessary to determine the four noise parameters. This can sometimes be a tricky business, especially if the quadrupole is at risk of instability. Most of the time, noise models are supplied by the foundry or component manufacturer. The results they give are usually based on measurements similar to those described here. However, it often happens that noise parameters outside the validity conditions of the model supplied need to be determined. In such cases, you need to perform the measurements yourself.

The main aim in designing a low-noise amplifier is to match the generator impedance giving maximum gain with that giving minimum noise. This can only be done if all four noise parameters are known.

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