Discussion
Statement I: A noise figure of 0 dB describes a noise-free device.
Statement II: A noise figure of 0 dB describes a device at absolute zero.
In the light of the above statements, choose the correct answer.
Answer: C
The first is true by definition and the second confuses two ideas: a noise figure of 0 dB means the device adds nothing to the noise already present at its input, which is an ideal rather than a temperature. Its equivalent noise temperature would be zero kelvin, but that describes the added noise, not the device's physical temperature. A cooled amplifier at 20 kelvin still has a small but non-zero noise figure. Distinguishing added noise from ambient noise, and equivalent temperature from physical temperature, resolves most confusion about this measure.
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