AN-6607 Datasheet, PDF (1/3 Pages) Fairchild Semiconductor – The Noise Figure Fallacy

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AN-6607
The Noise Figure Fallacy
Noise Figure (NF) can be one of the most misleading
specifications confronting the engineer today. Noise Figure
is defined as the ratio of total output noise power to the
output noise power of the source.
(1)
A minimum NF exists for any amplifier, but is usually far
removed from the actual operating conditions. This is where
the problem begins. Lowering the NF doesn't always lower
the noise which is what the engineer is really interested in.
NF only gives the designer insight into the ratio of the
amplifier noise to the source noise, not the input noise of the
amplifier or the signal to noise ratio.
Amplifier noise performance is adequately described by
modeling the noise sources as a series voltage generator and
a shunt current generator with a series voltage generator for
the source resistance noise.
With the spectral density given by e²ⁿ:
Figure 1. Simplified Amplifier Noise Model
The amplifier noise data is found on vendor data sheets in
the form of e_n and i_n vs. frequency for bipolar transistors and
e_n vs. frequency for FETs and FET amplifiers.
Current noise depends on amplifier input bias current which
is only a few pico-amps for FETs and is therefore negligible.
However, bipolar transistor amplifiers have bias currents into
the micro-amp range where current noise is significant.
The thermal noise of the source resistance is given by
Nyquist's relation.
Figure 2. Thermal Noise vs. Resistance
Using the model of Figure 1, an expression of noise figure
in terms of the noise generators can be developed.
The noise power of the source can be found by using
Nyquist's relation.
(2)
© 1977 Fairchild Semiconductor Corporation
Rev. 1.0 • 6/30/15
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