AN-6602 Datasheet, PDF (1/7 Pages) Fairchild Semiconductor – Low Noise JFET

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AN-6602
Low Noise JFET –
The Noise Problem Solver
Introduction
The most versatile low noise active device available to the
designer today is the Junction Field-Effect Transistor
(JFET). JFETs are virtually free of the problems which have
plagued bipolar transistors—limited bandwidth, popcorn
noise, a complex design procedure to optimize noise
performance. In addition, JFETs offer low distortion and
very high dynamic range.
Most designers think of JFETs for very high source
impedances. However, modern devices offer the designer
performance improvements over bipolar transistors in Noise
Figure (NF) for all but lowest impedance (<5000Ω) sources
and even then may provide improved performance If
popcorn noise, bandwidth or circuit component noise is a
consideration (see Figure 1).
Review of Basics
Before guidelines are established for designing low noise
JFET amplifiers, a method of noise characterization must be
chosen. Designers are confronted with a multitude of
different noise parameters such as Noise Figure (NF), noise
voltage and current densities, noise temperature, noise
resistance, etc. Designers are primarily concerned with
signal to noise (S/N) ratios preferring noise voltage, (e_n) and
current (i_n) density.
Noise generally manifests itself in three forms: thermal
noise, shot noise and flicker or "1/f" noise. Thermal noise
arises from thermal agitation of electrons in a conductor and
is given by Nyquist's relation:
Therefore, the purpose of this article is to review low noise
design procedures and indicate the simplicity of designing
high performance low noise amplifiers with low cost JFETs.
The noise of a resistor may be represented as a spectral
density (V²/Hz) or more commonly in uV/√ N Hz or nV/√
Hz and is given by:
Figure 1. Bipolar and JFET Transistor Noise
Comparison
© 2014 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 7/16/15
It is sometimes more convenient to represent thermal noise
as noise current instead of a noise voltage. One needs only
to consider the Norton equivalent yielding a noise current
density.
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