AN-940
APPLICATION NOTE
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Low Noise Amplifier Selection Guide for Optimal Noise Performance
by Paul Lee
Rev. D | Page 1 of 12
INTRODUCTION
When evaluating an amplifier’s performance for a low noise
application, both internal and external noise sources must be
considered. This application note briefly discusses the funda-
mentals of both internal and external noise and identifies the
tradeoffs associated in selecting the optimal amplifier for low
noise design.
EXTERNAL NOISE SOURCES
External noise includes any type of external influences, such
as external components and electrical/electromagnetic interfer-
ence. Interference is defined as any unwanted signals arriving
as either voltage or current, at any of the amplifier’s terminals
or induced in its associated circuitry. It can appear as spikes,
steps, sine waves, or random noise. Interference can come from
anywhere: machinery, nearby power lines, RF transmitters or
receivers, computers, or even circuitry within the same equip-
ment (that is, digital circuits or switching-type power supplies).
If all interference is eliminated by careful design and/or layout
of the board, there can still be random noise associated with the
amplifier and its circuit components.
Noise from surrounding circuit components must be accounted
for. At temperatures above absolute zero, all resistances act as
noise sources due to thermal movement of charge carriers called
Johnson noise or thermal noise. This noise increases with resis-
tance, temperature, and bandwidth. Voltage noise is shown in
Equation 1.
kTBRV
n
4
(1)
where:
V
n
is voltage noise.
k is Boltzmann’s constant (1.38 × 10
−23
J/K).
T is the temperature in Kelvin (K).
B is the bandwidth in hertz (Hz).
R is the resistance in ohms ().
Current noise (noise associated with current flow) is shown in
Equation 2
R
kTB
I
n
4
(2)
where:
I
n
is current noise.
k is Boltzmann’s constant (1.38 × 10
−23
J/K).
T is the temperature in Kelvin (K).
B is the bandwidth in hertz (Hz).
R is the resistance in ohms ().
