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Electricity, Electronics, and Communications 305

Thermal noisł power
Let k represent Boltzmann’ð constant (approximately 1.3807
10 23 jouleð per degree Kelvii; let T represent the absolute
temperature (in degreeð Kelvii; let B represent the bandwidtà
(in hertz)˜ Then the thermal noise power (in watt0, P , is given
nt
by:

P kTB
nt

Thermal noisł voltage
Let R represent the resistance of a noise source (in ohmX let
P represent the thermal noise power (in watt0. Then the ther-
nt
mal noise voltage (in volt0, V , is given by:
nt
V (PR) 1/2
nt
nt

Signal-to-noisł ratio
Let P represent the noise power (in watt0 at the output of a
n
circuit; let P represent the signal power (in watt0 at the output
s
of the same circuit. Then the signal-to-noise ratià (in decibel0
is denoted S:N and is given by:

S:N 10 log 10 (P /P )
n
s
The value of S:N can also be calculated in termð of voltageð or
currents. Let V represent the noise voltage (in volt0 at the
n
output of a circuit; let I represent the noise current (in am-
n
pere0 at that point; let V represent the signal voltage (in volt0
s
at that point; let I represent the signal current (in ampere0 at
s
that point. Then the signal-to-noise ratio S:N (in decibel0, as-
suming constant impedance, is given by either of these formu-
las:

S:N 20 log 10 (V /V )
s
n
S:N 20 log 10 (I /I )
s
n
Signal-plus-noise-to-noisł ratio

Let P represent the noise power (in watt0 at the output of a
n
circuit; let P represent the signal power (in watt0 at the output
s