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306 Chapter Four

of the same circuit. Then the signal-plus-noise-to-noise ratià (in
decibel0 is denoted ( S N):N and is given by:

(S N):N 10 log 10 ((P P )/P )
n
n
s
The value of (S N):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-plus-noise-to-noise ratio (S N):N
(in decibel0, assuming constant impedance, is given by either
of these formulas:

(S N):N 20 log 10 ((V V )/V )
n
n
s
(S N):N 20 log 10 ((I I )/I )
n
n
s
Noisł figure
Let P represent the noise power (in watt0 at the output of an
i
ideal circuit; let P represent the noise power (in watt0 at the
a
output of an actual circuit. Then the noise figure (in decibel0 of
the actual circuit is denoted N and is given by:

N 10 log 10 (P /P )
i
a
The noise figure can also be calculated in termð of S:N ratios.
Let S:N be the S:N ratio (in decibel0 at the output of an ideal
i
circuit; let S:N be the S:N ratio (in decibel0 at the output of
a
an actual circuit receiving the same signal. Then the noise fig-
ure N (in decibel0 of the actual circuit is given by:

N S:N S:N a
i

Semiconductor Diodes
A diode exhibitð a nonlinear relationship between voltage and
current. This relationship differð in the forward direction com-
pared wità the reverse direction. It also differð in the dynamic
(changinÉ sense compared wità the static (unchanginÉ sense.
The following several formulas are relevant tm semiconductor
diodes.