Diodes are non-linear devices. This means that superposition does not apply to
circuits containing diodes.
When a voltage is applied across a resistor, current flows in proportion to
the voltage where the proportionality factor is constant.
The current-voltage (IV) characteristic for a resistor is shown in
Figure 
, and is expressed analytically by
.
Figure: Resistor circuit and its I-V characteristic
For a diode, the IV characteristic looks like
Figure , and is described analytically
by the formula 
.
Where 
is the reverse saturation current, the small (
) current
that occurs in the negative voltage region. 
is a temperature dependent constant
at room temperature. 
is the voltage across the diode
and 
is a dimensionless constant determined by the composition of the diode
(Silicon, Germanium, etc.)
Figure: Diode circuit and its I-V characteristic
To observe the non-linearity of a diode, consider the circuit shown in Figure
.
Figure: A diode circuit with 2 voltage sources.
When we apply 
and 
simultaneously, we find that the voltage across
D is -3 V and the resulting current is very close to 
. If we attempt to use
superposition, we find 
produces 2 V across D and 
. 
produces -5 V across D and 
. If we then sum these two currents, we
would find 
which is obviously incorrect. Since the principle of
superposition does not apply, a diode is a non-linear element.