| Introduction to Diodes |
We can start to figure out what happens in this circuit by examining what
happens in the circuit in the two situations.
-
We can assume that the
diode is ON and
check whether that assumption is consistent with what else we know - KCL,
KVL and the diode.
-
We can assume that the
diode is OFF
and check whether that assumption is consistent with what else we know
- KCL, KVL and the diode.
-
We are using the method
of contradiction to solve this problem.
Let's
assume that the diode is ON. If the diode is ON, then, we can consider
it so be a short circuit. Here is the circuit with the diode
and symbols for the diode voltage and current.
We've replaced the diode with a short circuit
below.
Since it's now a short circuit, Vd has to
be zero. Let's think this through.
-
The diode is ON
and the voltage across it is zero.
-
The current through the
diode, Id, must be postive.
It can't be negative. Current through a diode can never be negative.
-
The current through the
diode, Id, is Vin/R, (use Ohm's Law) so you cannot have a negative
input voltage.
-
That means that our assumption
that the diode is ON has to be false for negative input voltages.
Let's
assume the diode is OFF. Then, the diode can be replaced by an open
circuit. Here's the equivalent circuit.
-
The diode is OFF
and the current through it is zero.
-
The voltage across the
diode, Vd, must be negative.
It can't be positive.
-
The voltage across the
diode, Vd, is just Vin, (use KVL) so you cannot
have a positive input voltage.
-
A positive input voltage
is inconsistent with the assumption the diode is OFF.
All
of the above is consistent. We have examined all the possibilities
for the diode (ON and OFF) and what we get is consistent so we must have
a good prediction of how the diode works in this circuit.
What can we conclude here?
-
If the input voltage is
positive, current flows through the diode, and the output voltage is equal
to the input voltage.
-
If the input voltage is
negative, no current flows through the diode, and the output voltage is
zero.
What
If The Circuit Is More Complex?
If the circuit is more complex, then we still need to remember that every
diode can be ON or it could be OFF. Here's a circuit with two diodes.
There are four combinations of diode states
that can occur in this circuit. Let us examine all four possibilities.
Here are the four combinations with each diode replaced by either a short
circuit or an open circuit, depending upon whether we assume the diode
is ON or OFF.
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