Question

Assuming that the silicon diode having resistance of $20\Omega $, the current through the diode is (knee voltage $0.7V$)

a) $0mA$
b) $10mA$
c) $6.5mA$
d) $13.5mA$

Answer 1

Hint: The forward voltage drop of an ideal diode is zero. Here, the resistance and the knee voltage of the diode are mentioned. This means that the diode is non ideal. Hence, we need to replace the diode with a voltage and resistor.

Complete step-by-step answer:
Here, we know that the diode conducts only when it is forward biased. Here the given diode is forward biased. We can say so because the p-junction of the diode is connected to $2V$ and the n-junction of the diode is connected to $0V$. In other words, the p-junction is connected to the higher potential while the n-junction is connected to the lower potential. Hence the diode is forward biased.
An ideal diode, when forward biased, can be replaced by a short circuit in the circuit. But if the diode is not ideal, then we have to put the forward drop of the diode along with its internal resistance in the circuit.
In the question, it is clearly mentioned that the diode has a resistance of $20\Omega $ and along with that the knee voltage of the diode is also mentioned. This indicates that the diode is not ideal.
Hence, we have to replace the diode with its forward voltage drop along with its internal resistance.
We know that the knee voltage is considered as the forward voltage drop.
Hence, the circuit can be redrawn as:

Now, we need to apply a simple KVL equation to find the current through the diode.
Let the current through the diode be $I$.
Using KVL equation, we can write:
$2-0.7-I\times 20-I\times 180=0$
$\Rightarrow 200I=1.3$
$\Rightarrow I=0.0065A$
$\Rightarrow I=6.5mA$
Hence, the current flowing through the diode, along with the whole circuit, is $6.5mA$. Thus, the correct option is (c).

Note: We replaced the diode with a resistor and a voltage source only because the diode was not ideal. It was clearly mentioned that the diode had a resistance and knee voltage which meant non ideal diode. Had the diode been ideal, we would have replaced the diode with a short circuit.